Models of the atom and Rutherford's experiment: WASSCE Physics

Everything is made of atoms — but what is inside an atom? A century of clever experiments revealed a tiny, dense nucleus surrounded by electrons in fixed energy levels, which explains why every element glows with its own colours.

Learning objectives

By the end of this lesson you should be able to:

describe the models of the atom from Thomson to Bohr
describe Rutherford's alpha-scattering experiment and its conclusions
state the structure of the atom: protons, neutrons, electrons, proton number and nucleon number
define isotopes

Models of the atom

Model	Main idea	Limitation
Dalton (1808)	atoms are tiny indivisible spheres	atoms are divisible (electrons were discovered)
Thomson's "plum pudding" (1904)	a sphere of positive charge with electrons embedded in it	could not explain alpha-particle scattering
Rutherford's nuclear model (1911)	a tiny, dense, positive nucleus with electrons orbiting it	could not explain why orbiting electrons do not spiral into the nucleus, or line spectra
Bohr's model (1913)	electrons move only in certain allowed orbits (energy levels); they emit or absorb light only when they jump between levels	works well for hydrogen only
Electron cloud (quantum) model	electrons are described by probability "clouds" (orbitals)	the modern model

Rutherford's alpha-scattering experiment

Geiger and Marsden, working with Rutherford, fired a beam of alpha particles at a very thin gold foil in a vacuum and counted the scattered particles with a zinc sulphide screen.

Observation	Conclusion
most alpha particles passed straight through	most of the atom is empty space
a few were deflected through small angles	there is a positive charge concentrated in the atom that repels the positive alpha particles
a very few (about 1 in 8000) bounced back	the positive charge and almost all the mass are concentrated in a tiny, dense nucleus

The nucleus is about $10^{-15}$ m across — about 100 000 times smaller than the atom (about $10^{-10}$ m).

In Sierra Leone

If an atom were the size of the National Stadium in Freetown, its nucleus would be about the size of a pea on the centre spot — yet it would contain almost all the atom's mass.

The structure of the atom

Particle	Location	Relative charge	Relative mass
Proton	nucleus	+1	1
Neutron	nucleus	0	1
Electron	orbits (energy levels)	−1	about $\dfrac{1}{1840}$

Protons and neutrons are together called nucleons.

Definition

The proton number (atomic number) $Z$ is the number of protons in the nucleus. It identifies the element.
The nucleon number (mass number) $A$ is the total number of protons + neutrons.
Number of neutrons $N = A - Z$ .

A nuclide is written $^{A}_{Z}\text{X}$ , for example $^{23}_{11}\text{Na}$ : 11 protons, 12 neutrons.

In a neutral atom, the number of electrons equals the number of protons.

Isotopes

Definition

Isotopes are atoms of the same element (same proton number) with different numbers of neutrons (different nucleon numbers).

Isotope	Protons	Neutrons
$^{1}_{1}\text{H}$ (hydrogen)	1	0
$^{2}_{1}\text{H}$ (deuterium)	1	1
$^{3}_{1}\text{H}$ (tritium)	1	2
$^{12}_{6}\text{C}$	6	6
$^{14}_{6}\text{C}$ (radioactive)	6	8

Isotopes of an element have the same chemical properties (same electrons) but different masses, and some are radioactive.

Worked example

How many protons, neutrons and electrons are in a neutral atom of $^{238}_{92}\text{U}$ ?

Protons $= 92$ ; electrons $= 92$ ; neutrons $= 238 - 92 = 146$ .

Key points

Models: Dalton → Thomson → Rutherford (nucleus) → Bohr (energy levels) → quantum.
Alpha scattering: atom mostly empty; tiny dense positive nucleus.
$Z$ = protons; $A$ = protons + neutrons; neutrons = $A - Z$ .
Isotopes: same $Z$ , different $A$ ; same chemistry.

WASSCE examination focus

WAEC tests this

Describe Rutherford's alpha-scattering experiment, state its observations and conclusions, compare atomic models, and define proton number, nucleon number and isotopes. Objective items often ask for the number of neutrons in a nuclide.

Quick check

What did the few alpha particles that bounced back from gold foil show?
How many neutrons are in $^{27}_{13}\text{Al}$ ?
What are isotopes?

Answers: 1. That the atom has a tiny, dense, positively charged nucleus. 2. $27 - 13 = 14$ . 3. Atoms of the same element with the same number of protons but different numbers of neutrons.

Models of the atom and Rutherford's experiment