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WACE Chemistry ATAR study materials
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WACE Chemistry ATAR practice questions

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Question 1Structure of the atom and the periodic table

Which subatomic particle determines the identity of an element?

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

Explanation

The number of protons in the nucleus, called the atomic number, is unique to each element and defines its identity. Changing the proton count converts the atom into a different element, whereas changing neutrons or electrons does not.

Question 2Structure of the atom and the periodic table

An atom of an element has the electron configuration 1s² 2s² 2p⁶ 3s². In which group of the periodic table does this element belong?

  1. Group 1
  2. Group 16
  3. Group 18
  4. Group 2 Correct answer.

Explanation

The configuration ends in 3s², giving two valence electrons in the outermost s subshell. Elements with two outer s electrons occupy Group 2 of the periodic table.

Question 3Structure of the atom and the periodic table

Which statement correctly describes isotopes of an element?

  1. They have the same number of neutrons but different numbers of electrons
  2. They have different numbers of protons but the same number of neutrons
  3. They have the same mass number but different atomic numbers
  4. They have the same number of protons but different numbers of neutrons Correct answer.

Explanation

Isotopes are atoms of the same element, so they share the same proton count and atomic number. They differ only in neutron number, which changes the mass number while leaving chemical identity intact.

Question 4Structure of the atom and the periodic table

Which of the following species contains 18 electrons?

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

Explanation

A chlorine atom has 17 electrons, so gaining one electron to form Cl⁻ gives 18 electrons. This matches the electron count of the noble gas argon.

Question 5Structure of the atom and the periodic table

Which block of the periodic table contains the transition metals?

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

Explanation

Transition metals have their differentiating electrons added to d orbitals, so they lie in the d block. The s block holds Groups 1 and 2, and the p block holds Groups 13 to 18.

Question 6Structure of the atom and the periodic table

Which property generally increases across a period from left to right?

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

Explanation

Across a period, nuclear charge rises while electrons enter the same shell, so the outer electrons are held more tightly. Removing an electron therefore requires more energy, raising first ionisation energy.

Question 7Structure of the atom and the periodic table

What is the mass number of an atom with 11 protons, 12 neutrons and 11 electrons?

  1. 23 Correct answer.
  2. 22
  3. 34
  4. 11

Explanation

The mass number is the sum of protons and neutrons in the nucleus, giving 11 + 12 = 23. Electrons are not included because their mass is negligible.

Question 8Structure of the atom and the periodic table

Which electron configuration represents the ground state of a neutral oxygen atom (Z = 8)?

  1. 1s² 2s² 2p⁴ Correct answer.
  2. 1s² 2s¹ 2p⁵
  3. 1s² 2s² 2p⁶
  4. 1s² 2s² 2p³ 3s¹

Explanation

Eight electrons fill the 1s orbital, then the 2s orbital, leaving four electrons for the 2p subshell. This gives 1s² 2s² 2p⁴, the ground-state configuration of oxygen.

Question 9Structure of the atom and the periodic table

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

  1. Magnesium has a full outer shell, making it a noble gas configuration
  2. Magnesium has a higher nuclear charge and its outer electron is in the same shell, so it is held more strongly Correct answer.
  3. Magnesium has a larger atomic radius, so its outer electron is further from the nucleus
  4. Magnesium has more electron shells, which increases the shielding of the outer electron

Explanation

Sodium and magnesium place their outer electrons in the same third shell, so shielding is similar. Magnesium's greater nuclear charge pulls the outer electron closer and more tightly, requiring more energy to remove it.

Question 10Structure of the atom and the periodic table

Which species has the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶?

  1. Al³⁺
  2. S²⁻
  3. Ne
  4. K⁺ Correct answer.

Explanation

Potassium has 19 electrons, and forming K⁺ removes one, leaving 18 electrons. These fill orbitals to give 1s² 2s² 2p⁶ 3s² 3p⁶, the argon configuration.

Question 11Structure of the atom and the periodic table

Across Period 3, the atomic radius decreases from sodium to chlorine. Which factor best accounts for this trend?

  1. The number of occupied electron shells decreases
  2. The effective nuclear charge increases while electrons enter the same shell Correct answer.
  3. The number of neutrons in the nucleus decreases
  4. The shielding by inner electrons increases strongly

Explanation

All Period 3 elements place their outer electrons in the third shell, so shielding stays roughly constant. Increasing nuclear charge across the period draws this shell inward, reducing atomic radius.

Question 12Structure of the atom and the periodic table

Which element has the highest electronegativity?

