IGCSE Chemistry 0620 · Topic 2: Atoms, Elements and Compounds
Atoms, Elements and Compounds
Every atom is built from three particles, and it’s the number of protons that decides which element it is. Once you know electron arrangement, you can predict how atoms bond — by transferring electrons (ionic) or sharing them (covalent).
1. Atomic Structure
Particle
Relative charge
Relative mass
Location
Proton
+1
1
Nucleus
Neutron
0
1
Nucleus
Electron
−1
very small (~1/2000)
Orbiting the nucleus in shells
Proton (atomic) number = number of protons
Nucleon (mass) number = protons + neutrons
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Worked example — finding subatomic particles
Magnesium has proton number 12 and nucleon number 24. Find the number of protons, neutrons, and electrons.
Protons = 12 (equal to proton number). Neutrons = 24−12 = 12. Electrons = 12 (equal to protons, in a neutral atom).
Worked example — relative atomic mass from isotope abundance
Chlorine exists as 75% chlorine-35 and 25% chlorine-37. Calculate its relative atomic mass.
Ar = (75×35 + 25×37) / 100 = (2625+925)/100 = 35.5
2. Bonding
Type
How it forms
Example
Ionic
Metal transfers electron(s) to non-metal, forming oppositely charged ions held by electrostatic attraction
NaCl
Covalent
Non-metal atoms share pairs of electrons
H₂O, CO₂
Metallic
Positive metal ions surrounded by a “sea” of delocalised electrons
Cu, Fe
Giant covalent structures
Diamond and graphite are both giant covalent structures of pure carbon, but with very different properties. In diamond, each carbon atom forms 4 strong covalent bonds in a rigid 3D lattice — making it extremely hard, with a very high melting point, and non-conducting. In graphite, each carbon atom forms only 3 covalent bonds, arranged in layers with weak forces between the layers (allowing them to slide over each other, making graphite soft and slippery) and one delocalised electron per atom (allowing graphite to conduct electricity, unlike diamond).
Common mistakes to watch for
Forgetting that the number of electrons only equals the number of protons in a neutral atom — an ion has gained or lost electrons.
Mixing up ionic bonding (electron transfer, forms ions) with covalent bonding (electron sharing, forms molecules).
Thinking metallic bonding involves fixed pairs of electrons — the electrons are delocalised (free to move throughout the whole structure), which is why metals conduct electricity.
Key Vocabulary Recap
Isotopes — atoms of the same element with different numbers of neutrons
Ion — a charged particle formed when an atom gains or loses electrons
Delocalised electrons — electrons free to move throughout a structure, not fixed to one atom
Syllabus Reference
This note covers IGCSE Chemistry 0620 Topic 2, Atoms, Elements and Compounds: atomic structure, isotopes, ionic bonding, covalent bonding (including giant covalent structures like diamond and graphite), and metallic bonding.
Practice questions
IGCSE Chemistry 0620 · Topic 2: Atoms, Elements and Compounds — Practice Question Bank (no answers)
Atoms, Elements and Compounds — Practice
Covers: atomic structure, isotopes, and ionic/covalent/metallic bonding. 10 questions.
Q1 Core
State the relative charge and relative mass of a proton, a neutron, and an electron.
Q2 Core
An atom of aluminium has a proton number of 13 and a nucleon number of 27. Find the number of protons, neutrons, and electrons.
Q3 Core
Define isotopes.
Q4 Extended
Chlorine exists as 75% chlorine-35 and 25% chlorine-37. Calculate its relative atomic mass.
Q5 Core
Describe how an ionic bond forms between a sodium atom and a chlorine atom.
Q6 Core
Describe how a covalent bond forms between two hydrogen atoms.
Q7 Extended
Explain why graphite can conduct electricity but diamond cannot, even though both are pure carbon.
Q8 Extended
Explain why graphite is soft and slippery, in terms of its structure.
Q9 Core
Describe metallic bonding, and explain why metals conduct electricity.
Q10 Core
Explain why ionic compounds have high melting points.
Worked solutions
Show worked solutions
IGCSE Chemistry 0620 · Topic 2: Atoms, Elements and Compounds — Solutions
Atoms, Elements and Compounds — Practice: Solutions
Q1
Charge and mass of proton, neutron, electron.
✓Proton: charge +1, mass 1. Neutron: charge 0, mass 1. Electron: charge −1, mass ≈1/2000 (negligible).
Q2
Al: proton number13, nucleon number27.
1
Neutrons = nucleon number − proton number
27−13=14
✓13 protons, 14 neutrons, 13 electrons
Q3
Define isotopes.
✓Isotopes are atoms of the same element (same proton number) with different numbers of neutrons (and therefore different nucleon numbers).
Q4
Ar of chlorine from isotope abundance.
1
Weighted average = Σ(% abundance × mass number) ÷ 100
(75×35 + 25×37)/100
✓35.5
Q5
Describe ionic bond formation Na-Cl.
✓The sodium atom transfers its single outer-shell electron to the chlorine atom. This forms a positively charged sodium ion (Na⁺) and a negatively charged chloride ion (Cl⁻), both with full outer shells. The oppositely charged ions are then held together by strong electrostatic attraction.
Q6
Describe covalent bond formation H-H.
✓Each hydrogen atom shares its single electron with the other, forming a shared pair of electrons (a single covalent bond) between the two nuclei. Both atoms now have a full outer shell (2 electrons, like helium).
Q7
Why does graphite conduct but not diamond?
✓In graphite, each carbon atom forms only 3 covalent bonds, leaving one delocalised electron per atom free to move along the layers and carry charge. In diamond, each carbon atom forms 4 covalent bonds, using all of its outer electrons in fixed bonds — there are no free (delocalised) electrons to conduct electricity.
Q8
Why is graphite soft and slippery?
✓Graphite is arranged in layers, with strong covalent bonds within each layer but only weak forces between the layers. These weak forces allow the layers to slide over each other easily, making graphite soft and slippery.
Q9
Describe metallic bonding; why do metals conduct?
✓Metallic bonding consists of a lattice of positive metal ions surrounded by a “sea” of delocalised electrons, which are free to move throughout the whole structure. These mobile delocalised electrons can carry electrical charge, which is why metals conduct electricity.
Q10
Why do ionic compounds have high melting points?
✓Ionic compounds form giant ionic lattices, with strong electrostatic forces of attraction acting in all directions between oppositely charged ions. A large amount of energy is needed to overcome these strong forces, giving ionic compounds high melting points.
