The Periodic Table: IGCSE Chemistry 0620

The Periodic Table is the highest-yield recall topic in 0620: a typical series carries 3-5 multiple choice marks plus a structured question of 4-6 marks predicting the properties of an unfamiliar element. That prediction skill is exactly what examiners say weak candidates lack: they memorise sodium and chlorine but freeze when asked about rubidium or astatine. The table exists so you never have to memorise every element; you learn one trend per group and apply it.

Arrangement of the Periodic Table and trends

Elements are arranged in order of increasing proton (atomic) number. A vertical column is a group; a horizontal row is a period. The two facts that drive every prediction question:

• the group number equals the number of outer-shell electrons (for Groups I to VII)
• elements in the same group have similar chemical properties because they have the same number of outer electrons

Across a period, elements change from metallic on the left to non-metallic on the right. The Core picture stops there. At Supplement level (S), you link the position to ion charge: Group I forms +1 ions, Group II forms +2, Group VI forms −2, Group VII forms −1. The electronic configuration explains the chemistry. That connection back to atomic structure is worth revising as one unit, because Paper 4 routinely asks “explain, using electronic configuration, why sodium and potassium react similarly”.

Group I: the alkali metals

Lithium, sodium and potassium are the three you describe. Core facts: they are soft metals (cut with a knife), have low density (lithium, sodium and potassium float on water) and low melting points compared with typical metals, and melting point decreases down the group. They react with water to give a metal hydroxide and hydrogen:

2Na + 2H2O → 2NaOH + H2

Reactivity increases down the group. Lithium fizzes steadily; sodium melts into a ball and darts across the surface; potassium ignites with a lilac flame. A trend question asks you to predict rubidium: more vigorous still, possibly explosive, and the prediction mark needs a comparative word (“more violently than potassium”).

(S) The explanation: going down the group, the outer electron is further from the nucleus, so it is held less strongly and lost more easily. Losing that one electron is the reaction, so easier loss means faster reaction.

Group VII: the halogens

The halogens are diatomic non-metals: Cl2, Br2, I2. Learn the appearance table cold. It is pure recall and appears almost every series:

Halogen	State at room temperature	Colour
Chlorine	Gas	Pale yellow-green
Bromine	Liquid	Red-brown
Iodine	Solid	Grey-black

Density and melting/boiling point increase down the group; reactivity decreases down the group, the opposite of Group I. The proof is displacement: a more reactive halogen displaces a less reactive one from a solution of its salt. Chlorine bubbled through potassium bromide solution turns it orange because bromine is displaced:

Cl2 + 2KBr → 2KCl + Br2

(S) The explanation mirrors Group I in reverse. Halogens react by gaining one electron. Down the group, the outer shell is further from the nucleus, so the incoming electron is attracted less strongly and gained less easily. Chlorine is more reactive than bromine because it gains an electron more easily.

Transition elements

Between Groups II and III sits the transition block: iron, copper, nickel and friends. The contrast with Group I is the exam angle: transition elements are hard, dense, have high melting points, and form coloured compounds (copper(II) sulfate is blue, iron(III) compounds are red-brown). Two more properties earn marks: they act as catalysts (iron in the Haber process, vanadium(V) oxide in the Contact process) and they form ions with variable oxidation numbers: iron forms both Fe2+ and Fe3+. Naming a specific example converts a vague answer into a scoring one. Their everyday behaviour as structural metals (reactions with acids, extraction, alloying) sits in the metals topic, so treat the two as one revision block.

Noble gases

Group VIII (sometimes written Group 0): helium, neon, argon. They are unreactive monatomic gases because they have a full outer electron shell: no electrons to lose, gain or share. Uses follow from inertness: argon provides an inert atmosphere in lamps, helium fills balloons because it has low density and does not burn. The 1-mark explanation question (“why is argon unreactive?”) is among the most reliably available marks on Paper 1 and 3.

Worked exam question

Astatine (At) is below iodine in Group VII. Predict, with reasons, two physical properties and one chemical property of astatine. [4]

Model answer: Astatine is a solid at room temperature, because melting and boiling points increase down Group VII and iodine is already a solid (1). It is dark-coloured/black, because the halogens get darker down the group (1). Astatine is less reactive than iodine (1) because reactivity decreases down Group VII: its outer shell is further from the nucleus, so it gains an electron less easily (1, Supplement reasoning).

Mark-by-mark: the first two marks are physical predictions tied to a trend: a bare guess (“it is a solid”) without the trend scores only if the trend is stated somewhere. The third mark is the reactivity comparison, and the comparative word is compulsory: “less reactive than iodine”, not “not very reactive”. The fourth mark on the Extended paper is the electron-attraction explanation. Core candidates would face this question without the final explanation mark.

The mistakes that cost marks

1. Mixing up the two reactivity trends. Group I increases down; Group VII decreases down. Candidates who learn one rule and apply it to both groups lose every prediction mark in the question. Anchor it: metals lose electrons (easier when further out), non-metals gain them (harder when further out).
2. Displacement answers without observations. “Chlorine displaces bromine” earns the chemistry mark but the question usually asks what you would see. The solution turns orange. No colour, no observation mark.
3. “Sodium is stored under oil because it is flammable.” The accepted reason is that it reacts with oxygen and water vapour in the air. Precision in the reason matters.
4. Writing halogens as atoms. Cl, Br and I in equations instead of Cl2, Br2, I2 breaks the equation and loses the formula mark. The halogens are diatomic, always.
5. Predicting without comparing. Trend questions are marked on comparatives: “more reactive than”, “higher melting point than”, “darker than”. An absolute statement leaves the examiner unable to award the trend mark.

How to phrase it for full marks

Student wording	Mark-scheme wording
”Potassium reacts really fast"	"Potassium reacts more vigorously than sodium; reactivity increases down Group I"
"The electron is easier to remove"	"The outer electron is further from the nucleus, so it is held less strongly and lost more easily"
"Chlorine is stronger than bromine"	"Chlorine is more reactive than bromine, so it displaces bromine from potassium bromide solution"
"Argon doesn’t react with anything"	"Argon has a full outer electron shell, so it does not lose, gain or share electrons"
"Transition metals are special"	"Transition elements form coloured compounds, act as catalysts and have variable oxidation numbers”

The pattern: state the trend, apply the comparative, and (for Extended) explain with electron shells and nuclear attraction. Three sentences, three marks. That trend-plus-explanation structure is one of the habits we drill in the how to get an A* guide.

The Malaysia note

Malaysian international schools usually teach this topic early in Year 10, alongside atomic structure, and because it feels like memorisation, students park it and never return. By the May/June series of Year 11 the displacement colours and the two opposite reactivity trends have blurred together. Students with SPM-track siblings sometimes borrow the KSSM habit of writing “kumpulan 1” logic in absolute terms; 0620 wants comparatives and trend language. In a free trial lesson our Chemistry specialists typically test this topic first, because ten minutes of questioning shows exactly which trends have survived and which need rebuilding before the exam.

Every sub-topic in The Periodic Table

• Arrangement of the Periodic Table and Trends
• Group I: The Alkali Metals
• Group VII: The Halogens
• Transition Elements
• Noble Gases