SPEC 4.1.2.1–4.1.2.3 • PERIODIC TABLE
The Periodic Table — Development & Metals vs Non-Metals
Why the table is "periodic", how Mendeleev built it, why his order was sometimes "wrong", and the metal/non-metal divide. Includes the three-part Mendeleev answer that wins the marks every time.
How the Periodic Table is Arranged
The modern periodic table arranges every known element in order of atomic (proton) number — smallest to largest. As atomic number increases, similar chemical properties recur at regular intervals. That's what makes the table "periodic".
Three Rules of Arrangement
Order = atomic (proton) number — not atomic mass
Vertical columns = groups — elements with similar chemical properties
Horizontal rows = periods — properties change gradually across
Mark-Scheme Language
"Arranged in order of atomic number / proton number." Do not write "arranged in order of mass" — that loses the mark every time.
Groups and Periods — Definitions
These are 1-mark definition questions. Both must be learnt word for word.
Group
A vertical column of the periodic table. Elements in the same group have the same number of outer shell electrons — which is why they have similar chemical properties.
Period
A horizontal row of the periodic table. The period number equals the number of occupied electron shells. E.g. Period 2 elements (Li → Ne) all have 2 occupied shells.
Examiner Alert
"Same number of electrons" loses the mark. The mark-scoring phrase is "same number of outer shell electrons".
Group Number = Outer Electrons
The link between group number and outer-shell electrons is the heart of the periodic table. It is what makes properties recur.
GroupOuter electronsExamplesBehaviour
11Li, Na, K…Lose 1 electron → 1+ ion (alkali metals)
22Be, Mg, Ca…Lose 2 electrons → 2+ ion
77F, Cl, Br…Gain 1 electron → 1− ion (halogens)
08 (He = 2)He, Ne, Ar…Already stable → unreactive (noble gases)
Exam Tip
Helium is the exception in Group 0 — it has only 2 outer electrons, which fills its first (and only) shell. Lesson 1.07 covers this trap in detail.
The Periodic Table at a Glance
Groups run down (1, 2, 3 … 7, 0). Periods run across. Metals are found to the left and towards the bottom of the table; non-metals are found towards the right and top.
⚠ In Period 4 of the real table, 10 transition metals (Sc → Zn) sit between Ca and Ga — omitted here for clarity
Metals (left + bottom)
Non-metals (top + right)
Notes on this diagram
Periods 1–3 shown in full; Period 4 shown without the central transition metal block. Hydrogen is plotted in Group 1 by convention but is a non-metal. The periodic table is provided in your exam, so you don't need to memorise positions — just know how to read group/period and the metal/non-metal divide.
Before Mendeleev — Newlands' Octaves (1865)
Several scientists tried to organise the elements before Mendeleev. The most famous earlier attempt was John Newlands in 1865.
Newlands' Idea
Arranged the known elements in order of atomic weight
Noticed that every 8th element had similar properties — he called this the Law of Octaves
Worked for the lighter elements but broke down for heavier elements
Assumed all elements had been discovered — there were no gaps
Rejected by the scientific community at the time
Newlands' work matters because it set up the central problem Mendeleev solved a few years later: how to handle elements that didn't fit, and elements that hadn't yet been discovered.
Mendeleev's Periodic Table (1869)
Mendeleev's table was a step change. Three decisions made it work:
1. Broke strict atomic-weight order
He placed elements where their chemical properties fit the group, even if that meant the atomic weights looked slightly out of order. He trusted chemistry over weight.
2. Left gaps for undiscovered elements
Instead of forcing all known elements to fit into a continuous sequence, he left blank spaces where he believed elements existed but had not yet been found.
3. Predicted the properties of those missing elements
Using the patterns of elements above, below, and beside the gap, he predicted what the missing elements' properties would be — density, melting point, formula of oxides, and so on.
The Gallium Test (1875)
The table needed a real-world test. It came six years after Mendeleev published.
Mendeleev's prediction (1871)
A missing element below aluminium ("eka-aluminium"): relative atomic mass ≈ 68, density ≈ 5.9 g/cm³, low melting point, oxide formula M₂O₃.
Gallium discovered (1875)
Relative atomic mass ≈ 69.7, density ≈ 5.91 g/cm³, melts in your hand at ~30 °C, oxide formula Ga₂O₃. Predictions matched.
Why this validated the table
If a table can predict properties of elements that haven't been found yet — and those predictions turn out to be right — the underlying organising principle must be real. Scandium (1879) and germanium (1886) were also predicted and found, building the case further.
The 3-Part Mendeleev Answer
For "explain why Mendeleev's table was accepted" or "give two reasons why the discovery of gallium helped Mendeleev's table become accepted", the mark scheme always wants the same structure.
PPoint: Mendeleev left a gap for an undiscovered element.
EEvidence: He predicted the properties of that missing element from the trends in the table.
EExplain: When the element (e.g. gallium) was later discovered, its actual properties matched the predictions — validating the table.
Marks Logic (P1 Nov21 Q02.5 — 2 marks)
1 mark for "gallium fitted in a gap Mendeleev had left". 1 mark for "gallium's properties were predicted correctly" (the mark scheme also accepts "matched the rest of the group"). One sentence each.
Examiner Alert — The Mendeleev Trap
Examiner Alert (P1 Nov21 Q02.5)
The most common mistake on this question is writing "Mendeleev left gaps" and stopping there. That alone does not score. The mark needs both the gap and the matched prediction.
✗ Loses Mark
"Mendeleev left gaps in his periodic table."
✔ Wins Mark
"Mendeleev left a gap for gallium and predicted its properties; when gallium was later discovered, its properties matched the predictions."
