Trends - explanations

• First ionisation anergy across a period increases because there are more protons in each nucleus so the nuclear charge in each element increases ...
• therefore the force of attraction between the nucleus and outer electron is increased, and ...
• there is a negligible increase in shielding because each successive electron enters the same energy level ...
• so more energy is needed to remove the outer electron.

• Atomic radius decreases across a period because..
• the number of protons in the nucleus increases so ...
• the nuclear charge increases ...
• there are more electrons, but the increase in shielding is negligible because each extra electron enters the same principal energy level ...
• therefore the  force of attraction between the nucleus and the electrons increases ...
• and so the atomic radius decreases.

• Ionisation energy down a group decreases because there are more filled energy levels between the nucleus and the outer electron these  shieldthe outer electron from the attraction of the nucleus ...
• the radius of the atom increases, so the distance between the nucleus and the outer electron increases ...
• therefore the force of attraction between the nucleus and outer electron is reduced ...
• so less energy is needed to remove the outer electron.

• Atomic radius increases down a group because there are more filled energy levels between the nucleus and the outer electrons ...
• therefore the outer electrons are more  shielded from the attraction of the nucleus ...
• so the electrons in the outer energy levels are further from the nucleus and so the atomic radius increases.

When a substance melts, some of the attractive forces holding the particles together are broken or loosened so that the particles can move freely around each other but are still close together. The stronger these forces are, the more energy is needed to overcome them and the higher the melting temperature.

When a substance boils, most of the remaining attractive forces are broken so the particles can move freely and far apart. The stronger the attractive forces are, the more energy is needed to overcome them and the higher the boiling temperature.