Sample answers for questions that are answered offline are shown below.

Question 6

• The atomic number of an element is the same as the number of protons in the nuclei of its atoms.

• The period number of an element is the same as the number of electron shells in its atoms.

• The last digit of the group number of a main group element is the same as the number of valence electrons in its atoms.

Question 8

• Sodium is a group 1 alkali metal, therefore very reactive. It combines readily with other elements to form compounds, so does not exist naturally in its elemental form.

• Sodium is commonly found as the compound sodium chloride (NaCl), either as rock salt (halite) or dissolved in oceans.

Question 9

• Answers include any of: technetium, promethium, polonium, astatine, radon, francium, radium, actinium, thorium, protactinium, uranium, neptunium, plutonium.

• Answers include any element with atomic number greater than 94.

Question 10

• Argon, being a noble gas, is very unreactive and therefore will not contribute to chemical degradation of the paper and ink. Oxygen in air would slowly react with the paper and ink, especially if there was also moisture in the air.

• Helium, or any other noble gas.

Question 14

Melting points starts relatively high in group 1 (solids at room temperature). They increase until group 14 (very high boiling points), then decrease to their lowest value in group 18 (gases at room temperature).

Extension Question 1

Since the compound is a solid, it is most likely an ionic compound. Ionic compounds are formed from a metal and a non-metal, so A would be a metal and B would be a non-metal. For the compound to have the formula AB₂, it would be composed of a metal that forms a +2 ion and a non-metal that forms a -1 ion. To form a -1 ion, B must be from group 17, which would make it a halogen. This means it would be a reactive substance that produces toxic vapours. It would be composed of diatomic molecules (containing two atoms). It could be either a gas, liquid or solid, but wouldn’t have a high melting point. Even though A forms +2 ions, it would not be from group 2 as these elements form white solids with halogens. Therefore it must be a transition metal, located somewhere in groups 3-12. This means it would most likely be a hard solid, with a high density and high boiling point.

Extension Question 2

The densities from highest to lowest are: fluorine, oxygen, nitrogen. Density = mass/volume. Since atomic mass increases across a period (due to extra protons and electrons) and atomic size decreases across a period (due to greater attraction between nuclei and electrons), atomic density therefore increases across a period. (Each of the above gases consist of molecules containing two atoms, therefore their densities can be compared directly.)

Extension Question 3

• Electronegativity is a measure of how strongly an atom attracts electrons.

• Electronegativity decreases down groups and increases across periods.

• The more reactive a metal is, the less attraction it has for electrons; therefore, the lower its electronegativity. The more reactive a non-metal is, the greater attraction it has for electrons; therefore, the higher its electronegativity.