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2.5 Explaining Trends

GCSE Chemistry · AQA · Chapter 2: The Periodic Table

A free GCSE Chemistry lesson for AQA (8462) from OBA School: using electronic structure to explain why reactivity increases down Group 1 but decreases down Group 7. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain how Group 1 and Group 7 elements react by losing or gaining electrons to get a stable arrangement
  • Explain why reactivity increases going down Group 1
  • Explain why reactivity decreases going down Group 7
  • Use shielding and the attraction of the nucleus to explain the trends (Higher tier)
  • Describe oxidation and reduction in terms of electrons and write half equations (Higher tier)

Key words

  • Stable arrangement: A full outer shell, like a noble gas. Atoms lose, gain or share electrons to get one.
  • Nucleus: The positively charged centre of an atom. It attracts the negative electrons.
  • Shielding: (Higher tier) Inner shells of electrons reduce the attraction between the nucleus and the outer electrons.
  • Oxidation: (Higher tier) Loss of electrons.
  • Reduction: (Higher tier) Gain of electrons.
  • Half equation: (Higher tier) An equation showing what happens to one substance, with electrons shown as e−.

Losing and gaining electrons

Atoms react to get a stable arrangement of electrons (a full outer shell). Group 1 atoms have one outer electron, so they lose one electron to form 1+ ions. Group 7 atoms have seven outer electrons, so they gain one electron to form 1− ions. Group 0 atoms already have a full outer shell, so they hardly react at all.

Key fact: The easier it is for a Group 1 atom to lose its outer electron, the more reactive it is. The easier it is for a Group 7 atom to gain an electron, the more reactive it is.

Why reactivity increases down Group 1

  • going down the group, each atom has one more occupied shell, so the outer electron is further from the nucleus
  • (Higher tier) there are more inner shells between the nucleus and the outer electron, so there is more shielding
  • so the attraction between the positive nucleus and the outer electron is weaker
  • so the outer electron is lost more easily, and the metal is more reactive

Exam tip: The nucleus does get more protons down the group, but the extra distance and shielding win. Always say what happens to the attraction and then to how easily the electron is lost.

Worked example: radius data

The atomic radius of sodium is 186 pm and of potassium is 227 pm. Calculate the percentage increase.
(227 − 186) ÷ 186 × 100 = 41 ÷ 186 × 100 = 22.04…

Answer: 22% larger, so the outer electron in potassium is further from the nucleus

Why reactivity decreases down Group 7

  • going down the group, the outer shell is further from the nucleus
  • (Higher tier) there is more shielding by inner shells
  • so the attraction between the nucleus and an electron joining the outer shell is weaker
  • so it is harder to gain an electron, and the halogen is less reactive

Worked example: explaining a displacement

Why does chlorine displace bromine from potassium bromide solution?
A chlorine atom is smaller than a bromine atom. Its outer shell is closer to the nucleus, so it attracts an extra electron more strongly.
So chlorine molecules take electrons from bromide ions: chlorine forms chloride ions and bromine is released.

Answer: chlorine gains electrons more easily than bromine, so it is more reactive

Three factors, any group

How strongly the nucleus attracts electrons in the outer shell depends on three factors. Use all three in a full answer.

  • distance: going down a group the outer shell is further from the nucleus → weaker attraction
  • shielding (Higher tier): more inner shells sit between the nucleus and the outer shell → weaker attraction
  • nuclear charge: more protons in the nucleus → stronger attraction, but this is outweighed by the other two

Worked example: applying the idea to other groups

Group 2 (metals): magnesium is 2,8,2 and calcium is 2,8,8,2. Calcium’s outer electrons are further from the nucleus and more shielded, so they are lost more easily.
Group 6 (non-metals): oxygen is 2,6 and sulfur is 2,8,6. Oxygen’s outer shell is closer to the nucleus with less shielding, so it attracts two extra electrons more strongly.

Answer: calcium is more reactive than magnesium; oxygen is more reactive than sulfur

Exam tip: Metals: bigger atom → electrons lost more easily → more reactive. Non-metals: bigger atom → electrons gained less easily → less reactive.

Oxidation and reduction in terms of electrons (Higher tier)

Oxidation is loss of electrons and reduction is gain of electrons (remember OIL RIG). We show these changes with half equations.

  • sodium atom → sodium ion; Half equation Na → Na⁺ + e−; Type oxidation (loss)
  • chlorine molecule → chloride ions; Half equation Cl₂ + 2e− → 2Cl−; Type reduction (gain)
  • iodide ions → iodine; Half equation 2I− → I₂ + 2e−; Type oxidation (loss)

When sodium burns in chlorine, sodium atoms are oxidised and chlorine is reduced. In the reaction 2Na + Cl₂ → 2NaCl, two electrons move from two sodium atoms to one chlorine molecule.

Common mistakes

  • Saying “potassium has more electrons so it loses them more easily”. Explain using distance from the nucleus (and shielding) and the attraction.
  • Using the same explanation direction for both groups. Bigger atoms lose electrons more easily (Group 1 more reactive) but gain electrons less easily (Group 7 less reactive).
  • Saying atoms “want” a full shell. Say atoms react to gain a stable arrangement.
  • Mixing up oxidation and reduction. Oxidation Is Loss, Reduction Is Gain (of electrons).
  • Writing half equations with electrons on the wrong side: in oxidation the electrons are products.

Revision questions

Try these, then check your answers below each one.

Question 1. Why is potassium more reactive than sodium? (Choose from: Its outer electron is further from the nucleus, so it is less strongly attracted and is lost more easily, It has more protons, so it attracts its outer electron more strongly, It has more outer electrons to lose, Its outer electron is closer to the nucleus, so it is lost more easily.)

Answer: Its outer electron is further from the nucleus, so it is less strongly attracted and is lost more easily

Working: Potassium has an extra shell. Its outer electron is further from the nucleus (and more shielded), so the attraction is weaker and it is lost more easily.

Question 2. Why is chlorine more reactive than bromine? (Choose from: Its outer shell is closer to the nucleus, so an extra electron is attracted more strongly, It has more electrons in its outer shell, Its outer shell is further from the nucleus, so electrons are gained more easily, It has a full outer shell.)

Answer: Its outer shell is closer to the nucleus, so an extra electron is attracted more strongly

Working: Chlorine atoms are smaller. The outer shell is closer to the nucleus (less shielding), so an incoming electron is attracted more strongly and gained more easily.

Question 3. Write the electronic structure of a sodium ion, Na⁺ (for example 2,8,1).

Answer: 2,8

Working: Na (2,8,1) loses its outer electron to form Na⁺ with structure 2,8 – a full outer shell.

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