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Cameroon GCE O-Level Chemistry study materials
Cameroon GCE O-Level Chemistry study resources

Cameroon GCE O-Level Chemistry practice questions

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Question 1Particulate nature of matter, states of matter and kinetic theory

Which statement best describes the arrangement and motion of particles in a gas?

  1. Particles are far apart, randomly arranged, and move rapidly in all directions. Correct answer.
  2. Particles are closely packed in fixed positions and only vibrate.
  3. Particles are close together but can slide past one another in a random pattern.
  4. Particles are held in a regular lattice and rotate about fixed points.

Explanation

In a gas the intermolecular forces are negligible, so particles are widely separated and occupy the whole container. They move rapidly and randomly, colliding with each other and with the walls. This random fast motion explains why gases fill any container and are easily compressed.

Question 2Particulate nature of matter, states of matter and kinetic theory

A sealed syringe contains air at room temperature. When the plunger is pushed in, the air volume decreases easily. Which property of gases explains this?

  1. Gas particles themselves shrink when pressure is applied.
  2. The spaces between gas particles are large, so particles can be pushed closer together. Correct answer.
  3. Gas particles dissolve into the walls of the syringe.
  4. Gas particles lose their kinetic energy and stop moving.

Explanation

Gases are highly compressible because most of the volume of a gas is empty space between particles. Applying pressure reduces the distance between particles without changing their own size. The particles keep moving; only the average spacing decreases.

Question 3Particulate nature of matter, states of matter and kinetic theory

Which change of state involves particles gaining energy and moving further apart?

  1. Freezing
  2. Evaporation Correct answer.
  3. Condensation
  4. Deposition

Explanation

Evaporation is a change from liquid to gas, so particles absorb energy and escape from the liquid surface into the space above. The average separation of the particles increases. Condensation, freezing and deposition are all the reverse processes, releasing energy.

Question 4Particulate nature of matter, states of matter and kinetic theory

A drop of ink placed at the bottom of a beaker of water slowly spreads until the water is uniformly coloured, even without stirring. What is this process called?

  1. Sublimation
  2. Diffusion Correct answer.
  3. Filtration
  4. Distillation

Explanation

Diffusion is the spontaneous spreading of particles from a region of higher concentration to one of lower concentration due to their random motion. Ink particles move through the water until they are evenly distributed. No stirring is needed because the particles have their own kinetic energy.

Question 5Particulate nature of matter, states of matter and kinetic theory

Which of the following is evidence that matter is made of tiny particles in constant motion?

  1. Water boils at 100 degrees Celsius at standard pressure.
  2. The smell of perfume spreads across a room. Correct answer.
  3. Ice floats on water.
  4. A solid block of iron keeps its shape.

Explanation

The spreading of perfume through air is diffusion, which can only be explained if perfume particles move randomly through the air particles. This motion is a direct consequence of the kinetic theory of matter. The other observations do not require particle motion for their explanation.

Question 6Particulate nature of matter, states of matter and kinetic theory

A gas is heated at constant volume. According to the kinetic theory, what happens to its particles?

  1. They combine to form a liquid.
  2. They move faster and collide with the container walls more often and more forcefully. Correct answer.
  3. They lose kinetic energy and settle at the bottom.
  4. They become larger and occupy more space.

Explanation

Heating transfers energy to the particles, increasing their average kinetic energy and therefore their speed. Faster particles hit the walls more frequently and with greater force, so the pressure rises. The size of the particles does not change.

Question 7Particulate nature of matter, states of matter and kinetic theory

Why does a liquid have a definite volume but no definite shape?

  1. Its particles are bonded permanently to the container walls.
  2. Its particles are close together but can slide past one another. Correct answer.
  3. Its particles are very far apart and move randomly.
  4. Its particles are fixed in a lattice but can rotate.

Explanation

In a liquid, particles are still close together with significant attractive forces, giving a fixed volume. However, they have enough energy to move past each other, so the liquid takes the shape of its container. This intermediate behaviour distinguishes liquids from solids and gases.

Question 8Particulate nature of matter, states of matter and kinetic theory

A sample of gas is compressed at constant temperature. Which statement correctly describes the effect on the particles?

  1. The particles stop moving because the space is smaller.
  2. The particles become smaller so more fit in the container.
  3. The average speed of the particles increases.
  4. The particles hit the walls more frequently, so pressure increases. Correct answer.

