The atom and isotopes

In a nutshell

This summary will review the structure of an atom and its components. Different types of atoms from the same element will also be reviewed, with worked examples.

Atomic structure

Atoms have a radius of around $1 \times 10^{-10} \, m$ and are made up of different components, responsible for the properties of the elements they form.

The atom

The atom is the smallest part of a chemical element that can exist. It is made from subatomic particles. This can be seen below.

Chemistry; Key concepts; KS4 Year 10; The atom and isotopes

part of the atom	location and contents
Nucleus	Central Positively charged Holds protons and neutrons Contains most of the mass of the atom
Shells	Circulate around the nucleus Holds electrons, allowing them to orbit the nucleus Contains very little mass of the atom

Subatomic particles

The different subatomic particles have different relative masses and charges. This is shown in the table below.

Subatomic particle	Relative Mass	Charge
Proton	$1$	$+1$
Neutron	$1$	$0$
Electron	$\frac{1}{1835}$	$-1$

Note: The overall charge on an atom is zero because the number of protons equals the number of electrons and the charges cancel out.

Atomic and mass numbers

These numbers are used to describe an atom.

Nuclear symbol

The nuclear symbol can tell you a lot about the element.

Example

Chemistry; Key concepts; KS4 Year 10; The atom and isotopes — A. The nuclear symbol

1. The element

2. Mass number

3. Atomic number

Atomic number - the amount of protons in the atom (the smaller number).
Mass number - the total amount of protons and neutrons in the atom (the larger number).

Note: You can calculate the number of neutrons in an atom by working out the difference between the two numbers (mass number subtract atomic number). The number of electrons is the same as protons (unless it is an ion).

Isotopes

Definition

These are different forms of the same element, with the same number of protons but different number of neutrons. This means that isotopes have the same atomic number but different mass numbers.

Example

$^{12}C$ and $^{13}C$ are a common pair of isotopes.

Isotope	COMPOSITION
* $^{12}C$ *	Six protons, six electrons, *six* neutrons
$^{13}C$	Six protons, six electrons, *seven* neutrons

Relative atomic mass

When referring to isotopes, we use a value called relative atomic mass ( $A_r$ ) instead of mass number since it takes into account the different abundances of different isotopes that make up an element.