Atomic Structure and Isotopes

An atom has a tiny nucleus of protons (charge $+1$ , mass $1$ ) and neutrons (charge $0$ , mass $1$ ), surrounded by electrons (charge $-1$ , mass about $\frac1{1840}$ ) in shells. The proton number $Z$ identifies the element; the mass number $A$ = protons + neutrons. Electrons fill shells 2, 8, 8, … . Isotopes are atoms of the same element with different numbers of neutrons; relative atomic mass is the abundance-weighted mean of their masses.

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Section 1 of 14

Learning objectives

State the relative charges and masses of protons, neutrons and electrons.
Work out numbers of subatomic particles from $Z$ and $A$ .
Write electronic configurations of the first 20 elements.
Define isotopes and calculate relative atomic mass.

Pause and think

Chlorine's relative atomic mass is $35.5$ , but no atom can have half a neutron. How?

Section 2 of 14

Core idea

Start hereHalf a neutron?

Look up chlorine on the Periodic Table and you'll see $35.5$ . No atom can contain half a particle, so what does the half mean? The answer is isotopes.

The big idea

Atoms have a nucleus of protons and neutrons with electrons in shells around it. The proton number defines the element; the number of neutrons can vary, giving isotopes. Relative atomic mass is the weighted average of the isotope masses.

1. Inside the atom

Proton: relative mass $1$ , charge $+1$ , in the nucleus.
Neutron: relative mass $1$ , charge $0$ , in the nucleus.
Electron: relative mass $\frac1{1840}$ , charge $-1$ , in shells.

\text{neutrons}=A-Z

A: mass (nucleon) number
Z: proton (atomic) number

Worked exampleCounting particles

An ion is written $^{32}_{16}\mathrm{S}^{2-}$ .

Goal: Protons, neutrons and electrons.
Strategy: Use $Z$ , $A-Z$ , then adjust electrons for the charge.

$p=16$
Proton number.
$n=32-16=16$
Mass number minus proton number.
$e=16+2=18$
A $2-$ charge means two extra electrons.

Answer16 protons, 16 neutrons, 18 electrons.

✓ CheckCharge

=16-18=-2

✓.

2. Electron shells

For the first 20 elements, electrons fill shells holding $2$ , $8$ , $8$ (then 2 more in the fourth). Calcium ( $Z=20$ ) is $2,8,8,2$ . The number of shells gives the period; the number of outer electrons gives the group.

3. Isotopes and relative atomic mass

Weighted mean for two isotopes

$\text{total mass of 100 atoms}=m_1p_1+m_2p_2$
Imagine 100 atoms: $p_1$ of mass $m_1$ , $p_2$ of mass $m_2$ .
$A_r=\frac{m_1p_1+m_2p_2}{100}$
Divide by the number of atoms.

Worked exampleChlorine

75% chlorine-35, 25% chlorine-37.

Goal: $A_r$ .
Strategy: Weighted mean.

$\frac{35\times75+37\times25}{100}$
Weight each mass by its abundance.
$=\frac{2625+925}{100}=35.5$
Evaluate.

Answer

A_r=35.5

✓ CheckNearer 35, the more abundant isotope ✓.

Key takeawaySame chemistry Isotopes have the same number of electrons arranged the same way, so they react identically. Only physical properties like mass and density differ.

Pause and think

Copper's $A_r$ is $63.5$ ; it has isotopes 63 and 65. Which is more common?

Summary

$Z$ protons, $A-Z$ neutrons.
Shells 2, 8, 8.
$A_r$ = weighted mean of isotopes.

Proton number: Number of protons in the nucleus.
Mass number: Total number of protons and neutrons.
Isotopes: Atoms of the same element with different numbers of neutrons.

HistoryRutherford. In 1911 Ernest Rutherford fired alpha particles at thin gold foil. Most went straight through, but a few bounced back, showing that atoms have a tiny, dense, positive nucleus.

Fun factMostly empty. If the nucleus were the size of a pea in the centre of a football stadium, the electrons would be specks in the outer stands. Atoms are about 99.9999999% empty space.

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