Explanation

At constant temperature the average kinetic energy and speed of the particles stay the same. Compressing the gas reduces the volume, so the particles are closer together and strike the walls more often per unit time, raising the pressure. The particle size is unchanged.

Question 9Particulate nature of matter, states of matter and kinetic theory

During melting, the temperature of a pure solid remains constant even though heat is being supplied. Why?

  1. The particles stop absorbing heat once melting begins.
  2. The solid is an insulator so heat cannot enter it.
  3. The heat is used to overcome attractive forces between particles rather than increase kinetic energy. Correct answer.
  4. The heat is lost to the surroundings.

Explanation

At the melting point, the energy supplied is used as latent heat to break the forces holding particles in the solid lattice. Since the average kinetic energy of the particles does not rise, the temperature stays constant until all the solid has melted. Only after melting does further heating raise the temperature.

Question 10Particulate nature of matter, states of matter and kinetic theory

A student places a crystal of potassium manganate(VII) in water. The purple colour spreads slowly through the water, and spreading is faster in warm water. Which conclusion is best supported?

  1. Warmer particles move faster, so diffusion occurs more quickly. Correct answer.
  2. Potassium manganate(VII) dissolves only in hot water.
  3. The colour is carried by gravity alone.
  4. Water particles are stationary until heated.

Explanation

Diffusion occurs because particles move randomly, and raising the temperature increases their average kinetic energy. Faster-moving particles spread the coloured ions through the water more quickly. The crystal dissolves in both hot and cold water; only the rate differs.

Question 11Particulate nature of matter, states of matter and kinetic theory

Which statement explains why a gas can be compressed much more than a liquid?

  1. Gas particles are much further apart, so there is more empty space to reduce. Correct answer.
  2. Liquid particles are heavier and cannot move.
  3. Gas particles attract each other more strongly than liquid particles.
  4. Gas particles are softer than liquid particles.

Explanation

In a gas, the distance between particles is much larger than the particle size, so most of the volume is empty space that can be reduced by pressure. In a liquid the particles are already close together, so there is little free space to compress. This difference in spacing is the key reason for the difference in compressibility.

Question 12Particulate nature of matter, states of matter and kinetic theory

Bromine vapour is placed in the bottom of a gas jar and air is above it. After some time the brown colour is seen throughout the jar. Which statement best explains this?

  1. Bromine reacts with the air to form a coloured gas.
  2. Bromine and air particles move randomly and mix by diffusion. Correct answer.
  3. The bromine condenses and then evaporates repeatedly.
  4. Bromine particles are lighter than air and rise.

Explanation

Diffusion is the random mixing of particles of different substances due to their kinetic energy. Bromine particles and air particles move in all directions and become evenly mixed, so the colour spreads throughout the jar. The process does not depend on bromine being lighter than air.

Question 13Particulate nature of matter, states of matter and kinetic theory

A liquid in an open dish evaporates and cools. Which statement explains the cooling?

  1. The liquid absorbs heat from itself to raise its temperature.
  2. The slowest particles escape, leaving faster particles behind.
  3. The fastest particles escape, lowering the average kinetic energy of the remaining liquid. Correct answer.
  4. All particles leave the liquid at once.

Explanation

Evaporation occurs when the most energetic particles at the surface escape into the vapour. Removing the fastest particles lowers the average kinetic energy of those left behind, so the temperature of the liquid falls. This is why evaporation produces cooling.

Question 14Particulate nature of matter, states of matter and kinetic theory

Two gas samples, one of hydrogen and one of oxygen, are at the same temperature. Which statement is correct?

  1. The hydrogen molecules have the same average kinetic energy as the oxygen molecules. Correct answer.
  2. The oxygen molecules have no kinetic energy.
  3. The hydrogen molecules have a higher temperature.
  4. The oxygen molecules move faster because they are heavier.

Explanation

Temperature is a measure of the average kinetic energy of particles. At the same temperature, the average kinetic energy of hydrogen and oxygen molecules is equal, regardless of their different masses. Because hydrogen molecules are lighter, they move faster on average for the same kinetic energy.

Question 15Particulate nature of matter, states of matter and kinetic theory

A sealed rigid container holds a gas at constant volume. The gas is cooled until it condenses. Which sequence correctly describes the changes in particle behaviour?

