Singapore-Cambridge N(A)-Level Combined Science, Chemistry: Atoms, Bonding and the Mole, from atomic structure and the periodic table through ionic and covalent bonding to the particle model and mole calculations

What this module is about

The Atoms, Bonding and the Mole module is the bedrock of N(A)-Level Combined Science Chemistry (SEAB 5105 and 5107). It works from the inside of the atom outwards: what an atom is made of, why atoms join together, how their arrangement explains the states of matter, and how chemists count atoms by weighing. The recurring idea is that the behaviour of every substance comes from its particles, their structure, their bonds and their arrangement.

This overview pulls the threads together and links to every dot point page in the module, each with its own worked answers and practice questions.

Inside the atom and the periodic table

The module opens with atomic structure and the periodic table. An atom has a tiny central nucleus of protons (positive) and neutrons (neutral), surrounded by electrons (negative) in shells. The proton number identifies the element and equals the number of electrons in a neutral atom; the nucleon number is the total of protons and neutrons, so the number of neutrons is the nucleon number minus the proton number. The periodic table arranges elements in order of proton number, into groups (columns) of similar properties and periods (rows).

Why atoms bond

Next comes chemical bonding. Atoms bond to achieve a stable, full outer shell of electrons. In ionic bonding a metal transfers electrons to a non-metal, forming positive and negative ions held together strongly, which gives high melting points and the ability to conduct electricity when molten or dissolved. In covalent bonding non-metal atoms share electrons to form molecules, which gives simple molecular substances low melting points and no electrical conductivity.

The particle model and separating mixtures

The third dot point is states of matter and separation. The particle model explains the three states: in a solid the particles are packed in a fixed arrangement and vibrate; in a liquid they are close but can move past each other; in a gas they are far apart and move freely. Changes of state are explained by adding or removing energy. The same model underpins separation methods such as filtration, evaporation, distillation and chromatography, each chosen to suit the mixture.

The mole: counting by weighing

The module finishes with the mole and chemical formulae. Relative atomic mass compares the mass of an atom with a standard, and relative formula mass is the sum of the relative atomic masses in a formula. The mole is the chemist's counting unit, and the key relationship is that the number of moles equals the mass divided by the relative formula mass, so atoms can be counted simply by weighing.

How this module is examined

• Get the atomic numbers right. State clearly which is the proton number and which is the nucleon number, and find neutrons by subtraction.
• Link bonding to properties. Ionic means transfer, ions, high melting point and conducts when molten or in solution; covalent means sharing, molecules, low melting point and no conduction.
• Show mole working. Write the relative formula mass, then divide the mass by it. Always include units in the mass.

Check your knowledge

A mix of recall and application questions covering the module. Attempt them under timed conditions, then check against the solutions, and use the dot point pages for fuller practice.

1. State the relative charges of a proton, a neutron and an electron. (2 marks)
2. An atom has a proton number of 11 and a nucleon number of 23. How many neutrons does it have? (1 mark)
3. Describe what happens to the electrons when sodium and chlorine form an ionic bond. (2 marks)
4. State two differences in properties between an ionic compound and a simple covalent substance. (2 marks)
5. Describe the arrangement and movement of particles in a gas. (2 marks)
6. Calculate the number of moles in 80 g of sodium hydroxide, NaOH. Use Na = 23, O = 16, H = 1. (2 marks)