The Periodic Table

Periodic Trends

Atomic Radius Decreases Across A Period.
Number of protons increases, so the positive charge of the nucleus increases.
Electrons are pulled closer to the nucleus, so atomic radius decreases.
Shielding doesn't change because the extra electrons gained are added to the outer energy level.
Ionisation Energy Increases Across A Period.
Number of protons increases, causing a stronger nuclear attraction.
Extra electrons are roughly the same energy level, so there is little shielding or extra distance to lessen the nuclear attraction.
Ionisation Energy Decreases Down A Group.
There are extra electron shells, so there is weaker nuclear attraction.
The shielding between the outer electrons and the nucleus overcomes the nucleus' positive charge.
Melting & Boiling Point Increase Across A Period.
Metals: The metallic bonds get stronger; increasing number of delocalised electrons and decreasing ionic radius. Higher charge density.
Macromolecular: Atoms are linked by strong covalent bonds, lots of energy needed to break the bonds.
Simple Molecular Substances: Van der Waals' forces are easily overcome.

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Group 2 elements lose their outer electrons to form 2+ ions.
Reactivity increases down group 2. When they react, they lose electrons. The easier it is to lose electrons, the more reactive the element.
Group 2 elements react with water to produce hydroxides.
- Metal Hydroxide + Hydrogen
They burn in oxygen to form solid, white oxides.
Group 2 oxides and hydroxides are bases.
They form alkaline solutions in water.
The oxides form more strongly alkaline solutions as you go down the group
Hydroxides become more soluble.
Thermal Decomposition: When a substance breaks down when heated.
Group 2 carbonates decompose to form the oxide and carbon dioxide.
Thermal stability increases down the group, so it takes more heat to decompose carbonates.
Group 2 compounds are used to neutralise acidity.
- Calcium Hydroxide [Ca(OH)2] is lime; used to neutralise soils.
- Magnesium Hydroxide [Mg(OH)2] is an antacid.
- H+ (aq) + OH- (aq) --> H2O (l)

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Benefits Of Chlorine In Water:
- It kills disease-causing microorganisms.
- It prevents reinfection further down the water supply.
- It prevents growth of algae.
Risks Of Chlorine In Water:
- Chlorine gas is very harmful if inhaled.
- Chlorine reacts with organic compounds in water to form chlorinated hydrocarbons.

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