Atomic Structure and Chemical Bonding (Singapore O-Level Chemistry 6092): subatomic particles, proton and nucleon number, isotopes and electronic configuration, and the three types of bonding that explain the properties of matter

What this topic is really about

Atomic Structure and Chemical Bonding is the structural foundation of the whole course. Once you know what is inside an atom and how its electrons are arranged, you can predict how it will bond, and once you know the bonding you can explain the properties. The unifying idea is that atoms bond to reach the stable electron arrangement of a noble gas, and the three ways they do this (transferring, sharing or pooling electrons) lead to the three structures that explain why some substances melt easily, some conduct electricity and some are hard. This guide draws the threads together and links to every dot point.

The complete set of dot-point pages for this topic, each with worked examples and questions, lives at /sg-o-level/chemistry/syllabus/atomic-structure-and-bonding.

Inside the atom

Atomic structure and isotopes builds the atom from protons, neutrons and electrons. The proton number defines the element; the nucleon number is protons plus neutrons; the number of neutrons is the difference. Isotopes share a proton number but differ in neutrons, so they are chemically identical because chemistry depends on electrons.

You should be fluent in writing the electronic configuration of the first twenty elements in shells, for example 2,8,1 for sodium. The number of electrons in the outer shell is the single most useful number in the topic, because it tells you the group, the likely charge of the ion, and how the atom will bond.

The three types of bonding

Ionic bonding is the transfer of electrons from metal to non-metal, forming oppositely charged ions held in a giant lattice. That lattice explains the high melting points and why ionic compounds conduct only when molten or dissolved, because the ions are then free to move.

Covalent bonding is the sharing of electron pairs between non-metals. The same bond produces two very different structures: simple molecular substances with weak forces between molecules and low melting points, and giant covalent structures such as diamond and graphite with strong bonds throughout and very high melting points.

Metallic bonding and structures describes a lattice of positive ions in a sea of delocalised electrons. The mobile electrons explain electrical conduction, the layers that can slide explain malleability, and the disruption of those layers explains why alloys are harder than pure metals.

How this topic is examined

• Draw clean dot-and-cross diagrams. Show only the outer shells, label the source of each electron, and get the numbers right; this is a frequent Paper 2 task.
• Always link structure to property. Do not just name a structure; explain the property from it (mobile ions or electrons for conduction, weak intermolecular forces for low melting point).
• Start from electronic configuration. Use the outer-shell count to predict ion charge, group and bonding type.

Check your knowledge

A mix of recall, reasoning and structure questions covering Atomic Structure and Chemical Bonding. Attempt them under timed conditions, then check against the solutions.

1. An atom has a proton number of 17 and a nucleon number of 35. State the number of protons, neutrons and electrons. (3 marks)
2. Define an isotope, and explain why isotopes of an element react in the same way. (3 marks)
3. Write the electronic configuration of a calcium atom (proton number 20). (1 mark)
4. Explain, in terms of structure and bonding, why sodium chloride conducts electricity when molten but not when solid. (3 marks)
5. Explain why an alloy is harder than the pure metal it is based on. (2 marks)