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# Properties of atomic nuclei

## Protons, neutrons and the strong nuclear force

An atom is the smallest unit of chemical elements. It consists of protons and neutrons in its centre (called the atomic nucleus), and electrons in orbitals around the centre.

The protons and neutrons are held together by the strong nuclear force (often just called the strong force). The strong nuclear force is a fundamental force like the electromagnetic force and the gravitational force.

The strong nuclear force is a very short-range attractive force. The force only affects particles smaller than the size of the nucleus (about $1-2\text{ fm}$).

The strong nuclear force offsets the repulsion between the positive charges of protons in the nucleus. Without it, most atoms would not be stable because of the repulsion between the positively charged protons.

At a nuclear scale, the strong nuclear force is approximately 100 times stronger than the electromagnetic force.

## Rutherford gold-foil experiment

The discovery of the nucleus in 1909 was the result of an experiment by a team of scientists under the direction of Ernest Rutherford.

This disproved the "plum pudding" atomic model, where electrons were assumed to be distributed evenly in a cloud of positive charge (the proton was not yet discovered).

The researchers shot alpha particles (essentially a helium atom deprived of its electrons, i.e. two protons and two neutrons) against an extremely thin gold foil.

They noticed that most of the alpha particles passed straight through. However, some of them were deflected from their paths and, most surprisingly to the researchers, a very small number were even reflected backwards.

Expected (left) and observed (right) results of the Rutherford gold foil experiment.

## Interpretation of the Rutherford gold-foil experiment

The charges on the atom and alpha particles are responsible for the deviation and reflection of the alpha particles. Alpha particles are positively charged.

Rutherford calculated the momentum of the alpha particles given their mass and speed. He found that they could only be reflected by a gold atom if almost the entire mass of the gold atom was concentrated in a very small space.

This tiny compact body was called the nucleus.

The nucleus surrounded by a cloud of orbiting electrons is called the "planetary model".

Expected (left) and observed (right) results of the Rutherford gold foil experiment.

## Chemical symbols

Each chemical element is different from other elements in the number of protons it contains in its atoms. An element can be represented by a unique chemical symbol $\ce{X}$ (e.g. $\ce{H}$ for a hydrogen atom, $\ce{He}$ for a helium atom).

The number of electrons and protons must be equal for an atom's charge to be zero. The number of electrons an atom contains in its neutral state determines the chemical characteristics of the atom.

## Nuclides

A nuclide is a type of atomic nucleus with a specific number of neutrons and protons (i.e. an atom without its electrons). The term was introduced to focus on the nucleus as a separate entity with its own specific reactions.

Nuclides are often represented by chemical symbols with a superscript and a subscript:$$\ce{^{A}_{Z}X}$$

$\ce{Z}$ is the proton (or atomic) number, which gives the number of protons in an atom. The proton number is unique for every chemical element.

$\ce{A}$ is the nucleon (or mass) number, which gives the total number of protons and neutrons (i.e. nucleons) in the atom.

The masses of atomic and subatomic particles are normally measured in $u$, called the atomic mass unit. It is obtained by taking the mass of $1/12$ of a carbon-12 atom (i.e. $1\text{ u}=m(\ce{^{12}_{6}C})/12=1.66\times 10^{-27}\text{ kg}$).

The masses of protons and neutrons are approximately equal ($1.007\text{ u}$ and $1.008\text{ u}$ respectively). The mass of an electron is very small relative to that of protons and neutrons ($5.5\times 10^{-4}\text{ u}$).

The nucleon number gives a good approximation of the relative mass of an atom.

## Isotopes

An isotope of a chemical element is distinguished from other isotopes of the same chemical element by the number of neutrons. In other words, different isotopes of a chemical element have the same number of protons but different numbers of neutrons.

The chemical differences among different isotopes of the same chemical element are mostly small. However, the isotopes have variable stability (i.e. susceptibility to decay).

Most elements have several isotopes but generally only one isotope is stable.

Hydrogen has three main isotopes which are present in relatively large quantities in the universe. The most common one by far (99.98% of all hydrogen) is $\ce{^{1}_{1}H}$, usually just referred to as hydrogen.

The isotope $\ce{^{2}_{1}H}$ is called deuterium and the isotope $\ce{^{3}_{1}H}$ is called tritium. These, in combination with $\ce{^{1}_{1}H}$ play an important role in nuclear fusion reactions in stars (e.g. producing sunlight).