1. What changes when a neutral atom forms a positive ion?
- It gains protons
- It loses electrons
- It loses neutrons always
Check the answer and reasoning
It loses electrons
Electron loss leaves more protons than electrons.
CHEM / IGCSE
Atomic structure, bonding and periodicity
Atoms contain protons, neutrons and electrons. Isotopes differ in neutron number; ions form when electrons are gained or lost. Electron arrangements help explain periodic patterns.
Learning objectives
- Explain atomic structure, bonding and periodicity using a concrete example.
- Apply the worked method and justify each step.
- Recognise the limitation described in the final check.
The big idea
Atoms contain protons, neutrons and electrons. Isotopes differ in neutron number; ions form when electrons are gained or lost. Electron arrangements help explain periodic patterns.
Ionic bonding is attraction between oppositely charged ions; covalent bonding shares electron pairs. Metallic bonding involves positive ions and delocalised electrons. Structure explains melting, conductivity and hardness, including diamond and graphite.
Equations and relationships
mass number = protons + neutrons
Nucleons determine isotope mass number.
Worked example
An atom has 11 protons and 12 neutrons. Find its mass number.
Follow the solution
Add nucleons: 11+12=23; electrons do not contribute to the mass number.
Investigate and explain
Predict the worked result before calculating. Change one input, record the new result, and explain which assumptions still hold.
Related interactive model. Compare its assumptions with the explanation; it is not a complete syllabus lesson.
Check your understanding
2. An atom has 11 protons and 12 neutrons. Find its mass number.
- 11
- 12
- 23
Check the answer and reasoning
23
Add nucleons: 11+12=23; electrons do not contribute to the mass number.
3. Why does molten sodium chloride conduct electricity?
- Its ions can move
- Its atoms split into nuclei
- It has free electrons like a metal
Check the answer and reasoning
Its ions can move
Mobile charged ions carry current in the melt; fixed lattice ions cannot do so in the solid.