Structure of the Atom (Edexcel A Level Chemistry): Revision Note

Structure of the Atom

• All matter is composed of atoms, which are the smallest parts of an element that can take place in chemical reactions

• Atoms are mostly made up of empty space around a very small, dense nucleus that contains protons and neutrons

• The nucleus has an overall positive charge

  • The protons have a positive charge and the neutrons have a neutral charge

• Negatively charged electrons are found in orbitals in the empty space around the nucleus

The basic structure of an atom (not to scale)

Subatomic Mass & Charge

• Subatomic particles are the particles an element is made up of and include protons, neutrons and electrons

• These subatomic particles are so small that their masses and charges in conventional units (such as grams and coulombs) are very small numbers that are inconvenient to use

• Instead, their masses and charges are compared to each other using ‘relative masses’ and ‘relative charges’

• These are not actual charges and masses but they are charges and masses of particles relative to each other

  • Protons and neutrons have a very similar mass so each is assigned a relative mass of 1 whereas electrons are 1836 times smaller than a proton and neutron

  • Protons are positively charged, electrons negatively charged and neutrons are neutral

• The relative mass and charge of the subatomic particles are:

Relative mass & charge of subatomic particles table

Atomic & Mass Numbers

• The atomic number (or proton number) is the number of protons in the nucleus of an atom and has symbol Z

  • The atomic number is equal to the number of electrons present in a neutral atom of an element

  • Eg. the atomic number of lithium is 3 which indicates that the neutral lithium atom has 3 protons and 3 electrons

• The mass number (or nucleon number) is the total number of protons and neutrons in the nucleus of an atom and has symbol A

• The number of neutrons can be calculated by:

Number of neutrons = mass number - atomic number

• Protons and neutrons are also called nucleons