Edexcel A Level Chemistry:复习笔记1.1.1 Structure of the Atom

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 it is not possible to measure their masses and charges using conventional units (such as grams and coulombs)
  • Instead, their masses and charges are compared to each other using ‘relative atomic masses’ and ‘relative atomic 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


Exam Tip

The relative mass of an electron is almost negligible. The charge of a single electron is -1.602 x 10-19  coulombs whereas the charge of a proton is +1.602 x 10-19  coulombs, however, relative to each other, their charges are -1 and +1 respectively.


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


Exam Tip



The mass (nucleon) and atomic (proton) number are given for each element in the Periodic Table