Ions and ionic bonding — return questions

1. explain / predict

Calcium bromide is an ionic compound made from calcium and bromine. (a) A pupil writes: "A bromine atom gains one electron, so the bromide ion is Br⁺." Explain why the pupil is incorrect. (b) A second pupil writes: "The ionic bond in calcium bromide is the two electrons that moved from the calcium atom to the bromine atoms." Explain why this pupil is incorrect. (c) Predict whether calcium bromide has high or low melting and boiling points, and whether it conducts electricity as a solid, when molten and when aqueous. (d) Supplement: explain, in terms of structure and bonding, your predictions for the melting point and for the boiling point.

(a) Why the first pupil is incorrect

(b) Why the second pupil is incorrect

(c) Your predictions

(d) Supplement: melting and boiling points explained

2. explain / deduce / identify

(a) A pupil draws a dot-and-cross diagram of potassium iodide, outer shells only, with potassium’s electrons as crosses and iodine’s as dots. The potassium ion, in square brackets with charge +, has 1 cross in its outer shell. The iodide ion, in square brackets with charge −, has 7 dots and 1 cross in its outer shell. Explain why the drawing is incorrect. (b) Supplement: a dot-and-cross diagram, outer shells only, shows a compound of metal M and non-metal X, with M’s electrons as crosses and X’s as dots. It shows two ions [M]⁺, each with an outer shell of 2 crosses, and one ion [X]²⁻ with an outer shell of 6 dots and 2 crosses. Deduce the Group of X. Then identify the element M, and explain how the diagram shows it.

(a) Why the drawing is incorrect

(b) Supplement: X and M