# Atomic Structure

A LevelAS LevelAQA

## Atomic Structure

Atoms are the building blocks of the universe and are the smallest unit of ordinary matter.

## Inside an Atom

Atoms are made up of protons, neutrons and electrons

Protons and neutrons are found in the nucleus and are known as nucleons. The nucleus is located in the centre of the atom. Electrons orbit around the nucleus. This is the nuclear model of the atom.

Each of the particles that make up an atom have different properties.

As the actual charges and masses of the constituent particles are tiny, the relative mass and charge are often used.

You will need to remember the relative mass and charge of each of the particles, the actual values will be given to you in the formula sheet. A stable atom is neutral (zero charge). This means it has equal numbers of protons and electrons, as they have equal but opposite charges.

 Mass (kg) Charge (C) Relative Mass Relative Charge Protons $1.673 \times 10^{-27}$ $+1.6 \times 10^{-19}$ $1$ $+1$ Neutrons $1.675 \times 10^{-27}$ $0$ $1$ $0$ Electrons $9.11 \times 10^{-31}$ $-1.6 \times 10^{-19}$ $0.005$ (Negligible) $-1$
A LevelAS LevelAQA

## Nuclide Number

A nuclide is an atom or group of atoms that have the same number of protons and neutrons

The nuclide notation summarises the atomic structure of an element.

The nucleon number, sometimes referred to as the mass number, gives the number of protons and neutrons in an atom. It has the symbol A.

Protons and neutrons have a relative mass of approximately $1$, whilst an electron has negligible relative mass. Therefore, the nucleon number gives the relative mass of an atom.

The atomic number, sometimes referred to as the proton number, gives the number of protons in an atom. It has the symbol Z. The proton number characterises the element, every element has a unique number of protons. A neutral atom has an equal number of protons and electrons. Electrons play an important role in an element’s chemical behaviour, therefore the number of protons gives information about the chemical properties of an element.

A LevelAS LevelAQA

## Atomic Structure Example Questions

In the centre of an atom, there is a nucleus containing protons and neutrons.

Electrons orbit around the nucleus.

Gold Standard Education

Relative mass of a proton $=1$

Relative mass of a neutron$=1$

Relative mass of an electron$=0$

Relative mass$= 4 + 5 = \bold{9}$

Gold Standard Education

Nucleon number $=281$

Proton number$=110$.

Therefore, the neutron number $=281-110=\bold{171}$