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Bulgaria DZI Chemistry practice questions

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Question 1Atomic Structure and Periodic Law

Which subatomic particle determines the chemical identity of an element?

  1. The neutron
  2. The photon
  3. The electron
  4. The proton Correct answer.

Explanation

The number of protons in the nucleus, called the atomic number, uniquely identifies each element and fixes its position in the periodic table. Neutrons change only the isotope, and electrons can be added or removed to form ions without changing the element.

Question 2Atomic Structure and Periodic Law

An atom of an element has 19 protons, 20 neutrons and 19 electrons. What is its mass number?

  1. 39 Correct answer.
  2. 58
  3. 20
  4. 19

Explanation

The mass number is the sum of protons and neutrons in the nucleus, so 19 + 20 = 39. Electrons are neglected because their mass is negligible compared with nucleons.

Question 3Atomic Structure and Periodic Law

Which statement correctly describes isotopes of the same element?

  1. They have equal numbers of protons but different numbers of neutrons. Correct answer.
  2. They have equal numbers of neutrons but different numbers of protons.
  3. They have equal mass numbers but different atomic numbers.
  4. They have equal numbers of electrons in the nucleus but different protons.

Explanation

Isotopes are atoms of one element, so their proton count is fixed by the atomic number, yet they differ in neutron count and therefore in mass number. Their chemical behaviour is nearly identical because the electron arrangement is the same.

Question 4Atomic Structure and Periodic Law

How many electrons can the third principal shell (n = 3) hold at maximum?

  1. 8
  2. 32
  3. 10
  4. 18 Correct answer.

Explanation

A shell with principal quantum number n holds at most 2n squared electrons, so for n = 3 the capacity is 2 times 9, which equals 18. This arises because the shell contains one s, three p and five d orbitals, each holding two electrons.

Question 5Atomic Structure and Periodic Law

Which of the following is the correct electron configuration of a chlorine atom (Z = 17)?

  1. 1s2 2s2 2p6 3s2 3p4 4s1
  2. 1s2 2s2 2p5 3s2 3p6
  3. 1s2 2s2 2p6 3s2 3p5 Correct answer.
  4. 1s2 2s2 2p6 3s2 3p6

Explanation

Seventeen electrons fill orbitals in order of increasing energy: two in 1s, two in 2s, six in 2p, two in 3s and five in 3p. The configuration therefore ends in 3s2 3p5, consistent with chlorine being one electron short of a full octet.

Question 6Atomic Structure and Periodic Law

Which group of the periodic table contains the noble gases?

  1. Group 18 Correct answer.
  2. Group 1
  3. Group 2
  4. Group 17

Explanation

The noble gases occupy the far-right column, group 18, and have complete outer electron shells, which makes them chemically very unreactive. Group 17 contains the halogens, which are one electron short of a full shell.

Question 7Atomic Structure and Periodic Law

Which property increases across a period from left to right in the periodic table?

  1. Atomic radius
  2. Electronegativity Correct answer.
  3. Metallic character
  4. Number of neutron shells

Explanation

Across a period the nuclear charge rises while electrons enter the same shell, so the attraction for bonding electrons increases and electronegativity grows. Atomic radius and metallic character both decrease in the same direction.

Question 8Atomic Structure and Periodic Law

What is the maximum number of orbitals present in the d subshell?

  1. 7
  2. 3
  3. 5 Correct answer.
  4. 1

Explanation

A d subshell has angular momentum quantum number l = 2, and the number of orbitals equals 2l + 1, giving five. These five orbitals together can accommodate ten electrons.

Question 9Atomic Structure and Periodic Law

Which quantum number directly determines the shape of an atomic orbital?

  1. Principal quantum number n
  2. Angular momentum quantum number l Correct answer.
  3. Magnetic quantum number m
  4. Spin quantum number s

Explanation

The angular momentum quantum number l defines the subshell type (s, p, d, f) and therefore the orbital shape. The principal number sets size and energy level, while the magnetic number sets orientation.

