Atoms: Definitions & Structure (Edexcel IGCSE Chemistry)
Revision Note
Written by: Caroline Carroll
Reviewed by: Stewart Hird
Atoms and molecules
It is important to understand the terms atom and molecule when referring to atomic structure, elements and compounds
Term | Definition |
|---|---|
Atom | The smallest particle of an element that contains electrons surrounding a nucleus that contains protons and neutrons |
Molecule | A group of two or more atoms chemically combined to form an identifiable unit which retains the properties and composition of the substance |
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Atomic structure
All substances are made of tiny particles of matter called atoms which are the building blocks of all matter
Each atom is made of subatomic particles called protons, neutrons, and electrons
The protons and neutrons are located at the centre of the atom, which is called the nucleus
The nucleus is positively charged
The electrons move very fast around the nucleus in orbital paths called shells
The mass of the electron is negligible, hence the mass of an atom is contained within the nucleus where the protons and neutrons are located
Atomic structure
Protons and neutrons are in the nucleus, and electrons in shells orbiting the nucleus
Subatomic particles are so small that it is not practical to measure their masses and charges using conventional units (such as grams or coulombs)
Instead, their masses and charges are compared to each other, and so are called ‘relative atomic masses’ and ‘relative atomic charges’
Protons and neutrons have a very similar mass, so each is assigned a relative mass of 1
Electrons are 2000 times smaller than a proton and neutron, and so their mass is often described as being negligible
These are not actual charges and masses, but rather charges and masses of particles relative to each other
Table of mass and charge of subatomic particles
Particle | Relative Mass | Charge |
|---|---|---|
Proton | 1 | +1 |
Neutron | 1 | 0 (neutral) |
Electron | 1/1840 | -1 |
Examiner Tips and Tricks
The mass of an electron can just be stated as 'negligible' or 'very small' in an exam. You do not need to learn the value.
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Calculating protons, neutrons and electrons
You need to know the following terms to describe the properties and characteristics of atoms
Term | Definition |
|---|---|
Atomic number | The number of protons in the nucleus of an atom |
Mass number | The number of protons and neutrons in the nucleus of an atom |
Isotope | Atoms of the same element which have the same number of protons but a different number of neutrons |
Relative atomic mass | The average mass of one atom of an element, taking into account the abundance of all the isotopes for the element. It is equal to 1/12th the mass of an atom of carbon-12. |
The atomic number is equal to the number of protons in an atom
Since atoms are neutral, then it is also the same as the number of electrons
The mass number is the number of protons plus neutrons
The number of neutrons can thus be calculated by subtracting the atomic number from the mass number
For example, lithium has an atomic number of , therefore it has 3 protons and 3 electrons
The mass number of lithium is 7, so it has 7 - 3 = 4 neutrons
Lithium therefore has:
3 protons
3 electrons
4 neutrons
Worked Example
Determine the number of protons, electrons and neutrons in an atom of element X with atomic number 29 and mass number 63.
Answer:
The number of protons of element X is the same as the atomic number
Number of protons = 29
The number of protons is always equal to the number of electrons
Number of electrons = 29
The number of neutrons is calculated by:
Number of neutrons = mass number – atomic number
Number of neutrons = 63 – 29
Number of neutrons = 34
Examiner Tips and Tricks
Both the atomic number and the mass number are given on the Periodic Table, but it can be easy to confuse them.
Think MASS = MASSIVE, as the mass number is always the bigger of the two numbers, the other smaller one is therefore the atomic number.
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