Ions and Ionic Bonds Exam Questions

(a) Na: 2,8,1 and Cl: 2,8,7 [1]

(b) Sodium loses one electron [1] and chlorine gains one electron [1] / Sodium transfers one electron to chlorine.

Examiner note: Say “electron transfer” not “electron sharing” — ionic bonding involves transfer. Also, only electrons in the outer shell are transferred.

• Mg2+ ion shown with no outer electrons, in square brackets with 2+ charge [1]
• Each Cl- ion shown with 8 outer electrons (7 original + 1 from Mg), in square brackets with 1- charge [1]
• Two Cl- ions shown [1]
• Dots and crosses used to distinguish transferred electrons [1]

Examiner note: Three common errors: (1) forgetting square brackets and charges on the ions, (2) showing only one Cl- instead of two, (3) not distinguishing between Mg electrons and Cl electrons (use dots for one and crosses for the other). The formula MgCl2 tells you the ratio: one Mg transfers one electron to each of two Cl atoms.

• Sodium chloride has a giant ionic structure/lattice [1]
• There are strong electrostatic forces of attraction between the oppositely charged ions (Na+ and Cl-) [1]
• A lot of energy is needed to overcome these strong forces [1]

Examiner note: “Strong bonds” alone is worth 0 marks. You must specify the nature of the force: electrostatic attraction between oppositely charged ions. Using the word “electrostatic” is strongly recommended.

• When solid, the ions are held in fixed positions in the lattice and cannot move [1]
• When molten or dissolved, the ions are free to move [1]
• The mobile ions can carry charge / move towards electrodes, allowing the substance to conduct electricity [1]

Examiner note: Do not say “electrons are free to move” — ionic compounds conduct through ion movement, not electron flow. This is different from metallic conduction. Also, dissolved means dissolved in water (aqueous), not melted.

(a) 2+ [1]

(b) Calcium has 2 electrons in its outer shell (electronic configuration 2,8,8,2) and loses both to achieve a stable electron configuration / full outer shell [1]

Examiner note: The charge equals the number of outer-shell electrons lost (for metals). Ca loses 2 electrons, so the ion has 2 more protons than electrons, giving a 2+ charge.

• Ca atom has configuration 2,8,8,2 — it loses 2 outer electrons [1]
• O atom has configuration 2,6 — it gains 2 electrons [1]
• Ca2+ ion shown with no outer electrons, in square brackets with 2+ charge [1]
• O2- ion shown with 8 outer electrons, in square brackets with 2- charge [1]

Examiner note: The ratio is 1:1 in CaO because calcium transfers exactly 2 electrons and oxygen needs exactly 2 electrons. This is different from MgCl2 where the ratio is 1:2. Always check the formula for the ratio.

• In ionic compounds, when layers are displaced, ions of the same charge come next to each other [1]
• The repulsion between like-charged ions causes the lattice to shatter [1]
• In metals, layers of atoms can slide over each other without disrupting the bonding because the delocalised electrons can adjust / the metallic bond is non-directional [1]

Examiner note: The key concept is that displacing a layer in an ionic lattice puts positive ions next to positive ions (and negative next to negative), causing repulsion. In a metal, the sea of electrons holds the layers together regardless of position.

(a) Li+: 2 [1] (same as helium) F-: 2,8 [1] (same as neon)

(b) Water molecules are polar and can attract/surround the ions, pulling them out of the lattice [1]. Hexane is non-polar and cannot interact strongly with the ions.

Examiner note: Both ions achieve noble gas electron configurations — Li+ has the same configuration as He, and F- has the same configuration as Ne. This stability is the driving force for ionic bond formation.