Periodic Table of Elements / Jadual Berkala Unsur
The Periodic Table tests arrangement by proton number, group and period trends, and properties of alkali metals, halogens, noble gases, transition metals and metalloids. You must predict reactivity and displacement from position.
Core patterns
- Period = number of occupied shells; group number for main groups = valence electrons.
- Down Group 1: atomic radius increases, the outer electron is farther from the nucleus and more shielded, so it is lost more easily; reactivity increases.
- Down Group 17: radius and shielding increase, attraction for an incoming electron decreases; reactivity decreases.
- Transition metals show variable oxidation states, coloured compounds, catalytic activity, high density and high melting points.
- Noble gases have full outer shells and are unreactive.
Worked example
Predict whether chlorine displaces bromine from potassium bromide. Chlorine is above bromine in Group 17, so it is more reactive and a stronger oxidising agent. Cl₂ + 2KBr → 2KCl + Br₂. The solution turns orange-brown as bromine forms. The reverse reaction does not occur because bromine cannot displace chlorine. Reasoning uses the trend down the group, not just the fact that chlorine is smaller.
Traps
Group number is not always valence electrons for transition metals. Metalloids lie along the staircase B–Si–Ge–As–Sb–Te; do not call every nonmetal a metalloid. Reactivity of Group 1 increases down the group, while Group 17 reactivity decreases. Noble gases have stable duplet or octet arrangements, not no electrons. In Period 3, Na and Mg are metals, Al is a metal, Si is a metalloid, P, S and Cl are nonmetals, and Ar is a noble gas.