1.2.1—Atomic structure

An atom has a dense, positively charged nucleus containing protons and neutrons. Negatively charged electrons occupy the space outside the nucleus. Protons and neutrons are nucleons.

Quantity	Meaning	Rule
Atomic number, Z	Number of protons	p = Z
Mass number, A	Protons plus neutrons	n = A − Z
Ion charge	Proton charge compared with electron charge	charge = p − e

For a neutral atom, electrons equal protons. For an ion, use the stated charge to determine the electron count.

Read Z first to obtain protons, subtract Z from A to obtain neutrons, then use the ion charge to check or calculate electrons.

Apply the symbols in a fixed order. For ³⁵₁₇Cl⁻, Z = 17 gives 17 protons, A − Z gives 18 neutrons, and the 1− charge means one more electron than protons, so there are 18 electrons. The ion charge changes electron count, never the element identity.

Particle	Location	Relative charge	Approximate relative mass
Proton	nucleus	+1	1
Neutron	nucleus	0	1
Electron	outside nucleus	−1	about 1/1836

Most atomic mass is concentrated in the nucleus. Atomic number identifies the element through proton count; ion formation changes electron count, not proton count.

How it is assessed

Questions ask learners to deduce proton, neutron, and electron counts from nuclear notation and ion charge, or construct a nuclear symbol from particle information.

Command terms

deduce / determine

What earns marks

Use atomic number for protons, mass number minus atomic number for neutrons, and the charge relationship to determine electrons; include all requested particle counts or the complete nuclear symbol.

Watch for

Using mass number as the neutron count, or treating a positive ion as having gained rather than lost electrons.

Representative question

[Maximum number: 2]

Calculate the number of protons, neutrons and electrons in the ${ }^{26} \mathrm{Mg}^{+}$ ion.

Protons:
Neutrons:
Electrons:

Retrieve the sequence: use nuclear notation to count particles, distinguish isotopes by neutron number, then read mass-spectrum positions and heights as isotope mass and abundance evidence.

When checking an answer, ask: Did I separate A, Z, and charge? Did I explain isotope properties through electron arrangement? Did I weight each isotope by its abundance?

1.2.1—Atomic structure

Reading Atomic Structure

Atomic and Ionic Particle Counts

Assessment in practice

The Nuclear Atom Summary

Question 1