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Question 1Periodic Table and Atomic Structure

How many protons, neutrons and electrons are present in a neutral atom of chlorine-37?

  1. 20 protons, 17 neutrons, 20 electrons
  2. 17 protons, 20 neutrons, 17 electrons Correct answer.
  3. 17 protons, 37 neutrons, 17 electrons
  4. 37 protons, 17 neutrons, 37 electrons

Explanation

Chlorine has atomic number 17, so every chlorine atom has 17 protons and a neutral atom has 17 electrons. The mass number 37 equals protons plus neutrons, giving 37 - 17 = 20 neutrons.

Question 2Periodic Table and Atomic Structure

Which subatomic particle determines the identity of an element?

  1. Neutron
  2. Electron
  3. Proton Correct answer.
  4. Nucleon

Explanation

The number of protons in the nucleus, the atomic number, is what defines an element and fixes its position in the periodic table. Changing the proton count converts the atom into a different element.

Question 3Periodic Table and Atomic Structure

In which block of the periodic table is the element with ground-state electron configuration 1s²2s²2p⁶3s²3p¹ found?

  1. d-block
  2. s-block
  3. f-block
  4. p-block Correct answer.

Explanation

The differentiating electron enters a 3p orbital, and elements whose last electron occupies a p subshell belong to the p-block. This element is aluminium, which sits in group 13 of the p-block.

Question 4Periodic Table and Atomic Structure

Which statement correctly describes the trend in first ionisation energy across period 3 from sodium to argon?

  1. It decreases steadily because atomic radius increases
  2. It increases overall, with dips at aluminium and sulfur Correct answer.
  3. It remains constant because the number of shells is unchanged
  4. It increases steadily with no exceptions

Explanation

Across a period, nuclear charge rises while shielding stays similar, so the outer electron is held more tightly and ionisation energy generally increases. The overall rise is interrupted at aluminium, where a 3p electron is removed, and at sulfur, where electron pairing in a 3p orbital increases repulsion.

Question 5Periodic Table and Atomic Structure

Which ground-state electron configuration is correct for a chromium atom?

  1. [Ar]3d⁴4s²
  2. [Ar]3d⁵4s¹ Correct answer.
  3. [Ar]3d³4s³
  4. [Ar]3d⁶

Explanation

Chromium is an exception to the expected filling order because a half-filled 3d subshell with a single 4s electron gives extra exchange stability. The configuration [Ar]3d⁵4s¹ is therefore preferred over [Ar]3d⁴4s².

Question 6Periodic Table and Atomic Structure

Why does the first ionisation energy of magnesium exceed that of aluminium?

  1. Magnesium has a larger atomic radius than aluminium
  2. The outer electron of magnesium is in a 3s orbital, which penetrates closer to the nucleus than a 3p orbital Correct answer.
  3. Aluminium has a larger nuclear charge than magnesium
  4. Aluminium has more electron shells than magnesium

Explanation

Magnesium loses a 3s electron, whereas aluminium loses a higher-energy 3p electron that is shielded more effectively by the 3s pair. The 3s electron is therefore held more strongly despite magnesium's smaller nuclear charge.

Question 7Periodic Table and Atomic Structure

Which property increases down group 2 from magnesium to barium?

  1. Atomic radius Correct answer.
  2. Melting point
  3. Electronegativity
  4. First ionisation energy

Explanation

Each step down group 2 adds a principal shell, so the atoms become larger and the atomic radius increases. The outer electrons are also more shielded, which explains the accompanying fall in ionisation energy.

Question 8Periodic Table and Atomic Structure

Which ion has the electron configuration 1s²2s²2p⁶?

  1. Na⁺ Correct answer.
  2. Mg²⁺
  3. F⁻
  4. O²⁻

Explanation

Sodium has 11 electrons, so Na⁺ retains 10 electrons, matching the 1s²2s²2p⁶ configuration of neon. The other ions listed each contain ten or more electrons in shells beyond the 2p subshell.

Question 9Periodic Table and Atomic Structure

Which statement best explains why the first ionisation energy of fluorine is greater than that of chlorine?

