Arrangement of the Periodic Table and Trends

One mark on nearly every 0620 paper hangs on a single sentence: the Periodic Table is arranged in order of increasing proton number. Students who write “atomic mass” lose it instantly, and the follow-up questions (predicting properties from a group, placing an unknown element) all collapse if the arrangement itself is misunderstood. This page covers the Core ideas that the rest of the Periodic Table topic is built on.

The arrangement: proton number, not mass

The modern Periodic Table lists elements in order of increasing proton number (atomic number). Cambridge expects the exact phrase. The historical clue that mass is the wrong answer sits in Period 3 to 4: argon (relative atomic mass 40) comes before potassium (relative atomic mass 39). Order by mass and they swap; order by proton number (18 then 19) and the table works.

Two definitions carry the structure:

Term	Meaning	What it tells you
Group	Vertical column	Number of outer-shell electrons (Groups I–VII)
Period	Horizontal row	Number of occupied electron shells

Sodium has electronic configuration 2,8,1: one outer electron puts it in Group I, three occupied shells put it in Period 3. Chlorine is 2,8,7: Group VII, Period 3. Given a configuration, you should be able to state group and period in under ten seconds, because Paper 1 and Paper 2 test exactly that.

Why elements in a group behave alike

Elements in the same group have the same number of outer-shell electrons, and outer electrons control chemical reactions. That single sentence is the scoring idea behind every “explain why lithium and sodium react similarly” question. Lithium (2,1) and sodium (2,8,1) both lose one electron to form a 1+ ion, so both react with water the same way. The detailed Group I chemistry is on the alkali metals page.

Group number also predicts ion charge for the main groups:

Group	Outer electrons	Ion formed
I	1	1+
II	2	2+
VI	6	2−
VII	7	1−

Metals lose electrons to form positive ions; non-metals gain electrons to form negative ions.

The trend across a period: metallic to non-metallic

Moving left to right across a period, elements change from metallic to non-metallic character. Period 3 shows it cleanly: sodium, magnesium and aluminium are metals; silicon sits in the middle; phosphorus, sulfur, chlorine and argon are non-metals. The dividing “staircase” runs down the right-hand side of the table.

The chemical consequence appears in the oxides. Metal oxides (Na2O, MgO) are basic; non-metal oxides (SO2, P4O10) are acidic. A question giving you an unknown element whose oxide turns universal indicator red is telling you the element is a non-metal on the right of its period.

Predicting properties of unfamiliar elements

Cambridge loves giving you an element you have never studied (rubidium, astatine, strontium) and asking you to predict its behaviour. The method is always the same:

1. Find its group: same outer electrons, so same type of reactions as the elements above it.
2. Apply the group trend: reactivity increases down Group I, decreases down Group VII; melting point falls down Group I and rises down Group VII.
3. Write the prediction as a comparison: “rubidium reacts more violently with water than potassium because it is lower in Group I.”

A prediction without the comparison word (more, less, faster, lower) reads as a guess and frequently fails to score.

Worked exam question

Element X has the electronic configuration 2,8,8,2. State the group and period of X, and predict the formula of its oxide. Explain your answer to each part. [4]

Model answer: X is in Group II (1) because it has two outer-shell electrons (1). X is in Period 4 because it has four occupied shells (1). X forms a 2+ ion, so its oxide is XO, containing X2+ and O2− (1).

Mark-by-mark: M1 is the bare group; M2 requires the link to outer electrons, so “because it ends in 2” is too vague. M3 needs shells counted, not guessed from position. M4 wants the formula XO with the reasoning that a 2+ ion balances one 2− oxide ion; X2O loses the mark.

The mistakes that cost marks

1. “Arranged by atomic mass.” The arrangement is by increasing proton number. This exact wording is a recurring one-mark question and the most common wrong answer in the topic.
2. Confusing group and period. Group = column = outer electrons; period = row = shells. Students who swap them lose every dependent mark in the question.
3. Predictions without comparisons. “Rubidium is reactive” scores nothing. “Rubidium is more reactive than potassium” scores, because trends are comparative by definition.
4. Forgetting Group VIII has full shells. The group-number-equals-outer-electrons rule covers Groups I to VII. Helium has 2 outer electrons and the other noble gases have 8, all full shells.

How examiners want it phrased

Student wording	Mark-scheme wording
”Elements are in order of size/mass"	"Elements are arranged in order of increasing proton number"
"Same group elements are similar"	"Elements in the same group have the same number of outer-shell electrons, so they have similar chemical properties"
"Sodium is in Period 3 because of where it is"	"Sodium (2,8,1) has three occupied shells, so it is in Period 3"
"It gets less metal-like across"	"Elements change from metallic to non-metallic character across a period”

Every answer in this subtopic reduces to counting protons, shells or outer electrons: state the count, then the conclusion. If configurations still feel slow to read, a free trial lesson with a Chemistry specialist will show you how quickly the table turns into a prediction machine.

Test yourself

Count protons, shells or outer electrons for each answer, then click to check.

Q1 (2 marks). Argon (relative atomic mass 40) comes before potassium (relative atomic mass 39) in the Periodic Table. Explain why.

Show answer

• the table is arranged in order of increasing proton number, not relative atomic mass [1] • argon has 18 protons and potassium has 19, so argon comes first [1]

Q2 (2 marks). Lithium and potassium react with water in a similar way. Explain why, in terms of electrons.

Show answer

• they are in the same group, so both have one outer-shell electron [1] • outer-shell electrons determine chemical properties; both lose that electron to form a 1+ ion [1]

Q3 (2 marks). Element Z is in Group VI. Predict the charge on the ion Z forms, and state whether the oxide of the metal at the far left of Z’s period is acidic or basic.

Show answer

• Z has six outer electrons, so it gains two to form a 2− ion [1] • the element at the far left is a metal, so its oxide is basic [1]