Common mistakes: The Periodic Table of Elements

Most marks in this chapter are lost in a few predictable ways: stating a trend without its reason, mixing up which direction reactivity runs in Group 1 versus Group 17, and confusing physical facts with chemical reasons. Work through each mistake below, then check your own answers for the same slips.

4.1 Development of the periodic table

Mistake 1, Saying the modern table is arranged by atomic mass

• What students write: “The modern periodic table is arranged by increasing atomic mass.”
• Why it loses marks: that was Mendeleev’s basis; the modern table follows Moseley.
• The correct version: the modern periodic table is arranged by increasing proton number (atomic number), following Moseley’s work.

Mistake 2, Calling Mendeleev’s gaps mistakes

• What students write: “Mendeleev left gaps because he made errors.”
• Why it loses marks: it reverses the meaning of his greatest achievement.
• The correct version: the gaps were left deliberately for elements not yet discovered, and Mendeleev predicted their properties, later confirmed.

4.2 Groups and periods

Mistake 3, Reading the group from the total electrons

• What students write: for an atom with 2.8.1, “it is in Group 11 because it has 11 electrons.”
• Why it loses marks: the group number comes from the valence electrons, not the total.
• The correct version: the group number equals the number of valence electrons (here 1, so Group 1); the period equals the number of shells (here 3).

Mistake 4, Saying shells increase across a period

• What students write: “Across a period, the number of shells increases.”
• Why it loses marks: it confuses the across-a-period trend with the down-a-group trend.
• The correct version: across a period the number of shells stays the same; only the nuclear charge and the number of valence electrons increase.

Mistake 5, Getting the atomic-size trend backwards

• What students write: “Atomic size increases across a period because there are more electrons.”
• Why it loses marks: the size actually decreases, and the reason is missing.
• The correct version: across a period the atomic size decreases, because the increasing nuclear charge pulls the same shells closer to the nucleus.

4.3 Group 1: alkali metals

Mistake 6, Stating the reactivity trend with no reason

• What students write: “Reactivity increases down Group 1.”
• Why it loses marks: the trend alone is one point; the explanation carries the rest.
• The correct version: reactivity increases down Group 1 because the atom is larger, the valence electron is further from the nucleus and held less tightly, so it is lost more easily.

Mistake 7, Saying the gas produced with water is oxygen

• What students write: “Sodium reacts with water to give sodium hydroxide and oxygen.”
• Why it loses marks: the wrong gas is named.
• The correct version: a Group 1 metal reacts with water to give a metal hydroxide and hydrogen gas.

4.4 Group 17: halogens

Mistake 8, Using the Group 1 rule for halogens

• What students write: “Reactivity increases down Group 17, like Group 1.”
• Why it loses marks: the direction is reversed for halogens.
• The correct version: reactivity decreases down Group 17, because the larger atom attracts an incoming electron less strongly, so it gains an electron less easily.

Mistake 9, Predicting no displacement

• What students write: “Adding chlorine to potassium bromide gives no change.”
• Why it loses marks: a more reactive halogen does react.
• The correct version: chlorine displaces bromine (chlorine is more reactive), the solution turns brown, and potassium chloride and bromine form.

Mistake 10, Muddling halogen colours and states

• What students write: “Bromine is a green gas and iodine is a brown liquid.”
• Why it loses marks: the physical descriptions are wrong.
• The correct version: chlorine is a greenish-yellow gas, bromine a reddish-brown liquid, and iodine a purplish-black solid; melting and boiling points rise down the group.

Mistake 11, Writing Group 7 instead of Group 17

• What students write: “The halogens are in Group 7.”
• Why it loses marks: the current syllabus uses the 1–18 numbering.
• The correct version: the halogens are in Group 17 of the modern periodic table.

4.5 Group 18: noble gases

Mistake 12, Giving “it is a gas” as the reason for inertness

• What students write: “Noble gases are unreactive because they are gases.”
• Why it loses marks: that is a physical fact, not a chemical reason.
• The correct version: noble gases are inert because they have a stable, full valence shell (helium a duplet of 2, the rest an octet of 8), so they have no tendency to lose, gain or share electrons.

4.6 Period 3 and transition elements

Mistake 13, Naming a transition-metal colour without the property

• What students write: “Copper sulfate is blue.”
• Why it loses marks: the question asks for the property, not just an observation.
• The correct version: transition elements form coloured compounds; they also show variable oxidation numbers, act as catalysts, and form complex ions, name the property the observation illustrates.

Mistake 14, Getting the Period 3 oxide trend backwards

• What students write: “Across Period 3 the oxides change from acidic to basic.”
• Why it loses marks: the direction is reversed.
• The correct version: across Period 3 the oxides change from basic (sodium, magnesium) through amphoteric (aluminium) to acidic (phosphorus, sulfur).

Mistake 15, Treating transition metals as very reactive like alkali metals

• What students write: “Transition elements are very reactive because they are metals.”
• Why it loses marks: it confuses alkali-metal behaviour with transition-metal behaviour.
• The correct version: transition elements are hard, strong, less reactive metals with high melting points and densities, very different from the soft, highly reactive alkali metals.

Mistake 16, Predicting a property without first stating the position

• What students write: jumping straight to “it is reactive” without mentioning the group, shells or valence electrons.
• Why it loses marks: the examiner awards the position-to-prediction reasoning, not just the final answer.
• The correct version: state the position (group and period), then the structure (shells and valence electrons), then the prediction, in that order.

Fixing these for good

Almost every mistake here is cured by one habit: never state a trend without the electron-arrangement reason behind it, and always check whether an element loses an electron (Group 1, reactivity down) or gains one (Group 17, reactivity up). A one-to-one teacher can mark a set of your trend answers and show you exactly where the reason is missing, the single biggest source of dropped marks in this chapter of the SPM Chemistry written papers.