  1. The outer electron of fluorine is in a shell closer to the nucleus and is less shielded Correct answer.
  2. Fluorine has more electron shells than chlorine
  3. Chlorine has a greater effective nuclear charge than fluorine
  4. Fluorine has a larger atomic radius than chlorine

Explanation

Fluorine's outer electron is in the n = 2 shell, closer to the nucleus and shielded by fewer inner shells than chlorine's n = 3 electron. The stronger attraction makes the electron harder to remove, so fluorine has the higher ionisation energy.

Question 10Periodic Table and Atomic Structure

Which pair of elements shows the most similar chemical properties?

  1. Na and Mg
  2. Na and Ar
  3. Na and K Correct answer.
  4. Na and Cl

Explanation

Chemical similarity is greatest within a group because the elements share the same number of valence electrons. Sodium and potassium both have one outer s electron, making them members of group 1.

Question 11Periodic Table and Atomic Structure

Which of the following species is isoelectronic with the argon atom?

  1. K⁺ Correct answer.
  2. Ca²⁺
  3. S²⁻
  4. Cl⁻

Explanation

Argon has 18 electrons. Potassium has 19 electrons, so K⁺ retains 18, giving the same configuration as argon. The other species listed contain 18 or 20 electrons, so only K⁺ matches argon exactly.

Question 12Periodic Table and Atomic Structure

Which factor is chiefly responsible for the decrease in first ionisation energy down group 1?

  1. Increasing electronegativity
  2. Decreasing number of valence electrons
  3. Increasing shielding by inner shells and greater distance from the nucleus Correct answer.
  4. Increasing nuclear charge

Explanation

Down the group each element has an additional filled inner shell, which shields the outer electron, and the outer electron also lies further from the nucleus. These effects outweigh the rising nuclear charge, so the outer electron is lost more easily.

Question 13Periodic Table and Atomic Structure

Which element has the highest electronegativity?

  1. Chlorine
  2. Fluorine Correct answer.
  3. Nitrogen
  4. Oxygen

Explanation

Electronegativity increases across a period and decreases down a group, placing fluorine at the top right of the trend. Fluorine's small size and high effective nuclear charge give it the greatest attraction for bonding electrons.

Question 14Periodic Table and Atomic Structure

Which statement correctly describes the trend in atomic radius across period 3 from sodium to chlorine?

  1. It decreases because nuclear charge increases while shielding stays roughly constant Correct answer.
  2. It increases because more electrons are added
  3. It stays the same because the elements are in the same period
  4. It decreases because the number of shells decreases

Explanation

Across period 3, protons are added to the nucleus while electrons enter the same n = 3 shell, so shielding changes little. The increasing effective nuclear charge pulls the outer electrons closer, reducing the radius.

Question 15Periodic Table and Atomic Structure

Which of these atoms has the largest first ionisation energy?

  1. Na
  2. Mg
  3. Si Correct answer.
  4. Al

Explanation

All four elements are in period 3, so the general trend is an increase in ionisation energy from left to right as effective nuclear charge rises. Silicon, furthest right of the four, has the highest value.

Question 16Periodic Table and Atomic Structure

Which ion has the smallest ionic radius?

  1. Mg²⁺
  2. Al³⁺ Correct answer.
  3. Na⁺
  4. F⁻

Explanation

These species are isoelectronic, all containing 10 electrons, so the radius depends on nuclear charge. Al³⁺ has the largest nuclear charge (13 protons) and therefore pulls the same electron cloud in most tightly.

Question 17Periodic Table and Atomic Structure

Which element is a transition metal with the ground-state configuration [Ar]3d⁸4s²?

  1. Iron
  2. Nickel Correct answer.
  3. Cobalt
  4. Copper

Explanation

The configuration has eight 3d electrons and two 4s electrons, giving a total of 28 electrons. Nickel has atomic number 28 and is a transition metal in period 4.

Question 18Periodic Table and Atomic Structure

Why do transition metals exhibit variable oxidation states?