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

Explanation

Electronegativity increases across a period and decreases down a group. Fluorine combines a small radius with a high nuclear charge, giving it the highest electronegativity of all elements.

Question 13Structure of the atom and the periodic table

An element has the electron configuration [Ar] 4s² 3d¹⁰ 4p³. To which group and period does it belong?

  1. Group 17, Period 4
  2. Group 15, Period 3
  3. Group 13, Period 4
  4. Group 15, Period 4 Correct answer.

Explanation

The highest principal quantum number is 4, placing the element in Period 4. The 4s and 4p electrons total five valence electrons, giving Group 15.

Question 14Structure of the atom and the periodic table

Which pair of species are isoelectronic?

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

Explanation

Isoelectronic species have the same total number of electrons. Na⁺ has 10 electrons after losing one, and neon also has 10 electrons, so they are isoelectronic.

Question 15Structure of the atom and the periodic table

Which statement about the d-block elements is correct?

  1. They all have a partially filled d subshell in every oxidation state
  2. They are all highly reactive non-metals
  3. They typically form coloured compounds and can show variable oxidation states Correct answer.
  4. They have their outer electrons exclusively in p orbitals

Explanation

Transition metals often have partly filled d orbitals that allow electrons to be lost in several ways, giving variable oxidation states. Electronic transitions involving these d orbitals also produce characteristic colours.

Question 16Structure of the atom and the periodic table

Which ion would be formed most readily by an element with the configuration 1s² 2s² 2p⁶ 3s² 3p⁵?

  1. A 1+ ion
  2. A 2− ion
  3. A 2+ ion
  4. A 1− ion Correct answer.

Explanation

This configuration is one electron short of the argon configuration, so gaining one electron completes the outer shell. The element therefore forms a 1− ion readily.

Question 17Structure of the atom and the periodic table

Which sequence correctly orders the first ionisation energies from lowest to highest?

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

Explanation

Ionisation energy generally rises across Period 3, but aluminium is lower than magnesium because its outer electron is in a 3p orbital, which is higher in energy and better shielded than the 3s orbital. The correct order is Na < Al < Mg < Si.

Question 18Structure of the atom and the periodic table

A sample of an element consists of two isotopes: one of mass 63 with abundance 70% and one of mass 65 with abundance 30%. What is the relative atomic mass?

  1. 65.0
  2. 63.6 Correct answer.
  3. 64.4
  4. 64.0

Explanation

Relative atomic mass is the abundance-weighted average of isotope masses. Calculating (63 × 0.70) + (65 × 0.30) gives 44.1 + 19.5 = 63.6.

Question 19Structure of the atom and the periodic table

Why is the second ionisation energy of calcium greater than its first?

  1. The nucleus loses a proton after the first ionisation
  2. The second electron is removed from a shell closer to the nucleus, so it experiences less shielding and stronger attraction Correct answer.
  3. The second electron is removed from a more shielded outer orbital
  4. Calcium gains a full outer shell after the first ionisation

Explanation

After the first electron is removed, the remaining outer electron is held by the same nuclear charge with one fewer electron providing repulsion. The second electron removed in calcium comes from a lower-energy, less shielded environment, so more energy is required.

Question 20Structure of the atom and the periodic table

Which factor best explains why atomic radius increases down Group 1?

  1. The nuclear charge decreases down the group
  2. The number of neutrons in the nucleus decreases
  3. Additional electron shells are added, increasing the distance of the outer electron from the nucleus Correct answer.
  4. Electronegativity increases down the group

Explanation

Each element down Group 1 has one more occupied shell than the one above it. This places the outer electron further from the nucleus and increases shielding, so the atomic radius grows.

Question 21Structure of the atom and the periodic table

Which species would have the largest radius?

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

Explanation

These species are isoelectronic with 10 electrons each, so size depends on nuclear charge. Fluoride has the lowest nuclear charge (9 protons) pulling on the same ten electrons, so it is the largest.

Question 22Structure of the atom and the periodic table

Which statement correctly describes the trend in first ionisation energy down Group 17?

  1. It increases because nuclear charge increases
  2. It decreases because the outer electron is further from the nucleus and more shielded Correct answer.
  3. It increases because atomic radius decreases
  4. It stays constant because all members have seven valence electrons

Explanation

Down Group 17, each element adds an electron shell, increasing the distance and shielding between the nucleus and the outer electron. Despite a higher nuclear charge, the outer electron is easier to remove, so ionisation energy falls.

Question 23Structure of the atom and the periodic table

An element has a first ionisation energy much lower than expected and forms a 1+ ion with a noble gas configuration. Which element is most likely?

