Atoms, Elements and Compounds: IGCSE Chemistry 0620

Topic 2 carries more 0620 marks than any other single topic because bonding and structure questions appear on every paper, every series: 4-6 multiple choice questions, a structured question of 5-8 marks on Paper 3 or 4, and dot-and-cross diagrams that are pure free marks once drilled. Examiner reports flag the same failure each year: students say sodium “wants” to lose an electron, or that ionic compounds are “strong”, instead of naming electrostatic attraction between oppositely charged ions. Vague bonding language is the single biggest mark-loser here.

Elements, compounds and mixtures (Core)

Three definitions, tested as 1-mark questions and as MCQ diagrams:

• An element contains only one type of atom (e.g. O2: one type of atom, even though the atoms are joined in pairs).
• A compound contains two or more elements chemically combined in fixed proportions (e.g. H2O, CO2).
• A mixture contains two or more substances not chemically combined, so the proportions can vary and the components keep their own properties.

The MCQ trap is the particle diagram: a box of identical two-atom molecules is an element; a box with two different molecule types is a mixture of compounds. Read the atoms, not the impression. Mixtures can be separated by physical methods (filtration, distillation, chromatography), which is where this topic hands over to experimental techniques.

Atomic structure and the Periodic Table (Core)

Every atom is built from three subatomic particles, and the table below is asked directly:

Particle	Relative mass	Relative charge	Location
Proton	1	+1	Nucleus
Neutron	1	0	Nucleus
Electron	1/1840 (negligible)	−1	Shells around nucleus

Proton number (atomic number) = number of protons = number of electrons in a neutral atom. Nucleon number (mass number) = protons + neutrons. So neutrons = nucleon number − proton number. For sodium-23: 11 protons, 11 electrons, 12 neutrons.

Electronic configuration fills shells 2, 8, 8 up to calcium. Two patterns earn quick marks: the number of outer-shell electrons equals the group number, and the number of occupied shells equals the period number. Elements in the same group have the same number of outer electrons, which is why they react in the same way, the link into the Periodic Table. Group VIII (noble gases) atoms have full outer shells, which is why they are unreactive.

Isotopes (Core, with Supplement reasoning)

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Chlorine-35 and chlorine-37 both have 17 protons; one has 18 neutrons, the other 20.

(S) Two extensions for Extended candidates. First, isotopes have the same chemical properties because they have the same number of electrons, and electrons control chemistry. Second, relative atomic mass is the weighted average mass of the isotopes, which is why chlorine’s Ar is 35.5, and why Ar values are not always whole numbers. Calculating Ar from isotope abundances belongs to stoichiometry.

Ions and ionic bonds (Core, with Supplement structure)

Metals lose their outer electrons to form positive ions; non-metals gain electrons to form negative ions. Both end up with the electronic configuration of a noble gas. Sodium (2,8,1) loses one electron to form Na+ (2,8); chlorine (2,8,7) gains one to form Cl− (2,8,8).

The ionic bond itself is the strong electrostatic attraction between oppositely charged ions. That exact phrase is the mark. “Sodium gives chlorine an electron and they stick together” is not.

Dot-and-cross diagrams for ionic compounds need three things: square brackets, the charge outside the bracket, and the correct electron count in the outer shell (8 for Cl−, empty outer shell shown as full inner for Na+, so draw the 2,8 shells).

(S) Ionic compounds form giant lattices: a regular alternating arrangement of positive and negative ions. The lattice explains the properties. High melting point: strong electrostatic attractions between oppositely charged ions need a lot of energy to overcome. Conducts when molten or dissolved but not when solid: ions are free to move only when the lattice breaks down. Note the charge carriers: ions, never electrons.

Simple molecules and covalent bonds (Core, with Supplement)

A covalent bond is a shared pair of electrons between two non-metal atoms. Core candidates draw dot-and-cross diagrams for H2, Cl2, H2O, CH4, NH3 and HCl. (S) Extended adds molecules with double and triple bonds: O2, CO2, N2, C2H4, plus CH3OH.

Simple molecular substances have low melting and boiling points. The reason is the most reliably fluffed explanation in the whole syllabus: the weak attractive forces between molecules (intermolecular forces) need little energy to overcome. The covalent bonds inside the molecules do not break when water boils. Write “weak covalent bonds” and the mark is gone. Covalent bonds are strong; the forces between molecules are weak. Simple molecules also do not conduct electricity because there are no free ions or electrons.

Giant covalent structures (S)

Three structures, all Supplement: diamond, graphite and silicon(IV) oxide.