  1. They contain only s electrons in their valence shell
  2. Their 3d and 4s electrons have similar energies and can both be lost Correct answer.
  3. Their nuclear charge is too low to hold electrons
  4. They have a full outer shell of electrons

Explanation

In transition metals the 3d and 4s orbitals are close in energy, so electrons from both can be removed to give several stable oxidation states. This contrasts with main-group metals, where loss is limited to the valence s and p electrons.

Question 19Periodic Table and Atomic Structure

Which statement about the elements of period 2 is correct?

  1. Lithium has a higher first ionisation energy than neon
  2. Fluorine has a larger atomic radius than lithium
  3. Nitrogen has a higher first ionisation energy than oxygen Correct answer.
  4. Beryllium has a lower first ionisation energy than lithium

Explanation

Nitrogen has a half-filled 2p subshell, and the extra exchange stability makes its outer electron harder to remove than oxygen's. Oxygen also has a paired 2p electron, whose repulsion slightly lowers the ionisation energy.

Question 20Periodic Table and Atomic Structure

Which observation best supports the model of electron shells in atoms?

  1. The mass of an atom is concentrated in the nucleus
  2. Isotopes of an element have different masses
  3. Atoms of noble gases are unreactive
  4. The emission spectrum of hydrogen consists of discrete lines Correct answer.

Explanation

Discrete spectral lines show that electrons can occupy only certain energy levels and emit photons of fixed energy when they drop between them. A continuous spectrum would be expected if electron energies were unrestricted.

Question 21Periodic Table and Atomic Structure

An element has successive ionisation energies that rise sharply after the fourth value. To which group does the element most likely belong?

  1. Group 1
  2. Group 7
  3. Group 2
  4. Group 4 Correct answer.

Explanation

A sharp jump in ionisation energy marks the removal of an electron from a new, inner shell. If the jump occurs after the fourth ionisation, the atom has four valence electrons, which identifies a group 4 element.

Question 22Periodic Table and Atomic Structure

Which species has the ground-state configuration 1s²2s²2p⁶3s²3p⁶3d⁵?

  1. Fe²⁺
  2. Cr³⁺
  3. Mn²⁺ Correct answer.
  4. Fe³⁺

Explanation

Manganese has atomic number 25, so Mn²⁺ has 23 electrons; removing the two 4s electrons leaves [Ar]3d⁵. This half-filled d subshell is the configuration described in the question.

Question 23Periodic Table and Atomic Structure

Which property shows a general increase across period 3 from sodium to chlorine?

  1. First ionisation energy Correct answer.
  2. Atomic radius
  3. Number of occupied electron shells
  4. Metallic character

Explanation

Across period 3 the effective nuclear charge rises while the outer electrons remain in the same shell, so first ionisation energy increases overall. Atomic radius and metallic character decrease, and the shell count stays at three.

Question 24Periodic Table and Atomic Structure

Which statement about the emission spectrum of hydrogen is correct?

  1. The spectrum is continuous because hydrogen has only one electron
  2. The lines are equally spaced in energy
  3. The lines converge at high energy because the electron transitions approach the ionisation limit Correct answer.
  4. The red line arises from a transition to the n = 3 level

Explanation

As transitions start from higher levels, the energy gaps between successive levels become smaller, so the spectral lines converge towards the series limit. That limit corresponds to the ionisation energy of the atom.

Question 25Chemical Bonding and Molecular Structure

Which type of bonding is present in a crystal of potassium chloride, KCl?

  1. Hydrogen bonding
  2. Metallic bonding
  3. Ionic bonding Correct answer.
  4. Non-polar covalent bonding

Explanation

KCl is formed from a metal (potassium) and a non-metal (chlorine). Electrons are transferred from K to Cl, producing K+ and Cl- ions held together by strong electrostatic attraction, which is the definition of ionic bonding.

Question 26Chemical Bonding and Molecular Structure

What is the shape of a methane molecule, CH4, according to VSEPR theory?

  1. Tetrahedral Correct answer.
  2. Trigonal pyramidal
  3. Bent
  4. Square planar

Explanation

The carbon atom in methane has four bonding pairs and no lone pairs. VSEPR theory states that four electron domains arrange themselves as far apart as possible, giving a tetrahedral shape with bond angles of about 109.5 degrees.

