Periodicity

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Periodicity: ionisation energyies and atomic radii

Trends across a period
Ionisation energy shows a general increase across each period.
Across each period the number of protons increase, so there is more attraction acting on the electrons. Electrons are added to the same shell, so the outer shell is drawn inwards slightly. There is the same number of inner shells, so electron shielding will hardly change

Across a period, the attraction between the nucleus and outer electrons increases, so more energy is needed to remove an electron. This means that the first ionisation energy increases across a period. There is also a decrease in atomic radius across a period, because the increased nuclear charge pulls the electrons in towards it.

Starting the next period, there is a sharp decrease in first ionisation energy between the end of one period and the start of the next period. This reflects the addition of a new shell, further from the nucleus, which leads to increased distance of the outermost shell from the nucleus and increased electron shielding of the outermost shell by inner shells. 

ACROSS A PERIOD, INCREASED NUCLEAR CHARGE IS THE MOST IMPORTANT FACTOR.

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Periodicity: ionisation energyies and atomic radii

Trends down a group
Down a group, first ionisation energy decreases. Down each group, the number of shells increase, so the distance of the outer electrons from the nucleus increases; hence, there is a weaker force of attraction on the outer electrons. There are more inner shells, so the shielding effect increases. The number of protons in the nucleus also increases, but the resulting increased attraction is far outweighed by the increase in distance and shielding. However, the atomic radius increases down a group, because less attraction means that the electrons are not pulled as close to the nucleus.

DOWN A GROUP, INCREASED DISTANCE AND SHIELDING ARE THE MOST IMPORTANT FACTORS INFLUENCING IONISATION ENERGY. 

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Periodicity: boiling points

Metal to non-metal
The most striking trends across the the periodic table are, metals to non-metals and solid to gas.
Na, Mg and Al are all clearly all metals, but Si is much harder to classify. Si has the shiny appearance of a metal, but is brittle. It conducts electricity, but very poorly. Silicon is an in-between element, usually classified as a metalloid.

Trends in melting and boiling points
Group 1 to 4 there is a general increase in boiling points.
Group 4 to 5 there is a sharp decrease in boiling point.
Group 5 to 8 there is a comparatively low boiling points.

There is a distinct change between group 4 and group 5 in both physical structure and the forces holding the structures together. This change is from giant structures to simple molecular structures and from strong forces to weak forces.
The trend in melting point is similar to the trend in boiling point. There is a sharp decrease between group 4 and group 5 marking the change from giant to simple molecular structure. 

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Periodicity: boiling points

The structure and bonding across Periods 2 and 3. The molecules making up the simple molecular structures are shown by their molecular formulae.

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