  1. A halogen in Group 17
  2. An alkali metal in Group 1 Correct answer.
  3. A noble gas in Group 18
  4. A transition metal in the d block

Explanation

Alkali metals have a single valence electron that is easily lost to achieve a stable noble gas configuration. Their low first ionisation energy reflects the weak hold on that outer electron.

Question 24Structure of the atom and the periodic table

Which statement about the aufbau principle and electron configurations is correct?

  1. Electrons pair in degenerate orbitals before occupying empty ones
  2. Each orbital can hold a maximum of one electron
  3. Electrons fill orbitals of higher energy before lower energy
  4. Electrons fill the lowest available energy orbitals first Correct answer.

Explanation

The aufbau principle states that electrons occupy the lowest-energy orbitals available before higher ones. This ordering underlies configurations such as filling 4s before 3d.

Question 25Chemical bonding and molecular shape

Which statement best describes the electrostatic attraction in an ionic bond?

  1. Attraction between nuclei and a shared electron cloud
  2. Delocalised electrons attracting metal cations
  3. Attraction between oppositely charged ions Correct answer.
  4. Sharing of electron pairs between two non-metals

Explanation

An ionic bond arises when electrons are transferred from a metal to a non-metal, producing cations and anions. The resulting oppositely charged ions attract each other electrostatically in a lattice. The strength of this attraction depends on ionic charge and separation distance.

Question 26Chemical bonding and molecular shape

Which pair of elements is most likely to form an ionic compound when combined?

  1. Sulfur and oxygen
  2. Carbon and oxygen
  3. Nitrogen and hydrogen
  4. Sodium and chlorine Correct answer.

Explanation

Ionic compounds typically form between a metal with low ionisation energy and a non-metal with high electron affinity. Sodium is a group 1 metal that readily loses an electron, while chlorine is a reactive non-metal that gains one. The large electronegativity difference favours electron transfer.

Question 27Chemical bonding and molecular shape

Which property is characteristic of a covalent molecular substance such as iodine?

  1. A rigid three-dimensional network of covalent bonds
  2. Electrical conductivity when solid
  3. Low melting point with weak forces between molecules Correct answer.
  4. High melting point from a giant ionic lattice

Explanation

Molecular covalent substances consist of discrete molecules held together by strong intramolecular bonds but attracted to neighbours only by weak intermolecular forces. These weak forces are overcome easily on heating, giving low melting and boiling points. They do not conduct electricity because there are no free charged particles.

Question 28Chemical bonding and molecular shape

How many valence electrons does a nitrogen atom have available for bonding, and how many covalent bonds does it typically form?

  1. Seven valence electrons; one bond
  2. Five valence electrons; five bonds
  3. Three valence electrons; three bonds
  4. Five valence electrons; three bonds Correct answer.

Explanation

Nitrogen is in group 15, so it has five valence electrons. To complete its octet it needs three more electrons, so it forms three covalent bonds, as in ammonia. The remaining two electrons form a lone pair.

Question 29Chemical bonding and molecular shape

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

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

Explanation

The central carbon has four bonding pairs and no lone pairs, so the four electron domains arrange themselves as far apart as possible. This gives a tetrahedral electron geometry and a tetrahedral molecular shape. The bond angles are approximately 109.5 degrees.

Question 30Chemical bonding and molecular shape

Which intermolecular force is present between all molecules, including non-polar ones such as methane?

  1. Ion-dipole attraction
  2. Covalent bonding
  3. Dispersion forces Correct answer.
  4. Hydrogen bonding

Explanation

Dispersion forces arise from temporary fluctuations in electron distribution that create instantaneous dipoles. Because all molecules have electrons, these forces operate between all molecules, though they are strongest in large, polarisable molecules. They are the only intermolecular force available to non-polar species.

Question 31Chemical bonding and molecular shape

Which element has the highest electronegativity on the Pauling scale?

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

Explanation

Electronegativity increases across a period and decreases down a group. Fluorine sits at the top right of the periodic table and has the highest value, about 4.0. Its small size and high nuclear charge attract bonding electrons strongly.

Question 32Chemical bonding and molecular shape

What is the shape of the ammonia molecule, NH3, and its approximate bond angle?

  1. Tetrahedral, about 109.5 degrees
  2. Bent, about 104.5 degrees
  3. Trigonal planar, about 120 degrees
  4. Trigonal pyramidal, about 107 degrees Correct answer.