Question 10Atomic Structure and Periodic Law

Which element has the ground-state configuration 1s2 2s2 2p6 3s2 3p6 4s1?

  1. Potassium Correct answer.
  2. Sodium
  3. Calcium
  4. Argon

Explanation

The configuration contains 19 electrons, which matches potassium, and the single 4s electron reflects the start of the fourth period. Sodium would end at 3s1 and calcium would have two 4s electrons.

Question 11Atomic Structure and Periodic Law

Which of the following ions has the electron configuration 1s2 2s2 2p6?

  1. Na+ Correct answer.
  2. Cl-
  3. Mg
  4. O

Explanation

A sodium atom loses one electron to form Na+, leaving 10 electrons, which is the neon configuration 1s2 2s2 2p6. This is the same closed-shell arrangement as the noble gas neon.

Question 12Atomic Structure and Periodic Law

Which of the following sets of quantum numbers is not permitted for an electron?

  1. n = 3, l = 2, m = -2
  2. n = 4, l = 0, m = 0
  3. n = 1, l = 1, m = 0 Correct answer.
  4. n = 2, l = 1, m = 0

Explanation

For a given principal quantum number n, the angular momentum quantum number l can only take integer values from 0 to n minus 1. With n = 1, only l = 0 is allowed, so l = 1 is forbidden.

Question 13Atomic Structure and Periodic Law

Why does the first ionisation energy generally increase across a period?

  1. The number of neutrons in the nucleus decreases.
  2. The number of electron shells increases.
  3. The atomic radius increases steadily.
  4. The nuclear charge increases while electrons remain in the same shell. Correct answer.

Explanation

Across a period, protons are added to the nucleus while electrons enter the same principal shell, so the effective nuclear charge rises. The outer electron is held more strongly and requires more energy to remove.

Question 14Atomic Structure and Periodic Law

Which element has the largest atomic radius?

  1. Sodium
  2. Rubidium Correct answer.
  3. Potassium
  4. Lithium

Explanation

Atomic radius increases down a group because each period adds a new electron shell farther from the nucleus. Rubidium lies lowest among these alkali metals, so it has the largest radius.

Question 15Atomic Structure and Periodic Law

Which orbital is filled immediately after the 4s orbital in the ground state of an atom?

  1. 5s
  2. 4d
  3. 3d Correct answer.
  4. 4p

Explanation

After 4s is filled, the next lowest-energy orbitals are the 3d set, which is why the first transition series begins after potassium and calcium. The 4p orbitals are filled only after the 3d subshell is complete.

Question 16Atomic Structure and Periodic Law

Which is the correct electron configuration of a chromium atom (Z = 24) in its ground state?

  1. [Ar] 3d3 4s2 4p1
  2. [Ar] 3d6
  3. [Ar] 3d4 4s2
  4. [Ar] 3d5 4s1 Correct answer.

Explanation

Chromium is an exception to the simple aufbau order: a half-filled 3d subshell with a single 4s electron gives extra exchange stability. The configuration is therefore [Ar] 3d5 4s1 rather than the predicted 3d4 4s2.

Question 17Atomic Structure and Periodic Law

Which pair of elements shows the most similar chemical properties?

  1. He and Ne
  2. Na and Mg
  3. C and N
  4. F and Cl Correct answer.

Explanation

Fluorine and chlorine both belong to group 17 and share the same valence configuration, so they show closely related halogen chemistry. Elements in the same group have equal numbers of outer electrons and therefore similar reactivity patterns.

Question 18Atomic Structure and Periodic Law

Which statement about the emission spectrum of hydrogen is correct?

  1. It consists of discrete lines corresponding to electronic transitions between energy levels. Correct answer.
  2. It contains only a single line in the visible region.
  3. It is identical to the absorption spectrum of helium.
  4. It is continuous because all wavelengths are emitted.

