Structure of an 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 surrounding a very small, dense nucleus that contains protons and neutrons
- The nucleus has an overall positive charge due to the presence of these protons
- Negatively charged electrons are found in shells in the empty space around the nucleus
Subatomic Particles
- 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:
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.
Atoms: Key Terms
- 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 suggests 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
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- Protons and neutrons are also called nucleons