O-Level Patterns in the Periodic Table: worked solution

Question

A section of the Periodic Table is shown, with only these elements labelled: H (Period 1); Ne in Group 0 of Period 2; Mg (Group II), Al (Group III) and S (Group VI) in Period 3; K (Group I) and Br (Group VII) in Period 4. Using only these elements (each may be used once, more than once, or not at all), identify one element that: (a) forms a neutral oxide, (b) exists as diatomic molecules, (c) forms 2+ ions, (d) has the greatest proton number, (e) has just one electron shell.

(a) forms a neutral oxide - H. Metal oxides such as MgO and K2O are basic, and non-metal oxides such as SO2 and SO3 are acidic. Al2O3 is amphoteric and neon, being a noble gas, forms no oxide at all. The oxide of hydrogen is water, H2O, which is pH 7, so hydrogen is the one with a neutral oxide. (b) exists as diatomic molecules - H (Br is also accepted). 'Diatomic' means the molecule contains two atoms joined together. Hydrogen exists as H2 and bromine as Br2. Neon is monatomic (single atoms), Mg, Al and K are giant metallic structures, and sulfur is S8. (c) forms 2+ ions - Mg. An atom loses its outer-shell electrons to reach a full outer shell, so a Group II atom loses 2 electrons and becomes 2+. Magnesium is the only Group II element listed: Mg -> Mg2+ + 2e-. (d) has the greatest proton number - Br. Proton number rises across a period and continues rising into the next period. Potassium starts Period 4 with proton number 19, while bromine is much further along the same period with proton number 35. So bromine has the most protons. (e) has just one electron shell - H. The Period number tells you how many occupied electron shells the atom has. Hydrogen is the only element listed from Period 1, so it is the only one with a single shell (electron arrangement 1).