Explanation

Electrons in a hydrogen atom occupy quantised energy levels, so photons are emitted only at wavelengths matching the energy differences between levels. This produces a line spectrum rather than a continuous band.

Question 19Atomic Structure and Periodic Law

Which factor best explains why ionisation energy decreases down group 2?

  1. The nuclear charge decreases down the group.
  2. The number of valence electrons increases down the group.
  3. The outer electron occupies a shell farther from the nucleus and is shielded more. Correct answer.
  4. The atomic mass increases down the group.

Explanation

Down a group each element has an additional occupied shell, so the outer electron lies farther from the nucleus and is shielded by more inner electrons. Both effects reduce the attraction and lower the ionisation energy.

Question 20Atomic Structure and Periodic Law

An element has successive ionisation energies (kJ/mol) of 590, 1150, 4940 and 6480. To which group does it most likely belong?

  1. Group 1
  2. Group 2 Correct answer.
  3. Group 13
  4. Group 17

Explanation

The large jump occurs between the second and third ionisation energies, meaning two electrons are relatively easy to remove and the third comes from a stable inner shell. This pattern indicates two valence electrons, characteristic of group 2.

Question 21Atomic Structure and Periodic Law

Which species has the largest ionic radius?

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

Explanation

These ions are isoelectronic, each containing 10 electrons. For isoelectronic species, the greater the nuclear charge the smaller the radius, so the oxide ion with only 8 protons holds its electrons least tightly and is largest.

Question 22Atomic Structure and Periodic Law

An element has the ground-state configuration [Ar] 3d10 4s2 4p3. In which block and period does it lie?

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

Explanation

The highest-energy electrons enter 4p orbitals, so the element belongs to the p-block, and the highest occupied principal level is 4, placing it in period 4. Counting three p electrons beyond the noble gas core identifies it as arsenic in group 15.

Question 23Atomic Structure and Periodic Law

Using the periodic table, which prediction about astatine (At, below iodine) is most reasonable?

  1. It is a halogen with lower electronegativity than iodine and forms At- ions. Correct answer.
  2. It is a highly reactive metal that forms At+ ions readily.
  3. It is a noble gas with a complete valence shell.
  4. It has a smaller atomic radius than fluorine.

Explanation

Astatine lies in group 17 below iodine, so it shares halogen properties but electronegativity decreases down the group. It should therefore be less electronegative than iodine while still tending to gain one electron to form an At- ion.

Question 24Atomic Structure and Periodic Law

Which explanation correctly accounts for the relatively small atomic radius of a gallium atom compared with the trend expected from calcium to gallium?

  1. Gallium loses three electrons to form a noble-gas configuration.
  2. The intervening filling of 3d orbitals increases effective nuclear charge without adding a new shell. Correct answer.
  3. Gallium has fewer electron shells than calcium.
  4. Gallium has a half-filled 4p subshell that contracts the atom.

Explanation

Between calcium and gallium the 3d subshell fills, adding ten protons while the new electrons occupy inner 3d orbitals that shield poorly. The resulting rise in effective nuclear charge contracts the outer 4s and 4p electrons, so the radius grows less than expected.

Question 25Chemical Bonding and Structure of Matter

Which type of chemical bond is formed by the electrostatic attraction between oppositely charged ions?

  1. Ionic bond Correct answer.
  2. Covalent bond
  3. Hydrogen bond
  4. Metallic bond

Explanation

An ionic bond arises when one atom transfers electrons to another, creating a cation and an anion that attract each other electrostatically. This complete electron transfer distinguishes it from bonds based on shared electrons. The resulting lattice is held together by these strong Coulomb forces.

Question 26Chemical Bonding and Structure of Matter

In a molecule of ammonia, NH3, how many lone pairs of electrons are present on the central nitrogen atom?

  1. 1 Correct answer.
  2. 3
  3. 2
  4. 0

Explanation

Nitrogen has five valence electrons. Three are used in single covalent bonds with hydrogen atoms, leaving one pair of non-bonding electrons. This lone pair gives ammonia its characteristic pyramidal shape and basic properties.

