The Trends in Elements in 1-20 Atomic Size Ionisation Energies

Hydrogen137 Helium2-- Lithium3134 Beryllium4129 Boron5 90 Carbon677 Nitrogen775 Oxygen873 Fluorine971 ElementAtomic NumberAtomic SizeCovalent Radius

Atomic Number versus Atomic Size Li F Na Cl K Atomic Number Atomic SizeAtomic Size

We can explain the pattern or trend shown in this graph by considering 2 areas 1. Number of Electron Shells (energy levels) 2. The nuclear charge of the atom The graph shows that 1. As we go across a period the atomic size decreases 2. As we go down a group the atomic size increases (The electron arrangement) Always consider this first This is given by the atomic number

Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Nuclear Charge 1 2 2, 1 2, 2 2, 3 2, 4 2, 5 2, 6 2, 7 2, 8 Electron Arrangement Atomic Size (Covalent radius)

Explanation of properties An increase in nuclear charge pulls the electrons closer to the nucleus giving a smaller atomic size An increase in the number of electron shells means an increase in the atomic size Hydrogen Lithium2, Beryllium2, 2129 Boron2,

Group 1 elements The extra shell of electrons also shields the outer electrons from the attraction of the positive nucleus This gives a larger atomic size Lithium2,1134 Sodium2,8,1154 Potassium2,8,8, As we go down the group the number of electron shells increases. The outer electrons are further from the nucleus, This gives a larger atom

If you are asked to compare the size of ions remember that the electron arrangement of an ion is not the same as the atom Why is the ionic radius of P 3- so much greater than that of Si 4+ P 15+ 2)8)5P )8)8 but The negative ion has 3 more electrons, giving a full shell Si 14+ 2)8)4Si )8 but The positive ion has 4 less electrons, one less shell The difference in size is caused by the extra shell of the negative ion

Ionisation Energies Ionisation energies can be explained by considering the same points 1. Number of Electron Shells (energy levels) 2. The nuclear charge of the atom E (g)E + (g) + e - (List the electron arrangement ; This will change for the ions) (This does not change when it becomes an ion)

ElementElectron Arrangement First I.E.(kJ mol -1 ) Nuclear Charge Lithium2, Beryllium2, Boron2, Carbon2, Nitrogen2, The trend is : As we go across a period the first ionisation energy increases

Explanation of properties An increase in nuclear charge pulls the electrons closer to the nucleus giving a smaller atomic size Lithium2, 1526 kJ3 + Beryllium2, 2905 kJ4 + The outer electrons are therefore more stongly held. This means more energy is needed to remove the outer electron

Explanation of properties An increase in number of shells means the outer electrons are further from the nucleus and shielded by the extra shell Lithium2, 1526 kJ3 + Sodium2, 8, 1502 kJ11 + The outer electrons are therefore less strongly held. This means less energy is needed to remove the outer electron This means a smaller ionisation energy

Second Ionisation Energies The difference in values between first and second ionisation energies can be explained by considering the same points 1. Number of Electron Shells (energy levels) 2. The nuclear charge of the atom M + (g)M 2+ (g) + e - Remember ions have different electron arrangements from their atoms

Explanation of Second IE An decrease in number of shells means the outer electrons are closer to the nucleus and no longer shielded by the extra shell The outer electrons are therefore more strongly held. This means more energy is needed to remove the outer electron This means a larger ionisation energy Sodium2, 8, 1502 kJ11 +Na Sodium2, kJ11 +Na +

The large increase on going from second to third ionisation energy is when the last electron in the shell has been removed The outer electrons are therefore more strongly held. This means more energy is needed to remove the outer electron This means a larger ionisation energy Magnesium2, 8, 2744 kJ12 +Mg Magnesium2, 8, kJ12 +Mg + Magnesium2, kJ12 +Mg 2+