  1. Particles keep the same speed but change colour.
  2. Particles break into smaller particles and lose mass.
  3. Particles speed up, move further apart, and become randomly arranged.
  4. Particles slow down, come closer together, and become held in fixed positions. Correct answer.

Explanation

Cooling reduces the average kinetic energy of the particles, so they move more slowly. As attractive forces become more important, the particles come closer together and eventually become fixed in a lattice when the substance solidifies. The particles themselves are unchanged chemically.

Question 16Particulate nature of matter, states of matter and kinetic theory

A balloon filled with air is taken from a warm room to a cold room. Assuming the pressure remains constant, which statement best explains why the balloon shrinks?

  1. The air particles become heavier in the cold.
  2. The air particles escape through the balloon wall.
  3. The balloon material contracts and squeezes the gas.
  4. The air particles lose kinetic energy, collide less forcefully, and the volume decreases. Correct answer.

Explanation

At constant pressure, cooling a gas reduces the average kinetic energy of its particles. Slower particles collide with the walls less forcefully, so the volume must decrease to keep the pressure equal to the outside pressure. This is why the balloon shrinks in the cold.

Question 17Atomic structure, isotopes and the Periodic Table

An atom of an element has the electron configuration 2, 8, 7. In which group of the Periodic Table is this element placed?

  1. Group II
  2. Group VIII
  3. Group XVII
  4. Group VII Correct answer.

Explanation

The group number of a main-group element is fixed by the number of electrons in its outermost shell. Here the outer shell holds seven electrons, so the element sits in Group VII. The total of 17 electrons shows it is chlorine, whose position in Group VII confirms the rule.

Question 18Atomic structure, isotopes and the Periodic Table

Which statement correctly describes the structure of the atom according to the nuclear model?

  1. A nucleus of electrons surrounded by a cloud of protons
  2. A uniform sphere of positive charge with electrons embedded in it
  3. A dense nucleus of protons and neutrons surrounded by orbiting electrons Correct answer.
  4. A solid ball of protons and neutrons with no internal space

Explanation

In the nuclear model the mass is concentrated in a tiny central nucleus containing protons and neutrons, while electrons occupy the space around it. This arrangement explains the deflection of alpha particles in Rutherford's experiment. The alternative descriptions belong to earlier or incorrect models of the atom.

Question 19Atomic structure, isotopes and the Periodic Table

The nucleus of an atom of sodium-23 contains 11 protons. How many neutrons does this nucleus contain?

  1. 12 Correct answer.
  2. 34
  3. 11
  4. 23

Explanation

The mass number equals the number of protons plus the number of neutrons. For sodium-23 the mass number is 23 and the proton number is 11, so the neutrons number 23 − 11 = 12. The proton number alone is the atomic number, not the neutron count.

Question 20Atomic structure, isotopes and the Periodic Table

Which particles are found in the nucleus of an atom?

  1. Neutrons and electrons
  2. Electrons only
  3. Protons and neutrons Correct answer.
  4. Protons and electrons

Explanation

The nucleus carries almost all the mass of the atom and contains the positively charged protons together with the neutral neutrons. Electrons are much lighter and occupy the shells around the nucleus. This separation of particles explains why the nucleus is positive while the atom as a whole is neutral.

Question 21Atomic structure, isotopes and the Periodic Table

An atom of an element has the electron configuration 2, 8, 8, 1. In which period of the Periodic Table does this element appear?

  1. Period 1
  2. Period 8
  3. Period 3
  4. Period 4 Correct answer.

Explanation

The period number equals the number of occupied electron shells. The configuration 2, 8, 8, 1 fills four shells, so the element is in Period 4. The single outer electron also shows it belongs to Group I, which is potassium.

Question 22Atomic structure, isotopes and the Periodic Table

Which of the following is the correct definition of an isotope?

  1. Atoms of the same element with different numbers of electrons
  2. Atoms of the same element with different numbers of neutrons Correct answer.
  3. Atoms with the same mass number but different proton numbers
  4. Atoms of different elements with the same number of neutrons

Explanation

Isotopes are atoms of the same element, so they share the same proton number, but they differ in neutron number and therefore in mass number. Chemical behaviour is nearly identical because the electron arrangement is the same. This definition distinguishes isotopes from ions and from isobars.

