What is the difference between proton number and nucleon number?

The proton number (atomic number) is the number of protons in the nucleus, and it defines which element an atom is. The nucleon number (mass number) is the total number of protons and neutrons in the nucleus. To find the number of neutrons, subtract the proton number from the nucleon number. In a neutral atom the number of electrons equals the number of protons, so the proton number also tells you the electron count.

How do I draw an electron shell arrangement?

Fill the shells from the inside out. The first shell holds up to 2 electrons, and the next two shells each hold up to 8 for the elements in the N(A) syllabus. For example, an atom with 11 electrons (sodium) is arranged 2, 8, 1. The number of electrons in the outermost shell is the most important number because it controls how the atom bonds and which group of the Periodic Table the element is in.

What is the difference between ionic and covalent bonding?

Ionic bonding is the transfer of electrons from a metal to a non-metal, forming positive and negative ions that are held together by strong electrostatic attraction in a giant lattice. Covalent bonding is the sharing of electrons between non-metal atoms to form molecules. The key consequence is structure: ionic compounds form giant lattices with high melting points, while simple molecular substances have weak forces between molecules and so melt and boil at low temperatures.

Why do ionic compounds have high melting points but simple molecular substances have low ones?

In an ionic compound the ions are held in a giant lattice by strong electrostatic forces, and a large amount of energy is needed to overcome them, so the melting point is high. In a simple molecular substance the atoms within each molecule are held by strong covalent bonds, but the forces between separate molecules are weak. Only these weak intermolecular forces need to be overcome to melt or boil, so the melting and boiling points are low.

How is Atomic Structure and Bonding assessed in N(A)-Level Science 5107?

It is examined in the two Chemistry papers. Paper 3 (multiple choice, 20 marks) tests working out numbers of protons, neutrons and electrons, identifying bonding types, and matching properties to structure. Paper 4 (structured, 30 marks) asks for dot-and-cross diagrams, electron arrangements, and explanations linking bonding to properties such as melting point and electrical conductivity. A Periodic Table is provided in the multiple choice paper.

SingaporeChemistry

Atomic Structure and Bonding (Singapore N(A)-Level Science Chemistry 5107): the structure of the atom, electron shells, and how ionic and covalent bonding explain the properties of compounds

A Singapore N(A)-Level Science Chemistry (SEAB 5107) overview of Atomic Structure and Bonding. The protons, neutrons and electrons inside an atom, proton and nucleon number, electron shell arrangements, and how transferring electrons gives ionic bonding while sharing electrons gives covalent bonding, with each bonding type linked to the properties of its compounds.

Generated by Claude Opus 4.86 min readSEAB-5107Updated 2026-06-13

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What this topic is really about
Inside the atom and the electron shells
Ionic bonding and giant ionic structures
Covalent bonding and simple molecules
How this topic is examined
Check your knowledge

What this topic is really about

Atomic Structure and Bonding is the foundation of N(A) Chemistry: it explains what substances are made of and why they behave the way they do. 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 predict the properties. This guide ties the three dot points together and links to each one for the worked answers and practice.

The complete set of dot-point pages for this topic lives at /sg-n-level/chemistry/syllabus/atomic-structure-and-bonding.

Inside the atom and the electron shells

Atomic structure and electron shells describes the atom as a tiny dense nucleus of protons and neutrons surrounded by electrons in shells. The proton number defines the element, and the nucleon number is the total of protons and neutrons. The outermost shell controls the chemistry, because atoms react to gain a full outer shell.

Ionic bonding and giant ionic structures

Ionic bonding and ionic compounds explains how a metal transfers electrons to a non-metal so that both reach a full outer shell. The result is a positive metal ion and a negative non-metal ion held in a giant lattice by strong electrostatic forces. This giant structure explains the high melting points, and the fact that ionic compounds conduct electricity only when molten or in solution, because only then are the ions free to move.

Covalent bonding and simple molecules

Covalent bonding and simple molecules covers the sharing of electrons between non-metal atoms. A dot-and-cross diagram shows the shared pairs that make each atom reach a full outer shell. Simple molecular substances such as water, methane and chlorine have strong bonds inside each molecule but only weak forces between molecules, so they have low melting and boiling points and do not conduct electricity.

How this topic is examined

Practise the particle counting. Convert between proton number, nucleon number, and the numbers of protons, neutrons and electrons quickly and reliably.
Draw clean dot-and-cross diagrams. Show only the outer-shell electrons, and use dots for one atom and crosses for the other so shared pairs are clear.
Link bonding to properties. Always justify a melting point or a conductivity claim by naming the structure (giant ionic lattice or simple molecular) and the forces involved.

Check your knowledge

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

An atom has proton number 17 and nucleon number 35. State the numbers of protons, neutrons and electrons. (3 marks)
Write the electron arrangement of a magnesium atom (proton number 12) and state which group it is in. (2 marks)
Describe how ionic bonding occurs between sodium and chlorine. (2 marks)
Explain why sodium chloride has a high melting point. (2 marks)
Explain why methane has a low boiling point even though its covalent bonds are strong. (2 marks)

Solutions

Q1: Protons: 17. Electrons: 17 (a neutral atom). Neutrons: $35 - 17 = 18$ .
Q2: Electron arrangement: 2, 8, 2. With 2 electrons in its outer shell, magnesium is in Group II.
Q3: A sodium atom transfers its single outer electron to a chlorine atom. Sodium becomes a $\text{Na}^+$ ion and chlorine becomes a $\text{Cl}^-$ ion, and the oppositely charged ions are held together by strong electrostatic attraction.
Q4: Sodium chloride is a giant ionic lattice in which the ions are held by strong electrostatic forces. A large amount of energy is needed to overcome these forces, so the melting point is high.
Q5: Methane is a simple molecular substance. The covalent bonds within each molecule are strong, but the forces between separate molecules are weak. Only the weak intermolecular forces need to be overcome to boil it, so the boiling point is low.

Sources & how we know this

Singapore-Cambridge GCE Normal (Academic) Level Science (Physics, Chemistry) and Science (Chemistry, Biology) (Syllabus 5107) — Singapore Examinations and Assessment Board (2026)
Cambridge Assessment International Education, working with SEAB on the Singapore-Cambridge GCE N(A)-Level — Cambridge Assessment International Education (2026)