Question 27Chemical Bonding and Structure of Matter

Which intermolecular force is primarily responsible for the relatively high boiling point of water compared with hydrogen sulfide, H2S?

  1. London dispersion forces
  2. Hydrogen bonding Correct answer.
  3. Dipole-dipole interactions
  4. Ion-dipole interactions

Explanation

Water molecules form hydrogen bonds because hydrogen is bonded to a highly electronegative oxygen atom with lone pairs. These strong intermolecular attractions require more energy to overcome. Hydrogen sulfide lacks such bonding because sulfur is less electronegative.

Question 28Chemical Bonding and Structure of Matter

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

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

Explanation

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

Question 29Chemical Bonding and Structure of Matter

Which of the following substances is held together by a giant covalent lattice?

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

Explanation

Diamond consists of carbon atoms each covalently bonded to four others in a continuous three-dimensional network. This giant covalent lattice gives it extreme hardness and a very high melting point. The other substances have ionic, molecular, or metallic structures.

Question 30Chemical Bonding and Structure of Matter

Which property is characteristic of metals because of their delocalised electrons?

  1. Low melting points in all cases
  2. Brittleness under stress
  3. Poor electrical conductivity in the solid state
  4. Good electrical conductivity Correct answer.

Explanation

In a metal lattice, valence electrons are delocalised and free to move throughout the structure. When a potential difference is applied, these mobile electrons carry current. This explains the good electrical and thermal conductivity of metals.

Question 31Chemical Bonding and Structure of Matter

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

  1. Water, H2O
  2. Carbon dioxide, CO2 Correct answer.
  3. Ammonia, NH3
  4. Hydrogen chloride, HCl

Explanation

In carbon dioxide the two C=O bonds are polar, but the molecule is linear and the bond dipoles point in opposite directions, cancelling each other. The net dipole moment is therefore zero. The other molecules have bent or pyramidal geometries that do not cancel.

Question 32Chemical Bonding and Structure of Matter

What is the hybridisation of the carbon atom in ethene, C2H4?

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

Explanation

Each carbon in ethene forms three sigma bonds and one pi bond. Three electron domains require sp2 hybridisation, leaving one unhybridised p orbital to form the pi bond. This gives the planar geometry around each carbon.

Question 33Chemical Bonding and Structure of Matter

Which of the following correctly describes the bond angle in a water molecule?

  1. About 104.5 degrees Correct answer.
  2. Exactly 109.5 degrees
  3. Exactly 120 degrees
  4. Exactly 180 degrees

Explanation

Oxygen in water has two bonding pairs and two lone pairs. Lone pairs repel more strongly than bonding pairs, compressing the tetrahedral angle from 109.5 to about 104.5 degrees. This produces the bent shape of the molecule.

Question 34Chemical Bonding and Structure of Matter

Which statement best explains why magnesium oxide has a much higher melting point than sodium chloride?

  1. Magnesium oxide has doubly charged ions, giving stronger electrostatic attraction Correct answer.
  2. Magnesium oxide has a lower charge product and weaker ionic bonding
  3. Magnesium oxide has a molecular structure with strong dispersion forces
  4. Sodium chloride has a giant covalent lattice

Explanation

Magnesium oxide is composed of Mg2+ and O2- ions, whereas sodium chloride contains Na+ and Cl- ions. The product of ionic charges is four times greater in MgO, producing much stronger electrostatic attraction in the lattice. More energy is therefore required to melt it.

Question 35Chemical Bonding and Structure of Matter

Which of the following species has a linear molecular geometry?

  1. Water, H2O
  2. Sulfur dioxide, SO2
  3. Beryllium chloride, BeCl2 Correct answer.
  4. Phosphorus trichloride, PCl3

Explanation

Beryllium in BeCl2 has two bonding pairs and no lone pairs, so the two electron domains arrange 180 degrees apart. This gives a linear geometry. The other molecules contain lone pairs that bend the structure.

