Atoms and Electrons


The Basics


  • Charge = +
  • Relative mass = 1
  • Relative charge= 1+


  • Charge = No charge
  • Relative mass = 1
  • Relative charge = 0


  • Charge = -
  • Relative mass = 1/2000
  • Relative charge = 1-
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Relative Atomic Mass: The weighted average mass of an atom compared to the mass of one twelfth of the mass of a carbon-12 atom.

Relative Isotopic Mass: The weighted average mass of an isotope compared to the mass of one twelfth of a carbon-12 atom.

Relative Molecular Mass: The weighted average mass of a molecule of a molecule compared to one twelfth of the mass of a carbon-12 atom.

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Calculating Relative Atomic Mass (Ar)

1. Take the mass number of the isotopes given and multiply them by their percentage abundance.

2. Divide by 100 to make a percentage.


35Cl = 75% an37Cl = 25%

Relative atomic mass = (35 x 75) + (37 x 25)   = 35.5 



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Ionisation Energies

Ionisation Energy- The energy required to remove an electron from each atom in a mole of gaseous atoms.

Na(g) ----> Na+(g) + e-                    (Example of equation for first ionisation energy)

Na+(g) ----> Na2+(g) + e-               (Example of equation for second ionisation energy)   

Factors affecting ionisation energy-

  • If the atomic radius is larger, the nuclear attraction felt by electrons on the outer shells is reduced, this decreases the first ionisation energy.
  • The greater the charge on the nucleus, the greater the attractive force on the outer electrons, this increases the first ionisation energy as it makes the electrons harder to lose.
  • The more electron shells there are, the greater the effect of electron shielding, this decreases the nucluear attraction on outer electrons, and actively repels them due to the other electrons sharing the same charge, thus decreasing the first ionisation energy.
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Working Out Number Of Moles


Moles = Mass (in grams)



Moles = volume x concentration 

(Divide volume by 1000 where concentration is given in decimetres)


Moles= volume


Divide by 24,000 if the answer needs to be in cm3

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