Atoms and isotopes

Key content

Atoms are tiny – they have a radius of about $1 \times 10^{-10}$ m. Our model of the atom has changed over time as scientists found new evidence.

The structure of an atom

An atom has a small, dense, positively charged nucleus made of protons and neutrons, surrounded by electrons.

Particle	Relative charge	Location
Proton	+1	Nucleus
Neutron	0	Nucleus
Electron	−1	Around the nucleus (in energy levels)

• The radius of the nucleus is less than $\tfrac{1}{10\,000}$ of the radius of the atom.
• Almost all of the mass of the atom is in the nucleus.
• Electrons are arranged at different distances from the nucleus (energy levels). Electrons can move to a higher energy level by absorbing electromagnetic radiation, or to a lower level by emitting it.

Mass number, atomic number and isotopes

• An atom has no overall charge, because the number of electrons equals the number of protons.
• The atomic number is the number of protons. All atoms of the same element have the same atomic number.
• The mass number is the total number of protons and neutrons.
• Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons.

Carbon-14 is written as $^{14}_{6}\text{C}$: mass number 14, atomic number 6, so it has $14 - 6 = 8$ neutrons. Carbon-12 ($^{12}_{6}\text{C}$) and carbon-14 are isotopes: the same bottom number (6 protons, so the same element and the same nuclear charge), but different top numbers, because carbon-14 has 2 more neutrons and so a greater mass.

If an atom loses outer electrons it becomes a positive ion.

The development of the model of the atom

• Atoms were once thought to be tiny spheres that could not be divided.
• The discovery of the electron led to the plum pudding model: a ball of positive charge with negative electrons embedded in it.
• In the alpha particle scattering experiment, alpha particles were fired at thin gold foil. Most passed straight through, but a few were deflected through large angles. This showed that the mass of the atom is concentrated in a tiny, positively charged nucleus. The plum pudding model was replaced by the nuclear model. The plum pudding model spread the positive charge and mass through the whole atom, so it predicted that every alpha particle would pass through with only tiny deflections; large deflections could only happen if the positive charge and mass were concentrated in a tiny nucleus. In the nuclear model the atom is mostly empty space, with a tiny positive nucleus and electrons outside it.
• Niels Bohr suggested that electrons orbit the nucleus at specific distances (energy levels). Experiments agreed with his calculations.
• Later experiments showed the positive charge of the nucleus is made of protons. About 20 years after the nucleus became an accepted scientific idea, the experimental work of James Chadwick provided evidence for the neutron in the nucleus.

New experimental evidence can lead to a scientific model being changed or replaced.

Common misconceptions