Atoms and Electrons


Atomic Structure

Fundamental (sub-atomic) Particles


  • Relative Charge = -1
  • Relative Mass = 1/2000


  • Relative Charge = +1
  • Relative Mass = 1


  • Relative Charge = 0
  • Relative Mass = 1

In the atom the protons and neutrons are packed together very tightly in the nucleus at the centre of the atom; these two types of particles together are called nucleons

The electrons are in different shells at various distances around the nucleus

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Atomic Number

'The atomic number of an element is the number of protons in one atom of that element'

  • All atoms of a given element have the same atomic number and each element has a different atomic number
  • A neutral atom must have the same amount of elecrtons as protons, so the atomic number of a neutral atom is also the number of electrons


'Isotopes are atoms of the same element with different numbers of neutrons'

'An isotope is an atom of an element with a specified number of neutrons'

'The mass number of an isotops is the number of nucleons in an atom of the isotope'

  • Isotopes of the same element have almost identical chemical and physical properties
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Atoms and Electrons


'An ion is an atom or group of atoms that has lost of gained one or more electrons'


'An orbital is a region of space in which there is a high probability of finding an electron'

'An orbital is a region of space that can hold up to two electrons with opposite spins'

Types of Orbital

  • Shapes denoted by a letter: s,p,d,f
  • An s orbital has a spherical boundary surface
  • A p orbital has a 'dumbbell' shape
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Shells and Subshells

  • A shell is a group of orbitals whose distance from the nucleus is about the same. The energy levels of the orbitals in a given shell are not always exactly the same, but are similar
  • As the number of shells increases, the orbitals within the shell become larger, so the average distance from the nucleus of the electrons increases and the energy of the electrons also increases
  • Within each shell there are subshells which are groups of orbitals of the same type
  • Subshells and the orbitals they include are named according to the shell they are within and the orbital type

The first shell can hold a maximum of 2 electrons

The second shell can hold a maximum of 8 electrons

The third shell can hold a maximum of 18 electrons

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Electron Configuration

  • In any given atom in it's ground state the electrons occupy as many orbitals as necessary, starting with those of the lowest energy
  • When there is more than one orbital in a subshell, electrons will preferentially go into different orbitals within the subshell rather than pair up in the same orbital where this is possible
  • This gives a lower energy state as it reduced repulsion between the electrons
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