Explanation

Nitrogen in ammonia has three bonding pairs and one lone pair, giving four electron domains arranged tetrahedrally. The lone pair repels the bonding pairs more strongly, compressing the H-N-H angle to about 107 degrees. The molecular shape, considering only atoms, is trigonal pyramidal.

Question 33Chemical bonding and molecular shape

Which molecule has a net dipole moment?

  1. CCl4
  2. BF3
  3. H2O Correct answer.
  4. CO2

Explanation

A molecule is polar only if its bond dipoles do not cancel by symmetry. Water has a bent shape with two polar O-H bonds, so the dipoles add to give a net dipole. The other molecules are highly symmetrical, so their individual bond dipoles cancel.

Question 34Chemical bonding and molecular shape

Why does magnesium oxide have a much higher melting point than sodium chloride?

  1. Its ions are larger and more polarisable
  2. Its ions carry double charges, strengthening the lattice Correct answer.
  3. It has a covalent network structure
  4. It contains hydrogen bonds between ions

Explanation

Lattice energy increases with the product of ionic charges and decreases with ion separation. Magnesium and oxide ions each carry a 2+ or 2- charge, so the electrostatic attraction is far stronger than in sodium chloride with 1+ and 1- ions. More energy is therefore needed to break the lattice.

Question 35Chemical bonding and molecular shape

Which species has a trigonal planar shape?

  1. CH4
  2. BF3 Correct answer.
  3. NH3
  4. H2O

Explanation

Boron in BF3 has three bonding pairs and no lone pairs, so the three domains spread out at 120 degrees in a plane. This gives a trigonal planar molecular shape. The other species have lone pairs or four domains that produce different geometries.

Question 36Chemical bonding and molecular shape

Which statement explains why graphite conducts electricity while diamond does not?

  1. Graphite has delocalised electrons between its layers Correct answer.
  2. Diamond has weaker covalent bonds than graphite
  3. Diamond contains delocalised electrons that are trapped
  4. Graphite contains mobile ions within its lattice

Explanation

Each carbon in graphite bonds to three neighbours, leaving one electron delocalised in a pi system that can move along the layers. These mobile electrons carry current. In diamond every valence electron is localised in a sigma bond, so no charge carriers are free to move.

Question 37Chemical bonding and molecular shape

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

  1. Dipole-dipole attraction
  2. Hydrogen bonding Correct answer.
  3. Dispersion forces
  4. Ion-dipole attraction

Explanation

Oxygen is highly electronegative and small, so the O-H bonds are strongly polar and hydrogen bonding occurs between molecules. Each water molecule can form up to four hydrogen bonds, requiring considerable energy to separate molecules. Sulfur is larger and less electronegative, so hydrogen sulfide has only weaker dipole-dipole and dispersion forces.

Question 38Chemical bonding and molecular shape

What is the electron-domain geometry and molecular shape of the water molecule?

  1. Tetrahedral electron geometry; linear molecular shape
  2. Linear electron geometry; linear molecular shape
  3. Trigonal planar electron geometry; bent molecular shape
  4. Tetrahedral electron geometry; bent molecular shape Correct answer.

Explanation

Oxygen in water has two bonding pairs and two lone pairs, giving four electron domains arranged tetrahedrally. When only the positions of atoms are considered, the shape is bent. Lone-pair repulsion reduces the H-O-H angle to about 104.5 degrees.

Question 39Chemical bonding and molecular shape

Which bond is most polar?

  1. H-I
  2. H-Cl
  3. H-F Correct answer.
  4. H-Br

Explanation

Bond polarity depends on the electronegativity difference between the bonded atoms. Fluorine is the most electronegative element, so the H-F bond has the largest difference and is the most polar. The difference decreases down group 17 as electronegativity falls.

Question 40Chemical bonding and molecular shape

Which compound would be expected to have the highest melting point?

  1. CO2
  2. HCl
  3. Cl2
  4. NaCl Correct answer.

Explanation

Sodium chloride is a giant ionic lattice held together by strong electrostatic attractions, so a large amount of energy is needed to melt it. The other substances are molecular, held together only by weak intermolecular forces. These are overcome at much lower temperatures.

Question 41Chemical bonding and molecular shape

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

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

Explanation

Ethene has four C-H single bonds and one C=C double bond. The double bond consists of one sigma and one pi bond, so the total is four plus one equals five sigma bonds and one pi bond. The pi bond forms from sideways overlap of unhybridised p orbitals.

Question 42Chemical bonding and molecular shape

Which statement best explains why the boiling points of the noble gases increase from helium to radon?

