IGCSE Chemistry: atomic structure and the Periodic Table

Sub-atomic particles

An atom is neutral because it has equal numbers of protons and electrons. Ions form when electrons are lost (positive ions, cations) or gained (negative ions, anions); the number of protons never changes in a chemical reaction.

Key ideas board by board

Cambridge 0620 Core (topic 2.2) covers the structure of the atom, relative charges and masses, proton number and mass number, and electronic configurations of elements and their ions with proton numbers 1 to 20. Core students must state that noble gases (Group VIII) have a full outer shell, that outer-shell electrons equal the group number in Groups I to VII, and that the number of occupied shells equals the period number. Isotopes are Core (2.3.1), but stating that isotopes have the same chemical properties because they have the same electronic configuration, and calculating relative atomic mass from abundances, are Supplement statements (2.3.3, 2.3.4).

Cambridge topic 8 links structure to the table: Group I metals (lithium, sodium, potassium) get softer, denser and more reactive down the group; Group VII halogens (chlorine, a pale yellow-green gas; bromine, a red-brown liquid; iodine, a grey-black solid) get denser and less reactive down the group; noble gases are unreactive monatomic gases because of their full outer shells. Transition elements have high densities and melting points, form coloured compounds and act as catalysts; Supplement adds variable oxidation numbers such as iron(II) and iron(III).

Edexcel 4CH1 covers the same atomic structure in statements 1.14 to 1.24, including calculating Ar from isotopic abundances for everyone (1.17), and calls the noble gases Group 0 rather than Group VIII. Group 1 and Group 7 trends sit in topic 2 (2.1 to 2.8C). Explaining the reactivity trends in terms of electronic configuration (2.4C for Group 1, 2.8C for Group 7) carries a C reference, so it is examined only in Paper 2.

Worked example 1: counting particles

Question: an aluminium ion is written with mass number 27 and proton number 13, and a 3+ charge. State the number of protons, neutrons and electrons, and the electronic configuration of the ion.

Protons = proton number = 13. Neutrons = mass number minus proton number = 27 - 13 = 14. The atom has 13 electrons; a 3+ ion has lost 3, so it has 10 electrons. Atom configuration 2,8,3; ion configuration 2,8, the same as neon.

For a negative ion, add electrons: a chloride ion (proton number 17, charge 1-) has 17 + 1 = 18 electrons, configuration 2,8,8.

Worked example 2: relative atomic mass from isotopes

Question: chlorine is 75% chlorine-35 and 25% chlorine-37. Calculate the relative atomic mass.

Ar = (35 x 75 + 37 x 25) ÷ 100 = (2625 + 925) ÷ 100 = 3550 ÷ 100 = 35.5.

Check: the answer must lie between the two isotope masses and closer to the more abundant one. 35.5 is between 35 and 37 and nearer 35, so it is sensible. A second example: boron is 20% boron-10 and 80% boron-11, so Ar = (10 x 20 + 11 x 80) ÷ 100 = (200 + 880) ÷ 100 = 10.8.

Common mistakes

• Saying isotopes have different numbers of protons or electrons. Only the neutron number differs.
• Adding the charge to the proton number when counting electrons in a positive ion. A 2+ ion has two fewer electrons, not two more.
• Putting 8 electrons in the first shell, or writing 2,8,9 for potassium. Potassium is 2,8,8,1.
• Explaining Group 1 reactivity as "more shells so more electrons to lose". The mark is for the outer electron being further from the nucleus and more shielded, so it is lost more easily.
• Saying halogens get more reactive down the group because they are bigger. They get less reactive: the outer shell is further from the nucleus, so an electron is gained less easily.
• Giving bromine's colour as brown gas at room temperature. It is a red-brown liquid.

Exam technique and mark-scheme language

Definitions are worth learning word for word: "isotopes are atoms of the same element with the same number of protons but different numbers of neutrons". A definition that says "different mass" without mentioning neutrons usually loses the mark. For "explain why elements in the same group react similarly", the answer is that they have the same number of electrons in their outer shell.

For displacement questions, a more reactive halogen displaces a less reactive one from its halide: chlorine + potassium bromide -> potassium chloride + bromine, Cl2 + 2KBr -> 2KCl + Br2. The observation is a colourless solution turning orange (bromine in water) or brown (iodine). Write the observation as a colour change, not "bromine is formed".

"Predict" questions on other Group I or VII elements want you to continue the trend with data: if the melting points of Li, Na and K are 181 °C, 98 °C and 63 °C, rubidium will be lower than 63 °C.

How one-to-one lessons help

This topic is the foundation for bonding, moles and electrolysis, and small slips here (wrong electron counts, mixed-up group names) spread into everything later. A tutor runs quick-fire configuration and particle-counting drills on the shared whiteboard, then sets trend-prediction and isotope questions from past papers for the student's board, marking each against the exact wording examiners accept.

Self-check

1. State the position, relative charge and relative mass of protons, neutrons and electrons.
2. Work out protons, neutrons and electrons for any atom or ion from its symbol.
3. Write the electronic configuration of any element from 1 to 20 and its ion.
4. Define isotopes and explain why isotopes react the same way.
5. Calculate Ar from percentage abundances.
6. Describe and explain the trends in Group I (1) and Group VII (7).
7. Write a halogen displacement equation and the colour change.