Diamond: each carbon atom bonds covalently to four others in a tetrahedral giant structure. Hard, very high melting point: a giant network of strong covalent bonds must be broken. Used for cutting tools. Does not conduct: all four outer electrons are in bonds.

Graphite: each carbon bonds to three others in flat hexagonal layers. The layers are held by weak forces, so they slide over each other. That makes graphite soft and a good lubricant. The fourth electron per atom is delocalised between the layers, so graphite conducts electricity and works as electrodes.

Silicon(IV) oxide has a diamond-like giant covalent structure (each Si bonded to four O, each O to two Si) and similar properties: hard, high melting point, non-conductor.

Diamond and graphite are the exam’s favourite “explain the difference in properties using structure and bonding” question, usually 4-6 marks, and a regular candidate for the 6-mark extended response, where structure-then-property sentence pairs score fastest.

Metallic bonding (S)

A metal is a lattice of positive ions in a “sea” of delocalised electrons. The metallic bond is the electrostatic attraction between the positive ions and the delocalised electrons. Two properties follow: metals conduct electricity because the delocalised electrons are free to move and carry charge, and metals are malleable because layers of ions can slide over each other without breaking the structure.

Worked exam question

Magnesium reacts with oxygen to form magnesium oxide, an ionic compound. Describe, in terms of electrons, what happens when magnesium reacts with oxygen, and explain why magnesium oxide has a high melting point. [4]

Model answer: Each magnesium atom loses two electrons (1) to form Mg2+ ions; each oxygen atom gains two electrons to form O2− ions (1). Magnesium oxide has a giant ionic lattice with strong electrostatic attraction between the oppositely charged ions (1), which requires a large amount of energy to overcome (1).

Mark-by-mark: mark 1 is electron loss by magnesium with the number two stated. Mark 2 is gain by oxygen forming correctly charged ions. Writing Mg+ or O− loses it. Mark 3 needs “electrostatic attraction between oppositely charged ions”; “strong bonds” alone is not enough. Mark 4 links that attraction to the energy needed to melt. Four sentences, four marks, no spare words.

The mistakes that cost marks

1. “Atoms want a full outer shell.” Atoms have no desires. Write what happens (electrons are transferred or shared), not why an atom “wants” it. Examiner reports reject anthropomorphic answers.
2. Calling intermolecular forces “weak covalent bonds”. Covalent bonds are strong. Low boiling points of molecular substances come from weak forces between molecules.
3. Saying ionic compounds conduct because “electrons move”. When molten or aqueous, the mobile charge carriers are ions. Electrons answer only works for metals and graphite.
4. Dot-and-cross diagrams missing brackets and charges on ions, or showing the transferred electron still on the metal. Check: Na+ shows 2,8, with the bracket and + charge.
5. Mixing up proton number and nucleon number, then computing neutrons wrongly. Neutrons = nucleon number − proton number, every time.

How to phrase it for full marks

Student wording	Mark-scheme wording
”Sodium wants to give away its electron"	"Each sodium atom loses one electron to form Na+"
"Ionic compounds have strong bonds"	"Strong electrostatic attraction between oppositely charged ions"
"Covalent bonds are weak so it boils easily"	"Weak attractive forces between molecules need little energy to overcome"
"Graphite has free electrons"	"One electron per carbon atom is delocalised and free to move, carrying charge"
"Isotopes are the same element but different"	"Same number of protons, different number of neutrons”

The habit to build: name the particle (atom, ion, electron, molecule), name the force, then state the consequence. Answers built that way match the mark scheme line by line.

The Malaysia note

International schools in Malaysia teach this topic early in Year 10, then build everything else on top of it, so by the May/June or Oct/Nov exam it feels familiar but the precise definitions have blurred. Students switching from KSSM Kimia also meet a vocabulary shift: “ikatan ion” maps cleanly, but the 0620 mark scheme demands the full “electrostatic attraction between oppositely charged ions” phrase that the Malaysian syllabus does not insist on. Bonding is the first topic our tutors audit in a free 1-hour trial lesson, because a student who phrases bonding correctly usually picks up 5-8 extra marks across Papers 2 and 4 with no new content learned.

Every sub-topic in Atoms, Elements and Compounds

• Elements, Compounds and Mixtures
• Atomic Structure and the Periodic Table
• Isotopes
• Ions and Ionic Bonds
• Simple Molecules and Covalent Bonds
• Giant Covalent Structures (Supplement)
• Metallic Bonding (Supplement)