Higher Chemistry KA 1 Periodicity & Bonding PPQs

2015;2

Which of the following equations represents the first ionisation energy of fluorine?

• F^-(g) --> F(g) + e^-

• F^-(g) --> 1/2F₂(g) + e^-

• F(g) --> F⁺(g) + e^-

• 1/2F₂(g) --> F⁺(g) + e^-

Answer explanation

Look on page 12 of your data booklet.

First ionisation:

E(g) --> E⁺(g) + e^-

Second ionisation:

E⁺(g) --> E²⁺(g) + e^-

2015;3

Which of the following atoms has least attraction for bonding electrons?

• Carbon

• Nitrogen

• Phosphorus

• Silicon

Answer explanation

Electronegativity is the ability of an atom to attract shared electrons in a bond.

• Electronegativity increases across a period (left to right)

• Electronegativity decreases down a group (top to bottom)

Nitrogen and Carbon are in Period 2 (small atoms = strong attraction).
Phosphorus and Silicon are in Period 3 (larger atoms = weaker attraction).

Although Phosphorus is below Nitrogen, Silicon is further left in the same period, meaning it has:

• A lower nuclear charge

• A larger atomic radius

• More shielding

So, Silicon has the weakest attraction for bonding electrons — the lowest electronegativity.

2018;4

The table shows the first 3 ionisation energies of aluminium.

Using this information, what is the enthalpy change, in kJmol^-1, for the following reaction?

Al⁺(g) --> Al³⁺(g) + 2e^-

• 1817

• 2395

• 4562

• 5140

Answer explanation

To calculate the enthalpy change for Al⁺(g) → Al³⁺(g) + 2e⁻, you need to add the 2nd and 3rd ionisation energies of aluminium:

• 2nd IE = 1817 kJ mol⁻¹

• 3rd IE = 2745 kJ mol⁻¹

Total energy required = 1817 + 2745 = 4562 kJ mol⁻¹

We do not include the 1st ionisation energy because the aluminium is already Al⁺, not neutral.

2021;2

The difference in the size of sodium and chlorine atoms is mainly due to the difference in the

• mass of each atom

• number of electrons

• number of neutrons

• number of protons

Answer explanation

• The size of an atom is mostly influenced by the electrostatic attraction between the nucleus and the electrons.

  • Chlorine has 17 protons, pulling the electrons in more tightly.

  • Sodium has 11 protons, so the pull on electrons is weaker.

  👉 Even though both atoms are in the same period (Period 3), the nuclear charge increases across the period, pulling electrons closer to the nucleus and making the atom smaller.

  So the main reason for the size difference is the greater number of protons in chlorine, leading to a stronger attraction and a smaller atomic radius.

• Why the other answers are wrong:

• Mass of atom: Doesn't directly affect atomic radius — mass affects density, not size.

• Number of electrons: Both atoms are neutral — this isn’t the key factor for size.

• Number of neutrons: Neutrons don’t affect attraction between nucleus and electrons.

2021;6

What is the enthalpy change, in kJ mol⁻¹, for the following reaction?

Be(g) --> Be²⁺(g) + 2e⁻

• 900

• 1757

• 2657

• 3514

Answer explanation

To remove two electrons from a beryllium atom, we need to add the 1st and 2nd ionisation energies:

• 1st IE = 900 kJ mol⁻¹

• 2nd IE = 1757 kJ mol⁻¹

900 + 1757 = 2657 kJ mol⁻¹

2022;2

The graph below shows the relative quantities of energy equivalent to successive ionisation energies for an element.

The most stable ion formed from an atom of this element has a charge of:

• 2+

• 3+

• 2-

• 3-

Answer explanation

The big jump in ionisation energy after the 2nd electron means only 2 electrons can be removed easily — they’re in the outer shell.
Removing more needs much more energy because you'd be taking electrons from a full inner shell.

So, the most stable ion is the one where only the 2 outer electrons are lost → charge of 2⁺

2023;8

Which of the following statements is true?

• The sodium atom is larger than the sodium ion.

• The chloride ion is smaller than the chlorine atom.

• The magnesium ion is larger than the magnesium atom.

• The oxygen atom is larger than the oxide ion.

Answer explanation

When an atom loses electrons to form a positive ion (cation):

• It loses an outer shell

• There are now more protons than electrons, so the remaining electrons are pulled in more tightly

• Result: the ion is smaller than the atom

Na → Na⁺

• Sodium loses 1 electron → smaller radius
  So Na atom is larger than Na⁺ ion