Question 23Atomic structure, isotopes and the Periodic Table

Chlorine consists of two isotopes, chlorine-35 and chlorine-37, in relative abundances of 75% and 25%. What is the relative atomic mass of chlorine?

  1. 35.5 Correct answer.
  2. 36.0
  3. 36.5
  4. 35.0

Explanation

The relative atomic mass is the weighted mean of the isotope masses. Taking (35 × 75) + (37 × 25) gives 3550, and dividing by 100 gives 35.5. This value is why chlorine's relative atomic mass is not a whole number.

Question 24Atomic structure, isotopes and the Periodic Table

An atom of an element has 17 protons and 18 neutrons. What is the symbol of this atom written with its mass number and atomic number?

  1. ³⁵₁₇Cl Correct answer.
  2. ¹⁷₁₈Cl
  3. ³⁵₁₈Cl
  4. ³⁶₁₇Cl

Explanation

The atomic number is the number of protons, 17, which identifies the element as chlorine. The mass number is the sum of protons and neutrons, 17 + 18 = 35, so the symbol is written with 35 above and 17 below. Writing the atomic number as 18 is wrong because that is the neutron count.

Question 25Atomic structure, isotopes and the Periodic Table

Why do isotopes of the same element have almost identical chemical properties?

  1. They have the same number of neutrons and therefore the same mass
  2. They have the same mass number and therefore the same density
  3. They have the same number of nucleons and therefore the same charge
  4. They have the same electron arrangement and therefore the same bonding behaviour Correct answer.

Explanation

Chemical properties depend on the number and arrangement of electrons, especially the outer-shell electrons. Isotopes of an element have identical proton numbers, so their electron configurations are the same. The difference in neutrons affects mass and nuclear stability, not ordinary chemical reactions.

Question 26Atomic structure, isotopes and the Periodic Table

An element has the electron configuration 2, 8, 2. Which statement about this element is correct?

  1. It is a metal in Group II that forms ions with a 2+ charge Correct answer.
  2. It is a non-metal in Group VI that forms ions with a 2− charge
  3. It is a halogen in Group VII that forms ions with a 1− charge
  4. It is a noble gas in Group VIII with a full outer shell

Explanation

The two outer electrons place the element in Group II, where the elements are metals. Losing those two electrons gives a stable configuration like the previous noble gas, so the ion carries a 2+ charge. This element is magnesium.

Question 27Atomic structure, isotopes and the Periodic Table

Which of the following correctly states the charge and approximate relative mass of an electron?

  1. Charge −1 and relative mass 1
  2. Charge −1 and relative mass about 1/1840 Correct answer.
  3. Charge +1 and relative mass 1
  4. Charge 0 and relative mass 1

Explanation

An electron carries a single negative charge and has a mass roughly 1/1840 that of a proton. Its negligible mass is why the mass of an atom is taken as the mass of its nucleus. Protons carry +1 charge and neutrons carry no charge, both with relative mass about 1.

Question 28Atomic structure, isotopes and the Periodic Table

In the modern Periodic Table, elements are arranged in order of increasing…

  1. relative atomic mass
  2. atomic number Correct answer.
  3. number of electron shells
  4. number of neutrons

Explanation

The modern table orders elements by atomic number, that is, by the number of protons in the nucleus. This ordering corrected anomalies in Mendeleev's mass-based arrangement, such as the placing of tellurium and iodine. The number of electron shells determines the period, not the overall sequence.

Question 29Atomic structure, isotopes and the Periodic Table

A sample of boron contains 20% boron-10 and 80% boron-11. What is the relative atomic mass of this sample?

  1. 10.2
  2. 11.0
  3. 10.8 Correct answer.
  4. 10.5

Explanation

The weighted mean is found from (10 × 20) + (11 × 80), which gives 1080, divided by 100 to give 10.8. The answer lies closer to 11 because boron-11 is the more abundant isotope. A simple average of 10 and 11 would be wrong because it ignores the abundances.

Question 30Atomic structure, isotopes and the Periodic Table

Which pair of species contains the same number of electrons?