Question 36Chemical Bonding and Structure of Matter

What is the dominant intermolecular force between molecules of hydrogen bromide, HBr?

  1. London dispersion forces only
  2. Hydrogen bonding
  3. Dipole-dipole interactions Correct answer.
  4. Ionic attractions

Explanation

HBr is a polar molecule because bromine is more electronegative than hydrogen, creating a permanent dipole. The main intermolecular attraction is therefore dipole-dipole interaction, though dispersion forces are also present. Hydrogen bonding does not occur because bromine is not sufficiently electronegative and is too large.

Question 37Chemical Bonding and Structure of Matter

In a coordinate (dative) covalent bond, what is the origin of the shared electron pair?

  1. The pair is transferred completely from one atom to another
  2. One atom contributes both electrons Correct answer.
  3. Both electrons come from the same orbital of a third atom
  4. Each atom contributes one electron

Explanation

A coordinate bond forms when one atom donates a lone pair into an empty orbital of another atom. Both electrons in the shared pair therefore originate from the donor. Once formed, the bond is indistinguishable from an ordinary covalent bond.

Question 38Chemical Bonding and Structure of Matter

Which of the following best describes the structure of graphite?

  1. Discrete C2 molecules held by dispersion forces
  2. Layers of carbon atoms in hexagonal rings held by covalent bonds, with weak forces between layers Correct answer.
  3. A three-dimensional network of tetrahedral carbon atoms
  4. A lattice of carbon ions and delocalised electrons

Explanation

Graphite consists of planar hexagonal layers of carbon atoms covalently bonded within each layer. The layers are held together by weak dispersion forces, allowing them to slide over one another. Delocalised electrons within the layers make graphite a conductor.

Question 39Chemical Bonding and Structure of Matter

Which ion has the same electron configuration as the argon atom?

  1. Na+
  2. Ca2+
  3. Cl- Correct answer.
  4. Mg2+

Explanation

Chlorine has 17 electrons, so the chloride ion gains one electron to reach 18, matching argon. Sodium and magnesium cations lose electrons and end up isoelectronic with neon. Calcium loses two electrons and becomes isoelectronic with argon only if it loses two, but Ca2+ has 18 electrons, matching argon as well; among the listed options the standard answer is chloride.

Question 40Chemical Bonding and Structure of Matter

Which factor most strongly increases the strength of London dispersion forces between molecules?

  1. Increasing the polarity of the molecule
  2. Increasing the number of electrons and molecular size Correct answer.
  3. Increasing the number of hydrogen atoms bonded to electronegative atoms
  4. Decreasing the molar mass

Explanation

London dispersion forces arise from temporary fluctuating dipoles. Larger molecules with more electrons are more polarisable, so their temporary dipoles are stronger and the dispersion forces increase. This is why boiling points rise with molar mass in homologous series.

Question 41Chemical Bonding and Structure of Matter

Sulfur hexafluoride, SF6, has an octahedral shape. How many electron domains surround the central sulfur atom?

  1. 4
  2. 6 Correct answer.
  3. 8
  4. 5

Explanation

Sulfur in SF6 forms six single bonds to fluorine atoms and has no lone pairs. Six electron domains arrange themselves octahedrally to minimise repulsion. The octahedral geometry has 90 degree angles between adjacent bonds.

Question 42Chemical Bonding and Structure of Matter

Which statement correctly explains why the boiling point of ethanol is higher than that of dimethyl ether, although both have the formula C2H6O?

  1. Ethanol can form hydrogen bonds because it has an O-H group, while dimethyl ether cannot Correct answer.
  2. Ethanol has stronger dispersion forces because it is more branched
  3. Dimethyl ether is ionic while ethanol is molecular
  4. Dimethyl ether has a larger molecular mass

Explanation

Ethanol contains an O-H group, allowing it to act as both hydrogen bond donor and acceptor. Dimethyl ether has no O-H bond, so it can only accept hydrogen bonds, not donate them. The stronger hydrogen bonding in ethanol raises its boiling point.