  1. Covalent bonds between atoms become stronger
  2. Hydrogen bonding becomes stronger down the group
  3. Permanent dipoles increase down the group
  4. Dispersion forces strengthen as electron number increases Correct answer.

Explanation

Noble gases are monatomic and non-polar, so the only intermolecular force is dispersion. As the number of electrons increases down the group, the electron cloud becomes more polarisable and instantaneous dipoles larger. Stronger dispersion forces then require more energy to overcome.

Question 43Chemical bonding and molecular shape

Which species has a bond angle closest to 109.5 degrees?

  1. NH4+ Correct answer.
  2. NO2-
  3. HCN
  4. SO2

Explanation

The ammonium ion has four bonding pairs around nitrogen and no lone pairs, so its four domains are arranged tetrahedrally with angles of about 109.5 degrees. The other species have fewer domains or lone pairs that alter the ideal angle.

Question 44Chemical bonding and molecular shape

Which factor best explains the very high melting point of diamond?

  1. Strong electrostatic attractions between ions
  2. Weak dispersion forces between layers
  3. Hydrogen bonding between carbon atoms
  4. A three-dimensional network of strong covalent bonds Correct answer.

Explanation

Each carbon atom in diamond is covalently bonded to four others in a rigid three-dimensional network. Melting requires breaking many strong covalent bonds, so the melting point is extremely high. The rigidity also makes diamond very hard.

Question 45Chemical bonding and molecular shape

The carbonate ion, CO3^2-, has three identical C-O bonds. Which bonding description best accounts for this observation?

  1. Delocalised pi electrons spread over three C-O regions Correct answer.
  2. Three separate single bonds with no pi electrons
  3. Two lone pairs donated into the carbon atom
  4. One C=O double bond fixed in position

Explanation

The carbonate ion contains one sigma framework and a delocalised pi system in which the pi electrons are shared across all three C-O regions. Resonance means each C-O bond has a bond order of about 1.33. This delocalisation makes the bonds equivalent in length and strength.

Question 46Chemical bonding and molecular shape

Which sequence correctly orders the species by increasing H-X-H bond angle?

  1. CH4 < NH3 < H2O
  2. H2O < CH4 < NH3
  3. H2O < NH3 < CH4 Correct answer.
  4. NH3 < H2O < CH4

Explanation

All three central atoms have four electron domains, so the ideal angle is 109.5 degrees. Lone pairs occupy more space than bonding pairs, so the angle decreases as the number of lone pairs rises. Water has two lone pairs, ammonia one and methane none, giving the order H2O < NH3 < CH4.

Question 47Chemical bonding and molecular shape

A binary compound of an unknown metal and oxygen has a very high melting point and conducts electricity only when molten. Which structure is most consistent with these observations?

  1. A metallic lattice
  2. A simple molecular lattice
  3. A covalent network solid
  4. A giant ionic lattice Correct answer.

Explanation

A giant ionic lattice melts at high temperature because of strong electrostatic attractions between ions. It conducts only when molten because the ions become mobile charge carriers, whereas in the solid they are fixed. A metal-oxygen compound with these properties fits an ionic model.

Question 48Chemical bonding and molecular shape

Which species has a square planar shape?

  1. CH4
  2. PCl5
  3. XeF4 Correct answer.
  4. SF4

Explanation

Xenon in XeF4 has four bonding pairs and two lone pairs, giving six electron domains. The two lone pairs occupy opposite positions, so the four fluorine atoms lie at the corners of a square plane. The lone pairs are positioned to minimise repulsion at 180 degrees to each other.

Question 49Chemical reactions, stoichiometry and the mole concept

How many particles are present in 0.500 mol of helium atoms? (Avogadro's constant = 6.02 × 10^23 mol^-1)

  1. 3.01 × 10^24
  2. 6.02 × 10^23
  3. 1.20 × 10^24
  4. 3.01 × 10^23 Correct answer.

Explanation

The number of particles equals the amount in moles multiplied by Avogadro's constant, so 0.500 × 6.02 × 10^23 gives 3.01 × 10^23 particles. Halving the mole amount halves the particle count relative to one mole.

Question 50Chemical reactions, stoichiometry and the mole concept

What is the molar mass of carbon dioxide, CO2? (C = 12.0, O = 16.0)

  1. 32.0 g mol^-1
  2. 44.0 g mol^-1 Correct answer.
  3. 56.0 g mol^-1
  4. 28.0 g mol^-1

Explanation

The molar mass is the sum of the relative atomic masses of all atoms in the formula. Carbon contributes 12.0 and the two oxygen atoms contribute 2 × 16.0 = 32.0, giving 44.0 g mol^-1.