  1. Na⁺ and Na
  2. Na⁺ and Ne Correct answer.
  3. Mg²⁺ and Ne
  4. Cl⁻ and Cl

Explanation

A sodium atom has 11 electrons, so losing one electron to form Na⁺ leaves 10. Neon has 10 electrons, so Na⁺ and Ne are isoelectronic. Counting electrons carefully in each species shows that the other pairs differ by one or more electrons.

Question 31Atomic structure, isotopes and the Periodic Table

An element has two isotopes with masses 63 and 65, and its relative atomic mass is 63.5. What is the approximate percentage abundance of the isotope of mass 63?

  1. 63.5%
  2. 50%
  3. 75% Correct answer.
  4. 25%

Explanation

Let the abundance of the lighter isotope be x per cent, so the heavier is (100 − x) per cent. Then 63x + 65(100 − x) = 6350, which simplifies to 2x = 150, giving x = 75. The element is copper, whose relative atomic mass of 63.5 confirms this ratio.

Question 32Atomic structure, isotopes and the Periodic Table

Which statement best explains why the first ionisation energy of magnesium is higher than that of sodium?

  1. Magnesium has a larger atomic radius than sodium
  2. The magnesium nucleus has a greater positive charge acting on the same number of shells Correct answer.
  3. Magnesium atoms have more electron shells than sodium atoms
  4. Magnesium atoms contain more neutrons than sodium atoms

Explanation

Across a period the nuclear charge increases while the outer electrons stay in the same shell, so the attraction on the outer electron grows. Magnesium therefore needs more energy than sodium to lose its first electron. The smaller atomic radius of magnesium also reflects this stronger attraction.

Question 33Atomic structure, isotopes and the Periodic Table

Which statement about the noble gases is correct?

  1. They have full outer electron shells and are unreactive Correct answer.
  2. They have seven outer electrons and form 1− ions readily
  3. They have half-filled outer shells and form covalent bonds easily
  4. They have one outer electron and form 1+ ions readily

Explanation

Group VIII elements have complete outer shells, so they neither gain nor lose electrons easily. This stable configuration makes them very unreactive and they exist mainly as single atoms. Helium is the special case with a full shell of two electrons.

Question 34Atomic structure, isotopes and the Periodic Table

A sample of an element consists of isotopes of mass 10 and 11, and its relative atomic mass is 10.8. Which conclusion can be drawn?

  1. The element contains more neutrons than protons in every atom
  2. About four-fifths of the atoms have mass 11 Correct answer.
  3. About four-fifths of the atoms have mass 10
  4. The two isotopes are present in equal amounts

Explanation

The value 10.8 lies much closer to 11 than to 10, so the heavier isotope must dominate the mixture. Setting up the weighted mean shows that roughly 80% of the atoms are of mass 11. This element is boron, whose main isotope is boron-11.

Question 35Atomic structure, isotopes and the Periodic Table

An atom of an element has the configuration 2, 8, 6. Which statement about its chemical behaviour is correct?

  1. It gains one electron to form a 1− ion and is a halogen
  2. It gains two electrons to form a 2− ion and is a non-metal Correct answer.
  3. It loses six electrons to form a 6+ ion and is a metal
  4. It has a full outer shell and is chemically inert

Explanation

An atom with six outer electrons needs only two more to reach the stable octet, so it gains two electrons and forms a 2− ion. Elements behaving this way are non-metals in Group VI, such as oxygen and sulfur. Losing six electrons would require far too much energy to be realistic.

Question 36Atomic structure, isotopes and the Periodic Table

The relative atomic mass of an element is 39.1, and its two isotopes have masses 39 and 41. Which set of approximate abundances fits these data?

  1. About 80% of mass 39 and 20% of mass 41
  2. About 50% of mass 39 and 50% of mass 41
  3. About 5% of mass 39 and 95% of mass 41
  4. About 95% of mass 39 and 5% of mass 41 Correct answer.

Explanation

The relative atomic mass of 39.1 is very close to 39, so the lighter isotope must be by far the most abundant. Testing the abundances shows that 95% of mass 39 with 5% of mass 41 gives (39 × 95) + (41 × 5) = 3910, or 39.1. The element is potassium.

Question 37Chemical bonding and structure (ionic, covalent, metallic)

Which statement correctly describes the electrostatic attraction present in a crystal of sodium chloride?

