Atomic Structure & Periodicity

An atom of an element is represented as ⁵⁶Fe³⁺ (atomic number 26). How many protons, neutrons and electrons does this ion contain?

• A26 protons, 30 neutrons, 23 electrons✓
• B26 protons, 26 neutrons, 26 electrons
• C26 protons, 30 neutrons, 26 electrons
• D30 protons, 26 neutrons, 23 electrons

Answer: Protons = atomic number = 26. Neutrons = mass number − protons = 56 − 26 = 30. A 3+ ion has lost 3 electrons, so electrons = 26 − 3 = 23.

Chlorine has two isotopes, ³⁵Cl and ³⁷Cl, with relative isotopic masses of 34.97 and 36.97. The relative atomic mass of chlorine is 35.5. What is the approximate percentage abundance of the ³⁵Cl isotope?

• A25%
• B50%
• C75%✓
• D90%

Answer: Let abundance of ³⁵Cl = x%, so ³⁷Cl = (100−x)%. Ar = [34.97x + 36.97(100−x)]/100 = 35.5. Solving: 34.97x + 3697 − 36.97x = 3550, so −2.00x = −147, x ≈ 73.5% ≈ 75%.

In a time-of-flight (TOF) mass spectrometer, which statement correctly describes why ions of the same charge but different mass reach the detector at different times?

• AThe accelerating field gives the heavier ions a higher velocity, so they cross the drift region ahead of the lighter ones
• BAll ions gain the same kinetic energy, so heavier ions travel more slowly through the drift region✓
• CA magnetic field deflects the heavier ions more strongly, so they take a longer path to the detector
• DLighter ions always gain more kinetic energy, so they cross the drift region more slowly and arrive last

Answer: During acceleration all ions of equal charge gain the same kinetic energy (KE = ½mv²). Because KE is fixed, a larger mass m gives a smaller velocity v, so heavier ions take longer to cross the field-free drift region and arrive later.

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

• A1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁴
• B1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶
• C1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵✓
• D1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 4p⁴

Answer: Chromium is an exception to the Aufbau filling order. A half-filled 3d sub-shell is more stable, so one 4s electron is promoted, giving [Ar] 4s¹ 3d⁵, i.e. 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵.

The successive ionisation energies (kJ mol⁻¹) of an element are: 738, 1451, 7733, 10540, 13630. In which group of the periodic table is this element?

• AGroup 1
• BGroup 2✓
• CGroup 13
• DGroup 14

Answer: There is a large jump between the 2nd (1451) and 3rd (7733) ionisation energies. This shows two electrons are easily removed before reaching a far more stable, closer-to-nucleus shell, so the element has 2 outer electrons and is in Group 2.

Which equation correctly represents the SECOND ionisation energy of magnesium?

• AMg(g) → Mg⁺(g) + e⁻
• BMg⁺(aq) → Mg²⁺(aq) + e⁻
• CMg(g) → Mg²⁺(g) + 2e⁻
• DMg⁺(g) → Mg²⁺(g) + e⁻✓

Answer: The nth ionisation energy refers to removing one mole of electrons from one mole of gaseous +(n−1) ions. The second ionisation energy starts from the gaseous 1+ ion: Mg⁺(g) → Mg²⁺(g) + e⁻.

The first ionisation energy of aluminium is slightly LOWER than that of magnesium. Which statement best explains this anomaly?

• AAluminium has a larger nuclear charge than magnesium (13 protons against 12), so its outer electron is held more loosely and needs less energy to remove
• BThe outer electron of aluminium is removed from a 3p sub-shell, which is higher in energy and better shielded than the 3s sub-shell of magnesium✓
• CMagnesium has a half-filled 3p sub-shell, and a half-filled sub-shell is extra stable, so it takes far more energy to remove the outer electron from magnesium
• DAluminium atoms are larger than magnesium atoms, so the outer electron lies further from the nucleus

Answer: In Mg the outer electron is removed from the 3s sub-shell, whereas in Al it is removed from the higher-energy 3p sub-shell, which is shielded by the 3s electrons. The 3p electron is therefore easier to remove, lowering the first ionisation energy despite Al's greater nuclear charge.

Going across Period 3 from sodium to argon, what is the general trend in atomic radius and the main reason for it?

• ADecreases, because increasing nuclear charge pulls the same shell of electrons closer✓
• BIncreases, because each successive element adds another electron shell further from the nucleus
• CStays constant, because the electrons are added to the same outer shell
• DIncreases, because shielding by the inner shells increases across the period

Answer: Across a period the number of protons (nuclear charge) increases while electrons are added to the same outer shell, so shielding is roughly constant. The stronger nuclear attraction pulls the outer shell in, so atomic radius decreases.

Which of the following correctly describes the trend in melting point across Period 3 from Na to Ar?