Question 27Chemical Bonding and Molecular Structure

Which statement correctly describes a coordinate (dative) covalent bond?

  1. Electrons are completely transferred from one atom to another
  2. Each atom contributes one electron to the shared pair
  3. The bond forms only between ions of opposite charge
  4. Both shared electrons come from the same atom Correct answer.

Explanation

In a coordinate bond, one atom donates an entire lone pair into an empty orbital of another atom, so both electrons in the shared pair originate from the donor. Once formed, the bond is indistinguishable from an ordinary covalent bond.

Question 28Chemical Bonding and Molecular Structure

Which molecule contains a polar covalent bond but has an overall dipole moment of zero?

  1. H2O
  2. NH3
  3. CO2 Correct answer.
  4. HCl

Explanation

CO2 is linear, so the two polar C=O bond dipoles point in opposite directions and cancel exactly, giving a net dipole moment of zero. The bonds are polar, but the molecular geometry makes the molecule non-polar overall.

Question 29Chemical Bonding and Molecular Structure

Which type of force holds separate molecules of iodine together in solid iodine?

  1. London dispersion forces Correct answer.
  2. Covalent bonds
  3. Ionic bonds
  4. Hydrogen bonds

Explanation

Iodine molecules, I2, are held internally by covalent bonds, but the separate molecules in the solid are held by weak London dispersion forces arising from temporary fluctuations in electron density. These weak forces explain iodine's low melting point.

Question 30Chemical Bonding and Molecular Structure

How many sigma and pi bonds are present in one molecule of ethene, C2H4?

  1. Five sigma and one pi Correct answer.
  2. Four sigma and two pi
  3. Four sigma and one pi
  4. Five sigma and two pi

Explanation

Ethene has four C-H sigma bonds plus one C-C sigma bond, giving five sigma bonds, and the C=C double bond contributes one pi bond. The total is therefore five sigma and one pi.

Question 31Chemical Bonding and Molecular Structure

What is the electron-pair geometry around the central atom in SF4?

  1. Octahedral
  2. Trigonal bipyramidal Correct answer.
  3. Tetrahedral
  4. Linear

Explanation

Sulfur in SF4 has four bonding pairs and one lone pair, giving five electron domains in total. Five domains adopt a trigonal bipyramidal electron-pair geometry, with the lone pair occupying an equatorial position.

Question 32Chemical Bonding and Molecular Structure

Which molecule has a bond angle closest to 104.5 degrees?

  1. CO2
  2. CH4
  3. BF3
  4. H2O Correct answer.

Explanation

Water has two bonding pairs and two lone pairs on oxygen. Lone-pair repulsion compresses the ideal tetrahedral angle to about 104.5 degrees, which is the value quoted for the H-O-H angle.

Question 33Chemical Bonding and Molecular Structure

Which species is isoelectronic with the neon atom?

  1. K+
  2. Mg2+ Correct answer.
  3. Na+
  4. Cl-

Explanation

Isoelectronic species have the same number of electrons. Neon has ten electrons, and Mg2+ has 12 protons minus 2 electrons lost, giving 10 electrons, so Mg2+ is isoelectronic with neon.

Question 34Chemical Bonding and Molecular Structure

Which statement best explains why the melting point of magnesium oxide is much higher than that of sodium chloride?

  1. MgO has weaker electrostatic attractions between its ions
  2. MgO contains covalent bonds while NaCl is ionic
  3. MgO has a larger lattice enthalpy because its ions are doubly charged and smaller Correct answer.
  4. MgO has a lower charge density on its ions than NaCl

Explanation

Lattice enthalpy depends on ionic charge and size, following the relationship that attraction increases with charge and decreases with distance. Mg2+ and O2- carry double charges and are smaller than Na+ and Cl-, so the electrostatic attraction in MgO is far stronger, giving a much higher melting point.

Question 35Chemical Bonding and Molecular Structure

Which of the following molecules is non-polar despite containing polar bonds?