  1. Attraction between shared electron pairs and two nuclei
  2. Attraction between oppositely charged ions arranged in a giant lattice Correct answer.
  3. Attraction between neighbouring molecules held by weak forces
  4. Attraction between delocalised electrons and positive metal ions

Explanation

Sodium chloride is an ionic compound: sodium atoms transfer an electron to chlorine, giving Na+ and Cl- ions. Each ion is surrounded by ions of opposite charge in a regular three-dimensional giant lattice, and the whole structure is held together by the strong electrostatic attraction between these oppositely charged ions.

Question 38Chemical bonding and structure (ionic, covalent, metallic)

In which of the following substances are the particles held together by metallic bonding?

  1. Iodine
  2. Diamond
  3. Copper Correct answer.
  4. Sodium chloride

Explanation

Copper is a metal: its atoms release their outer electrons into a sea of delocalised electrons, and the resulting positive ions are held by electrostatic attraction to this electron cloud. This arrangement of positive ions in a mobile electron sea is the definition of metallic bonding.

Question 39Chemical bonding and structure (ionic, covalent, metallic)

Which pair of elements is most likely to react together to form an ionic compound?

  1. A metal of low ionisation energy and a non-metal of high electron affinity Correct answer.
  2. Two non-metals of similar electronegativity
  3. Two metals with similar atomic radii
  4. A noble gas and a halogen

Explanation

Ionic bonding requires transfer of electrons from one atom to another, which happens readily when a metal with low ionisation energy loses electrons and a non-metal with high electron affinity gains them. The large difference in tendency to hold electrons gives complete transfer and stable ions.

Question 40Chemical bonding and structure (ionic, covalent, metallic)

Which property is characteristic of a simple molecular covalent substance such as carbon dioxide?

  1. It has a low melting point because only weak forces between molecules are overcome Correct answer.
  2. It conducts electricity when molten
  3. It is a giant lattice of ions
  4. It has a very high melting point

Explanation

In a simple molecular substance the strong covalent bonds are inside each molecule, but the molecules are held to one another only by weak intermolecular forces. On heating, these weak forces are broken first, so the substance melts and boils at low temperature without breaking the covalent bonds.

Question 41Chemical bonding and structure (ionic, covalent, metallic)

Which type of bond is formed when two chlorine atoms combine to form a Cl2 molecule?

  1. A single covalent bond Correct answer.
  2. An ionic bond
  3. A metallic bond
  4. A double covalent bond

Explanation

Each chlorine atom needs one more electron to complete its outer shell, so the two atoms share one pair of electrons. One shared pair between two atoms constitutes a single covalent bond.

Question 42Chemical bonding and structure (ionic, covalent, metallic)

Magnesium oxide has a very high melting point of about 2850 degrees Celsius. Which explanation best accounts for this?

  1. It contains small molecules held by strong covalent bonds
  2. It has delocalised electrons that hold the ions together
  3. It is a giant ionic lattice with ions of charge 2+ and 2-, giving very strong electrostatic attraction Correct answer.
  4. It contains weak intermolecular forces that require much energy to break

Explanation

Magnesium oxide is a giant ionic lattice containing Mg2+ and O2- ions, so each ion pair is held by a doubly charged electrostatic attraction. Because the charges are high and the ions are small, the attraction is very strong and a large amount of energy is needed to separate the ions, giving an exceptionally high melting point.

Question 43Chemical bonding and structure (ionic, covalent, metallic)

Solid sodium chloride does not conduct electricity, but molten sodium chloride does. What is the reason for this difference?

  1. In the solid the electrons are delocalised; when molten they are localised
  2. In the solid the ions are fixed in the lattice; when molten the ions are free to move Correct answer.
  3. In the solid the ions are neutral; when molten they become charged
  4. Melting converts covalent bonds into ionic bonds

Explanation

Conduction requires charged particles that are free to move. In solid sodium chloride the ions are locked in fixed positions in the lattice, but on melting the lattice breaks down and the ions become mobile, so the melt conducts electricity by ion migration.

Question 44Chemical bonding and structure (ionic, covalent, metallic)

Diamond and graphite are both made of carbon, yet diamond is extremely hard and does not conduct electricity while graphite is soft and conducts. Which statement best explains this?