Second Trap
The mark scheme accepts "properties matched the rest of the group". It does not accept "fits the properties of adjacent elements" — adjacent elements are in different groups, and groups are what determine properties.
Why Mendeleev's Order Was Sometimes "Wrong"
Mendeleev arranged elements by atomic weight. Sometimes that gave the wrong order. The classic example is tellurium (Te) and iodine (I).
ElementProtonsAverage A_rOrder by atomic weightOrder by chemistry
Tellurium (Te)52127.62nd (heavier)1st (Group 6 — like O, S, Se)
Iodine (I)53126.91st (lighter)2nd (Group 7 — like F, Cl, Br)
Strict atomic-weight order would put iodine before tellurium. But iodine clearly belongs with the halogens (Group 7) and tellurium with the Group 6 elements. Mendeleev placed them by chemistry, not weight.
Note on values
The A_r values shown are modern values (provided on the AQA periodic table in the exam). Mendeleev had less precise values, but the Te/I pair was already inverted in his day — the chemistry / atomic-weight conflict was real even then.
Isotopes Explained the Anomaly
Mendeleev couldn't explain why mass order broke down. The discovery of isotopes (and protons) decades later did.
The Resolution
The real ordering principle is proton number, not atomic weight
Tellurium has more high-mass isotopes (notably Te-128 and Te-130) than iodine, so its average A_r is higher even though it has fewer protons
Once protons (and isotopes) were understood, the modern table ordered everything by atomic number — and the tellurium/iodine "problem" disappeared
Synthesis
Mendeleev got the chemistry right by trusting the patterns. The science underneath caught up later: isotopes explained why his exceptions were necessary, and proton number became the official organising principle.
Metals vs Non-Metals — The Mark-Scheme Definitions
This is the cleanest definition pair on the spec. AQA wants one specific phrase for each.
Metal
An element that reacts to form positive ions.
Non-metal
An element that does not form positive ions — instead it forms negative ions or shares electrons.
Examiner Alert
The chemical definition is about ion formation, not physical properties. "Metals are shiny" is a property — not the definition. If the question asks "why is X a metal?" the answer is "because it forms positive ions".
Metal vs Non-Metal Properties
Beyond the definition, you need the typical physical and chemical properties.
FeatureMetalsNon-metals
PositionLeft and towards the bottomTowards the right and top
Ion typePositive ions (lose electrons)Negative ions, or share electrons
AppearanceShiny (lustrous)Dull
ConductivityGood conductors of heat and electricityGenerally poor conductors (insulators)
MechanicalMalleable, ductileBrittle if solid
State at 20 °CSolid (except mercury)Mix of solids, liquids, gases
Proportion of elementsMajority (~75%+)Minority
Application Tip
If AQA gives you a mystery element with properties (e.g. dull, low melting point, doesn't conduct), match each property to the column. Several "non-metal" properties → element is in the right and top region of the table.
Practice Question 1
two
2 MARKS
Source: P1 Nov21 Q02.5
P
Reason 1: Gallium fitted into a gap that Mendeleev had left in the periodic table for an undiscovered element. [1 mark]
E
Reason 2: The properties of gallium that were measured when it was discovered matched the properties Mendeleev had predicted (the mark scheme also accepts "matched the rest of the group"). [1 mark]
E
Why this scores: Both elements of the scoring pattern — the gap and the matched prediction — appear. "He left gaps" alone does not score.
Practice Question 2
Element Q is a dull solid with a melting point of 44 °C. It does not conduct electricity. Which section of the periodic table is most likely to contain element Q? Explain your answer using two pieces of evidence from the description.3 MARKS
P
Section: Element Q is on the right side of the periodic table — the non-metal region (towards the right and top). [1 mark]
E
Evidence 1: Q is dull — non-metals are dull, whereas metals are shiny. [1 mark]
E
Evidence 2: Q does not conduct electricity — non-metals are generally poor conductors / insulators, whereas metals conduct electricity. (Low melting point also supports non-metal.) [1 mark]
P
Most likely identity: White phosphorus (P, Group 5, Period 3) — melts at ~44 °C, dull non-metal solid. Identifying the non-metal region is what scores; naming P specifically is not required.
Common Errors — Quick Check
If a sentence on your answer matches the left column, swap it for the right one before you move on.
Loses the markWins the mark
"Arranged in order of (atomic) mass / atomic weight""Arranged in order of atomic / proton number "
"Periodic because patterns""Patterns recur at regular intervals as atomic number increases"
"Same number of electrons""Same number of outer shell electrons "
"Mendeleev left gaps""Left a gap, predicted properties, predictions matched when element was discovered"
"Properties match adjacent elements""Properties match the rest of the group "
"Metals are shiny" (as the definition)"Metals are elements that form positive ions " (definition); shininess is a property
Summary
Arrangement (4.1.2.1)
Atomic (proton) number = ordering principle
Group = vertical column = same outer-shell electrons
Period = horizontal row = same number of occupied shells
"Periodic" = similar properties recur at regular intervals
Development (4.1.2.2)
Newlands (1865): octaves — broke down for heavier elements
Mendeleev (1869): broke strict atomic-weight order; left gaps ; predicted properties
Gallium discovery (1875) matched predictions → table validated
Tellurium/iodine inversion explained later by isotopes — proton number is the real principle
Metals vs Non-metals (4.1.2.3)
Metal: reacts to form positive ions (left + towards bottom of table)
Non-metal: does not form positive ions — gains or shares electrons (towards right + top)
Most elements are metals
Top Exam Tip
The Mendeleev question always wants the three-part answer: (1) gap, (2) prediction, (3) matched on discovery. Memorise it as one sentence and write it out the same way every time.
Next: How the Periodic Table is Arranged