  1. CCl4 Correct answer.
  2. NH3
  3. SO2
  4. CHCl3

Explanation

CCl4 is tetrahedral, so its four identical C-Cl bond dipoles are arranged symmetrically and cancel, leaving no net dipole moment. The molecule is therefore non-polar even though each individual bond is polar.

Question 36Chemical Bonding and Molecular Structure

Which description of the bonding in a metal is correct?

  1. Neutral atoms are held by shared electron pairs between specific pairs of atoms
  2. Molecules are held together by hydrogen bonds between metal atoms
  3. Positive ions are fixed in a lattice surrounded by delocalised electrons Correct answer.
  4. Alternating positive and negative ions are held by electrostatic attraction

Explanation

Metallic bonding is modelled as a lattice of positive metal ions immersed in a sea of delocalised valence electrons. The attraction between the ions and the mobile electrons holds the structure together and accounts for properties such as electrical conductivity and malleability.

Question 37Chemical Bonding and Molecular Structure

Which molecule exhibits hydrogen bonding between its molecules?

  1. CH3F
  2. PH3
  3. CH3OH Correct answer.
  4. H2S

Explanation

Hydrogen bonding requires a hydrogen atom bonded directly to a highly electronegative N, O or F atom. In methanol, CH3OH, the O-H group provides this, so hydrogen bonds form between molecules.

Question 38Chemical Bonding and Molecular Structure

Which sequence correctly orders the species by increasing carbon-oxygen bond length?

  1. CO < CH3OH < CO2
  2. CH3OH < CO2 < CO
  3. CO < CO2 < CH3OH Correct answer.
  4. CO2 < CO < CH3OH

Explanation

Bond order controls bond length: a higher bond order means a shorter, stronger bond. Carbon monoxide has a triple bond (bond order 3), CO2 has double bonds (bond order 2), and methanol has single C-O bonds (bond order 1), so length increases in the order CO, CO2, CH3OH.

Question 39Chemical Bonding and Molecular Structure

What is the hybridisation of the carbon atoms in ethyne, C2H2?

  1. sp2
  2. sp3
  3. sp3d
  4. sp Correct answer.

Explanation

Each carbon in ethyne forms two sigma bonds, one to hydrogen and one to the other carbon, and two pi bonds. Two sigma bonds correspond to two hybrid orbitals, which means sp hybridisation with a linear geometry and a bond angle of 180 degrees.

Question 40Chemical Bonding and Molecular Structure

Which ion is predicted to have a bond angle of exactly 180 degrees?

  1. H3O+
  2. NH4+
  3. NO2+ Correct answer.
  4. CO3 2-

Explanation

The nitronium ion, NO2+, has a central nitrogen with two double bonds and no lone pairs, giving two electron domains. Two domains produce a linear arrangement, so the O-N-O angle is exactly 180 degrees.

Question 41Chemical Bonding and Molecular Structure

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

  1. Magnesium has more shielding from inner electrons
  2. Magnesium has a larger atomic radius
  3. Magnesium has a greater nuclear charge with the outer electron in the same shell Correct answer.
  4. Magnesium has its outer electron in a shell closer to the nucleus

Explanation

Across a period the outer electrons remain in the same shell, but the nuclear charge increases. Magnesium's extra proton increases the attraction on the 3s electrons without adding a new shielding shell, so more energy is needed to remove an electron than for sodium.

Question 42Chemical Bonding and Molecular Structure

Which statement about the delocalisation of electrons in a carbonate ion, CO3 2-, is correct?

  1. Two C-O bonds are double bonds and one is single
  2. One C-O bond is a double bond and two are single bonds of fixed length
  3. All three C-O bonds are single bonds
  4. All three C-O bonds are identical with bond order between one and two Correct answer.

Explanation

The carbonate ion has three resonance structures, and the true structure is a resonance hybrid. The pi electrons are delocalised over all three C-O positions, so the three bonds are identical and each has a bond order of about 1.33.

Question 43Chemical Bonding and Molecular Structure

Which compound is expected to have the highest boiling point?

