Periodic Table — order by proton number
The Periodic Table lists every known element in order of increasing proton (atomic) number: hydrogen (1 proton) first, helium (2 protons) second. A period is a horizontal row — every element in it has the same number of occupied electron shells (Period 3, sodium to argon, has three). A group is a vertical column, numbered I–VII with the noble gases as Group 0/VIII — every element in it has the same number of outer-shell (valence) electrons.
Group number = outer-shell electrons
Elements sit in the same group because they have the same number of outer-shell (valence) electrons; for main Groups I–VII the group number gives that count directly. Chemical properties are governed mainly by the outer-shell arrangement (the bonding electrons) — so same-group elements react in similar ways and form ions of the same charge. Hence a shared formula pattern: every Group I metal chloride is , every Group II chloride is .
Across a period — metallic to non-metallic
Across a period, proton number rises and outer-shell electrons increase by one each step, while the shell count stays the same. Elements on the left (few outer electrons) readily lose them, forming positive ions — metallic. Those on the right readily gain or share electrons to fill the outer shell — non-metallic. Border-zone elements are intermediate (metalloid). The trend resets at the next period, where a new empty outer shell begins.
Drawn from real examiner reports.
2,8,7 means 7 outer electrons, not 17
In a comma-separated electronic structure only the LAST number is the outer-shell electron count. For 2,8,7 that is 7 — the earlier figures describe full inner shells. Adding them (2+8+7 = 17) gives chlorine's total electron/proton number, not its outer-shell count. Claim "17 outer electrons" and the whole group-similarity explanation collapses.
Flagged Jun 2022 P31 Q11ai — electronic structure (e.g. 2,8,1) misread as the total electron/proton number
Period = row, group = column
A period is a horizontal ROW: its elements share the same number of electron SHELLS. A group is a vertical COLUMN: its elements share the same number of OUTER-SHELL ELECTRONS. Swap the two and you reverse which property the elements being compared actually have in common, so an otherwise sound explanation is attached to the wrong pattern.
Ordered by proton number, not mass
Asked to describe the arrangement, candidates write "elements in order of mass" or simply "grouped by similar properties". The ordering principle is increasing proton (atomic) number, not relative atomic mass — and a full description also needs the period (rows, same shells) and group (columns, same outer electrons) distinction to meet a 3-mark tariff.
"Same electrons" misses the mark
Explaining why two elements behave alike, "they have the same number of electrons" earns nothing — no two elements do. The credit is for the same number of OUTER-SHELL electrons. Examiners record this exact omission where shared chemical properties are justified as "same protons/neutrons/electrons" with the outer-shell point left out.
Flagged Jun 2022 P42 Q8bi; Jun 2023 P42 Q8b; Nov 2023 P42 Q2aiii — recorded in the digest's C2 isotopes entry, the same explanatory move this leaf requires
Balance charges, not ion counts
Building a formula from group numbers, match total positive charge to total negative charge — not the number of ions. Group III needs THREE , giving ; "AlCl" pairs the ions but not the charge. The reverse slip is over-subscripting a pair that already balances: with is , never "Li2Cl2".
Describing when asked to explain
C8.1 sets a "describe" and an "explain" statement side by side. "Elements become less metallic across the period" or "they are all reactive non-metals" is a description. An "explain" question wants the reason: the same number of outer-shell electrons, and that properties depend on outer-shell arrangement. True chemistry aimed at the wrong command word scores nothing.
(Extended) Trend direction flips mid-answer
Given a table of a property against proton number, candidates often state the trend correctly in part (a) and then predict the opposite direction for the next element in part (b). A prediction must CONTINUE the pattern just described. The reasoning mark needs that pattern too — "because it is further down the group", with no reference to the observed values, does not earn it.
The four-step group-similarity answer
(1) They are in the same group. (2) So they have the same number of outer-shell electrons — the group number, for main Groups I–VII. (3) Chemical properties are determined mainly by the outer-shell arrangement. (4) So they react in similar ways and form ions of the same charge.
Answer describe and explain separately
For "describe the change across a period", give the pattern only: metallic on the left to non-metallic on the right as proton number increases. Save the electron-based reason for the follow-on "explain" part. Repeating one point under both wastes time and earns neither mark.
(Extended) Reading a group data table
Take the values in the order given (increasing proton number) and check each is consistently higher or lower than the last. State the DIRECTION explicitly ("decreases down the group"), not single values. To predict the next member, continue that pattern and cite it as the reason.
Cambridge 0654 spec reference: Section C8 "The Periodic Table", sub-topic C8.1 "Arrangement of elements". Core: describe the Periodic Table as elements arranged in order of increasing proton (atomic) number, in periods and groups; describe the change from metallic to non-metallic character across a period; explain why elements in the same group have similar chemical properties in terms of their electronic configuration. Extended (Supplement): identify trends in a group given supplied information about its elements.
| Term | Mark-scheme-precise meaning |
|---|---|
| Periodic Table | A list of all known elements arranged in order of increasing proton (atomic) number |
| Period | A horizontal row of the Periodic Table; every element in a period has the same number of electron shells |
| Group | A vertical column of the Periodic Table (numbered I-VII, plus Group 0/VIII for the noble gases); every element in a group has the same number of outer-shell (valence) electrons |
| Outer-shell (valence) electron | An electron in the highest occupied energy level (shell) of an atom -- for main Groups I-VII, the group number equals this count |
| Metallic character | The tendency of an atom to lose outer-shell electrons and form a positive ion |
| Non-metallic character | The tendency of an atom to gain (or share) outer-shell electrons and form a negative ion or covalent bond |
How to read an electronic structure. A structure such as (chlorine) lists the number of electrons in each shell, innermost first. Only the LAST number (here, 7) is the outer-shell electron count that determines the group -- the earlier numbers describe full, inner shells and are not part of that count. (See "Don't mix these up" below -- this is the single most common slip in this topic.)
Define how elements are arranged in the Periodic Table.
Lithium and sodium are both in Group I of the Periodic Table. Each has 1 outer-shell electron and readily forms an ion with a 1+ charge.
Use this information to write the formula of lithium chloride (chloride ion, ). (1 mark)
