2. The atomic radius decreases across Period 3 because the force of attraction between the nucleus and the electrons increases.

3. The number of protons in the nucleus and the nuclear charge increase.

11. There are more electrons, but a negligible increase in shielding because each extra electron enters the same shell.

12. The first ionisation enthalpy increases across Period 3. The force of attraction between the nucleus and the outer electron increases.

13. The first ionisation enthalpy increases across Period 3. The force of attraction between the nucleus and the outer electron increases.

14. The first ionisation enthalpy increases across Period 3. The force of attraction between the nucleus and the outer electron increases.

15. The number of protons in the nucleus and the nuclear charge increase.

20. There are more electrons, but little increase in shielding as each extra electron enters the same shell.

21. The density of an element is its mass per unit volume. It is a measure of how closely its particles are packed.

22. The density of the metals increases across the period.

23. Silicon has a densely packed giant covalent structure.

24. Phosphorus and sulphur exist as the simple molecules P 4 and S 8, and are less dense.

25. Chlorine and argon are both gases at room temperature, so their densities are very low.

26. When a substance melts or boils, attractive forces holding the particles together must be overcome.

27. The stronger the attractive forces are, the more energy is needed to overcome them.

28. Sodium, magnesium and aluminium are all metals. They have strong metallic bonding.

29. Silicon has a giant covalent structure in which its atoms are joined by strong covalent bonds.

30. There are weak instantaneous dipole–induced dipole attractions between phosphorus, sulphur and chlorine molecules.

31. There are weak instantaneous dipole–induced dipole attractions between argon atoms. Little energy is needed to overcome them.

32. Sodium, magnesium and aluminium are metals with delocalised electrons that can move and carry charge.

33. Sodium, magnesium and aluminium are metals with delocalised electrons that can move and carry charge.

34. Sodium, magnesium and aluminium are metals with delocalised electrons that can move and carry charge.

35. Each aluminium atom contributes more delocalised electrons than sodium or magnesium.

36. Silicon is a semiconductor. It is used in computer circuitry.

37. Phosphorus, sulphur and chlorine are simple molecules with no free electrons. Argon exists as single atoms.