  1. Propan-1-ol, C3H7OH Correct answer.
  2. Propanone, CH3COCH3
  3. Ethoxyethane, C2H5OC2H5
  4. Butane, C4H10

Explanation

Propan-1-ol molecules can form intermolecular hydrogen bonds through the O-H group, which are stronger than the dipole-dipole and dispersion forces in the other compounds. Stronger intermolecular forces require more energy to overcome, giving the highest boiling point.

Question 44Chemical Bonding and Molecular Structure

Which species has a shape described as seesaw?

  1. SF4 Correct answer.
  2. BF3
  3. XeF4
  4. PCl5

Explanation

SF4 has four bonding pairs and one lone pair around sulfur, giving five electron domains in a trigonal bipyramidal arrangement. With the lone pair occupying an equatorial position, the resulting molecular shape is described as seesaw.

Question 45Chemical Bonding and Molecular Structure

Which observation provides the strongest evidence that a molecule has delocalised pi electrons rather than localised alternating single and double bonds?

  1. The compound is insoluble in water
  2. The compound has a characteristic smell
  3. The compound decolourises bromine water
  4. All carbon-carbon bond lengths in the ring are intermediate and equal Correct answer.

Explanation

If single and double bonds alternated, they would have measurably different lengths. Equal, intermediate bond lengths throughout the ring show that the pi electrons are spread over all the bonds, which is the signature of delocalisation.

Question 46Chemical Bonding and Molecular Structure

A molecule has a central atom from group 16 bonded to four fluorine atoms with two lone pairs remaining on the central atom. Which molecular shape is predicted?

  1. Trigonal bipyramidal
  2. Tetrahedral
  3. Square planar Correct answer.
  4. Seesaw

Explanation

Four bonding pairs plus two lone pairs give six electron domains, so the electron-pair geometry is octahedral. Placing the two lone pairs opposite each other minimises repulsion, leaving the four bonds in a square planar arrangement, as in XeF4.

Question 47Chemical Bonding and Molecular Structure

Which comparison of the bond angles in NH3, NH4+ and NH2- is correct?

  1. NH4+ > NH3 > NH2- Correct answer.
  2. NH3 > NH4+ > NH2-
  3. NH4+ > NH2- > NH3
  4. NH2- > NH3 > NH4+

Explanation

All three species have four electron domains, so the ideal angle is tetrahedral. NH4+ has no lone pairs, giving about 109.5 degrees; NH3 has one lone pair, reducing the angle to about 107 degrees; NH2- has two lone pairs, reducing it further to about 104 degrees.

Question 48Stoichiometry, Formulas and Equations

What is the relative molecular mass of carbon dioxide, CO₂? (Aᵣ: C = 12, O = 16)

  1. 32
  2. 28
  3. 56
  4. 44 Correct answer.

Explanation

Relative molecular mass is the sum of the relative atomic masses of all atoms in the formula. CO₂ contains one carbon (12) and two oxygens (2 × 16 = 32), giving 12 + 32 = 44. This is a dimensionless relative value, not a mass in grams.

Question 49Stoichiometry, Formulas and Equations

How many moles of sodium ions are present in 0.50 mol of sodium sulfate, Na₂SO₄?

  1. 2.0 mol
  2. 1.0 mol Correct answer.
  3. 0.50 mol
  4. 0.25 mol

Explanation

Each formula unit of Na₂SO₄ contains two sodium ions, so the amount of Na⁺ is twice the amount of the compound. Multiplying 0.50 mol by 2 gives 1.0 mol of Na⁺. The sulfate ion is not needed for this count.

Question 50Stoichiometry, Formulas and Equations

Which of the following is the correct formula of aluminium oxide?

  1. AlO
  2. Al₃O₂
  3. Al₂O₃ Correct answer.
  4. AlO₂

Explanation

Aluminium forms Al³⁺ ions and oxide is O²⁻. Charges must balance, so the lowest common multiple of 3 and 2 requires two Al³⁺ and three O²⁻, giving Al₂O₃. Cross-multiplying the charge magnitudes (3 × 2 and 2 × 3) confirms this ratio.