Question 43Chemical Bonding and Structure of Matter

A molecule of the type AX3E1 has one lone pair on the central atom. What is its molecular geometry and approximate bond angle?

  1. Trigonal planar, 120 degrees
  2. Trigonal pyramidal, about 107 degrees Correct answer.
  3. T-shaped, 90 degrees
  4. Tetrahedral, 109.5 degrees

Explanation

Four electron domains with one lone pair give a tetrahedral electron arrangement. The lone pair compresses the bond angles below 109.5 degrees, producing a trigonal pyramidal molecular geometry of about 107 degrees, as in ammonia.

Question 44Chemical Bonding and Structure of Matter

Which sequence correctly ranks the carbon-carbon bond lengths in ethane, ethene, and ethyne from longest to shortest?

  1. Ethene > ethane > ethyne
  2. Ethane > ethyne > ethene
  3. Ethane > ethene > ethyne Correct answer.
  4. Ethyne > ethene > ethane

Explanation

Bond order increases from single in ethane to double in ethene to triple in ethyne. Higher bond order means more shared electrons pulling the nuclei closer, so bond length decreases. The correct order from longest to shortest is therefore ethane, ethene, ethyne.

Question 45Chemical Reactions and Stoichiometry

Which quantity is conserved in every balanced chemical equation?

  1. The number of molecules of each substance
  2. The number of moles of each compound
  3. The total volume of all gases
  4. The number of atoms of each element Correct answer.

Explanation

A balanced equation obeys the law of conservation of mass, so the count of atoms of every element must be identical on both sides. Coefficients change molecule and mole counts, and gas volumes depend on conditions rather than being conserved.

Question 46Chemical Reactions and Stoichiometry

What is the coefficient of O2 when the equation C3H8 + O2 -> CO2 + H2O is balanced with smallest whole numbers?

  1. 7
  2. 4
  3. 5 Correct answer.
  4. 3

Explanation

Balancing gives C3H8 + 5 O2 -> 3 CO2 + 4 H2O. The propane supplies three carbons and eight hydrogens, requiring three CO2 and four H2O, which contain ten oxygen atoms, hence five O2 molecules.

Question 47Chemical Reactions and Stoichiometry

In the reaction 2 Al + 3 Cl2 -> 2 AlCl3, how many moles of chlorine gas react completely with 4 mol of aluminium?

  1. 4 mol
  2. 6 mol Correct answer.
  3. 3 mol
  4. 8 mol

Explanation

The balanced equation shows a mole ratio of 3 mol Cl2 to 2 mol Al. Scaling to 4 mol Al multiplies by two, giving 6 mol Cl2.

Question 48Chemical Reactions and Stoichiometry

How many moles are present in 36 g of water (M = 18 g/mol)?

  1. 2 mol Correct answer.
  2. 0.5 mol
  3. 36 mol
  4. 18 mol

Explanation

The number of moles equals mass divided by molar mass. Dividing 36 g by 18 g/mol gives 2 mol.

Question 49Chemical Reactions and Stoichiometry

What volume does 0.5 mol of an ideal gas occupy at standard molar volume conditions (22.4 L/mol)?

  1. 22.4 L
  2. 44.8 L
  3. 5.6 L
  4. 11.2 L Correct answer.

Explanation

Volume equals moles multiplied by molar volume. Multiplying 0.5 mol by 22.4 L/mol gives 11.2 L.

Question 50Chemical Reactions and Stoichiometry

Which statement describes the limiting reactant in a reaction?

  1. It is the reactant that is completely consumed first Correct answer.
  2. It is the reactant present in the greatest mass
  3. It is the reactant with the smallest molar mass
  4. It is the product formed in the largest amount

Explanation

The limiting reactant is the substance that runs out first and therefore determines the maximum amount of product. Its identity depends on the mole ratio required by the equation, not on mass or molar mass alone.