  1. Graphite contains free carbon atoms between its layers
  2. Diamond has ionic bonds while graphite has covalent bonds
  3. In diamond each carbon is covalently bonded to four others in a rigid network, while in graphite each carbon is bonded to three others in layers with delocalised electrons Correct answer.
  4. Diamond has weaker covalent bonds than graphite

Explanation

In diamond every carbon atom forms four single covalent bonds in a rigid three-dimensional network, so the structure is very hard and has no free electrons. In graphite each carbon bonds to only three others, forming layers, and the fourth electron is delocalised along the layers, allowing conduction and allowing layers to slide, which makes graphite soft.

Question 45Chemical bonding and structure (ionic, covalent, metallic)

Which of the following substances will conduct electricity in the solid state?

  1. Solid carbon dioxide
  2. Solid sodium chloride
  3. Solid iodine
  4. Solid magnesium Correct answer.

Explanation

Magnesium is a metal, so its solid structure contains delocalised electrons that are free to move throughout the lattice and carry charge. That is why metals conduct electricity even when solid.

Question 46Chemical bonding and structure (ionic, covalent, metallic)

Which statement best explains why magnesium is malleable while sodium chloride is brittle?

  1. Magnesium ions are held by a mobile electron sea so layers can slide, while in sodium chloride sliding layers bring like charges together and the crystal splits Correct answer.
  2. Sodium chloride contains delocalised electrons that repel each other
  3. Magnesium has weaker bonds than sodium chloride in every direction
  4. Magnesium is covalent and sodium chloride is ionic

Explanation

In a metal the delocalised electrons are not tied to particular ions, so when layers of ions are displaced they can still be held by the electron sea, allowing the metal to bend. In an ionic lattice, displacing a layer brings ions of the same charge next to one another, and the resulting repulsion splits the crystal.

Question 47Chemical bonding and structure (ionic, covalent, metallic)

An element X forms a chloride with the formula XCl2, and X is in Group 2 of the periodic table. What is the charge on the ion of X in this compound?

  1. 1-
  2. 2+ Correct answer.
  3. 2-
  4. 1+

Explanation

Each chloride ion carries a single negative charge, Cl-. Since two chloride ions balance one ion of X in XCl2, the ion of X must carry a charge of 2+. This is consistent with Group 2 elements, which lose two outer electrons to form 2+ ions.

Question 48Chemical bonding and structure (ionic, covalent, metallic)

Which dot-and-cross description correctly represents the bonding in a water molecule?

  1. Oxygen transfers two electrons to each hydrogen, forming ions
  2. Oxygen shares two pairs of electrons with one hydrogen and none with the other
  3. Each hydrogen shares one pair of electrons with oxygen, giving oxygen two bonding pairs and two lone pairs Correct answer.
  4. Each hydrogen shares two pairs of electrons with oxygen, giving a double bond to each hydrogen

Explanation

Oxygen has six outer electrons and needs two more, while each hydrogen needs one. Oxygen therefore shares one pair with each of the two hydrogen atoms, forming two single covalent bonds, and retains two non-bonding lone pairs to complete its octet.

Question 49Chemical bonding and structure (ionic, covalent, metallic)

Why does ammonia, NH3, have a lone pair of electrons on the nitrogen atom?

  1. Nitrogen has gained an extra electron from hydrogen
  2. Ammonia contains a double bond between nitrogen and one hydrogen
  3. Nitrogen uses only three of its five outer electrons in bonding, leaving two as a lone pair Correct answer.
  4. Nitrogen forms three ionic bonds with hydrogen

Explanation

Nitrogen has five outer electrons. In ammonia it shares three of these, one with each hydrogen, to form three single covalent bonds, and the remaining two electrons stay as a non-bonding lone pair on nitrogen. This lone pair allows ammonia to act as a base and to form dative bonds.

Question 50Chemical bonding and structure (ionic, covalent, metallic)

Which statement about the bonding in a molecule of carbon dioxide is correct?

  1. It contains one triple bond and one single bond
  2. It contains two double bonds, each made of two shared pairs of electrons Correct answer.
  3. It contains two single bonds and two lone pairs on carbon
  4. It is held together by ionic attraction between carbon and oxygen

Explanation

Carbon has four outer electrons and each oxygen needs two, so carbon shares two pairs with each oxygen atom. This gives two carbon-oxygen double bonds, O=C=O, with each bond consisting of two shared electron pairs.