• AIt rises to a maximum at silicon, drops sharply at phosphorus, and is lowest at argon✓
• BIt rises steadily all the way from sodium to argon as the metallic bonding gets stronger and stronger
• CIt peaks at sodium, where the metallic lattice is strongest, and then it falls steadily to argon
• DIt is highest at chlorine, because the strong covalent (Cl–Cl) bonds must be broken to melt it

Answer: Metallic bonding strengthens Na→Al; silicon is a giant covalent (macromolecular) solid with the highest melting point. From phosphorus onwards the elements are simple molecules (P₄, S₈, Cl₂) held by weak London forces, so melting points fall sharply, and argon (monatomic) has the lowest.

Which species has the smallest radius?

• ANa⁺
• BO²⁻
• CF⁻
• DMg²⁺✓

Answer: Na⁺, Mg²⁺, F⁻ and O²⁻ are all isoelectronic (10 electrons). The species with the most protons exerts the greatest pull on the electron cloud. Mg²⁺ has 12 protons (more than Na⁺ 11, F⁻ 9, O²⁻ 8), so it has the smallest radius.

What does the mass number of an atom count?

• AThe protons and neutrons in the nucleus✓
• BThe protons and electrons in the atom
• CThe neutrons and electrons in the atom
• DThe protons, neutrons and electrons

Answer: Mass number counts nucleons — protons plus neutrons. Electrons weigh almost nothing by comparison and are not included. The proton count alone is the atomic number, which fixes the element.

Why does first ionisation energy fall down a group?

• AGreater atomic radius and more shielding outweigh nuclear charge✓
• BGreater atomic radius and less shielding outweigh nuclear charge
• CSmaller atomic radius and more shielding outweigh nuclear charge
• DNuclear charge falls steadily as the group is descended

Answer: Each step down adds a full electron shell, so the outer electron is further from the nucleus and screened by more inner electrons. Nuclear charge does increase, but distance and shielding win, so less energy is needed to remove the electron.

Why does first ionisation energy dip between nitrogen and oxygen?

• AOxygen's paired 2s electrons repel, easing removal
• BOxygen has a lower nuclear charge than nitrogen
• COxygen's paired 2p electrons repel, easing removal✓
• DOxygen's outer electron sits in a higher shell

Answer: Nitrogen's three 2p electrons occupy separate orbitals. Oxygen's fourth must pair up, and repulsion between two electrons sharing one orbital makes it easier to remove despite the extra proton.

What are isotopes of an element?

• AAtoms with equal neutrons but differing protons
• BAtoms with equal protons but differing neutrons✓
• CAtoms with equal protons but differing electrons
• DAtoms with equal electrons but differing protons

Answer: Isotopes share the atomic number, so they are chemically identical, but differ in mass. Atoms differing in electron count are ions, and differing proton count means a different element entirely.

Why does atomic radius decrease across a period?

• ANuclear charge rises while shielding stays roughly constant✓
• BNuclear charge falls while shielding stays roughly constant
• CNuclear charge rises and shielding rises at the same rate
• DElectrons are added to successively higher energy shells

Answer: Each element adds one proton and one electron to the same outer shell, so shielding barely changes while the nucleus pulls harder. The outer electrons are drawn in and the atom gets smaller.

Why is the second ionisation energy always larger than the first?

• AThe electron is pulled from a negatively charged ion
• BThe electron is pulled from a positively charged ion✓
• CThe second electron always comes from an inner shell
• DThe nuclear charge increases after the first removal

Answer: Once one electron has gone the species is positive, so the remaining electrons feel a stronger net attraction and are harder to remove. The number of protons does not change — the charge imbalance does.

How many electrons fill a complete d subshell?

• ATen, across five orbitals✓
• BSix, across three orbitals
• CFourteen, across seven orbitals
• DTwo, within a single orbital

Answer: A d subshell has five orbitals holding two electrons each. p holds six across three orbitals, f holds fourteen across seven, and s holds two in one.

Why does a mass spectrum of chlorine gas show a peak at m/z 74?

• AIt is the ³⁷Cl–³⁷Cl molecular ion✓
• BIt is the ³⁵Cl–³⁷Cl molecular ion
• CIt is the ³⁵Cl–³⁵Cl molecular ion
• DIt is a fragment containing three chlorine atoms

Answer: Chlorine exists as Cl₂ with two isotopes, giving molecular ions at 70, 72 and 74. The 74 peak is both atoms being the heavier ³⁷Cl, which is the least likely combination and so the smallest of the three peaks.

What is relative atomic mass?

• AThe exact mass of the single most abundant isotope of an element
• BThe total mass of the protons and the neutrons within one atom
• CThe mass of one mole of atoms expressed in grams
• DThe weighted mean mass compared with one twelfth of carbon-12✓

Answer: Relative atomic mass averages the isotopes by abundance, which is why values such as chlorine's 35.5 are not whole numbers. The standard is carbon-12, defined as exactly 12.