IMAT Chemistry · Atomic Structure

Protons, Neutrons, Electrons and Ions

An atom is a tiny nucleus of protons and neutrons with electrons around it; the atomic number fixes the element, and an ion is an atom that has gained or lost electrons.

All Atomic Structure notes

Why this matters

Counting the particles in an atom or ion is the most frequent task in this chapter: it was asked in 2011, 2012, 2016, 2017, 2019, 2020, 2022 and 2023, often with an ion, a polyatomic ion or a mass number written in algebra. The 2016 and 2018 papers asked which species share the same electron arrangement.

Concept 1 of 3: The three subatomic particles: charge, mass and position

Almost all of an atom's mass sits in a nucleus about a hundred thousand times smaller than the atom itself. The protons give the nucleus its positive charge, the neutrons add mass without charge, and the very light electrons fill the space around it. In a neutral atom the electrons exactly cancel the protons.

Definition

The atomic number $Z$ is the number of protons. It defines the element: every carbon atom has 6 protons.
The mass number $A$ is the number of protons plus neutrons (the nucleons).
A neutral atom has as many electrons as protons.
A nuclide is written ${}^{A}_{Z}\mathrm{X}$ , for example ${}^{23}_{11}\mathrm{Na}$ : 11 protons and 12 neutrons.
In chemical reactions only electrons move. The nucleus (protons and neutrons) does not change.

Particle	Relative charge	Relative mass	Where it is
Proton	+1	1	Nucleus
Neutron	0	1	Nucleus
Electron	−1	About 1/1840 (almost zero)	Shells (energy levels) around the nucleus

The proton and neutron masses are almost equal; the electron is about 1840 times lighter than either.

Practice this conceptself-check · 3 quick reps

Ions form by moving electrons, never protons

A positive ion has MORE protons than electrons because it has lost electrons. It has not gained protons: the nucleus never changes in a chemical reaction. Options that explain a positive charge by extra protons, or by a difference between protons and neutrons, are wrong.

Concept 2 of 3: Counting protons, neutrons and electrons from a nuclide symbol

The symbol

{}^{A}_{Z}\mathrm{X}^{q}

holds three numbers, and each particle comes from one simple subtraction. Neutrons are whatever the mass number has left after the protons. Electrons are the protons adjusted by the charge: a positive charge means electrons were removed, a negative charge means electrons were added.

Definition

For a particle written ${}^{A}_{Z}\mathrm{X}^{q}$ :

protons $= Z$
neutrons $= A - Z$
electrons $= Z - q$ . For $q = +2$ subtract 2; for $q = -2$ add 2.
For a polyatomic ion, add the atomic numbers of all its atoms, then apply the charge: $\mathrm{OH^-}$ has $8 + 1 + 1 = 10$ electrons.
The same rules work when $A$ and $Z$ are given in algebra: just subtract the expressions.

Neutrons and electrons

N = A - Z \qquad n_{e} = Z - q

$A$ mass number (protons + neutrons)
$Z$ atomic number (protons)
$N$ number of neutrons
$q$ charge on the particle, with its sign
$n_e$ number of electrons

Worked example

Find the numbers of protons, neutrons and electrons in

{}^{37}_{17}\mathrm{Cl}^{-}

and in

{}^{27}_{13}\mathrm{Al}^{3+}

Practice this conceptself-check · 4 quick reps

Subtract a positive charge, add a negative one

Electrons equal

Z - q

. A 2+ ion has two FEWER electrons than protons, a 2− ion two MORE. Reversing the sign is the commonest slip, and IMAT always offers the reversed count as an option.

The mass number is not the number of neutrons

The mass number counts protons and neutrons together. The neutrons are

A - Z

. An option equal to the mass number itself is there to catch students who skip the subtraction.

Concept 3 of 3: Isoelectronic species: the same number of electrons

Electrons arrange themselves according to how many there are, not which nucleus they belong to. So any two particles with the same electron count have the same ground-state arrangement, even if one is an ion and the other a neutral atom. Count electrons for each particle and match the numbers.

Definition

Particles are isoelectronic when they have the same number of electrons, and so the same electron arrangement in their lowest energy state.

The 10-electron set (the arrangement of neon): $\mathrm{N^{3-}}$ , $\mathrm{O^{2-}}$ , $\mathrm{F^-}$ , $\mathrm{Ne}$ , $\mathrm{Na^+}$ , $\mathrm{Mg^{2+}}$ , $\mathrm{Al^{3+}}$ .
The 18-electron set (the arrangement of argon): $\mathrm{P^{3-}}$ , $\mathrm{S^{2-}}$ , $\mathrm{Cl^-}$ , $\mathrm{Ar}$ , $\mathrm{K^+}$ , $\mathrm{Ca^{2+}}$ .
Isoelectronic particles still differ in protons, so they differ in size and chemistry (see the Periodic Table chapter for ionic radius).

Worked example

Which of these have the same electron arrangement as an argon atom (

Z = 18

\mathrm{K^+}

(

Z = 19

\mathrm{Ca^{2+}}

(

Z = 20

\mathrm{Cl^-}

(

Z = 17

\mathrm{S^{2-}}

(

Z = 16

\mathrm{Na^+}

(

Z = 11

Practice this conceptself-check · 3 quick reps

Isoelectronic does not mean identical

\mathrm{O^{2-}}

and

\mathrm{Na^+}

have the same 10 electrons in the same arrangement, but different numbers of protons and neutrons. Options saying two isoelectronic particles have the same number of protons, or the same size, are wrong.

Two ions of the same element are not isoelectronic

\mathrm{O^-}

and

\mathrm{O^{2-}}

have the same nucleus but 9 and 10 electrons. Being the same element is a distractor: only the electron count matters.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Counting protons, neutrons and electrons from a nuclide symbol
Neutrons and electrons
$N = A - Z \qquad n_{e} = Z - q$

Reference tables (1)

The three subatomic particles: charge, mass and position3 rows

Particle	Relative charge	Relative mass	Where it is
Proton	+1	1	Nucleus
Neutron	0	1	Nucleus
Electron	−1	About 1/1840 (almost zero)	Shells (energy levels) around the nucleus

The proton and neutron masses are almost equal; the electron is about 1840 times lighter than either.

Watch out for (5)

Ions form by moving electrons, never protons
→ The three subatomic particles: charge, mass and position
Subtract a positive charge, add a negative one
→ Counting protons, neutrons and electrons from a nuclide symbol
The mass number is not the number of neutrons
→ Counting protons, neutrons and electrons from a nuclide symbol
Isoelectronic does not mean identical
→ Isoelectronic species: the same number of electrons
Two ions of the same element are not isoelectronic
→ Isoelectronic species: the same number of electrons

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