Chemistry C3

The periodic table

• Dalton: arranged elements in order of mass;
• Newlands: produced a table of octaves but tried to fit everything in;
• Mendeleev: arranged elements in order of atomic mass but left spaces for undiscovered elements;
• Modern periodic table arranged in order of atomic number.

Reactivity in the alkali metals and halogens

Alkali metals - Want to lose an electron. Reactivity increases going down the group. As the atoms get bigger, the -ve electron is attracted less to the +ve nucleus so is easier to lose. This is because:

• The distance between them gets bigger;
• There are more shells to 'shield' the +ve charge of the nucleus.

Alkali metal properties: very reactive (stored in oil), low density (some float on water, very soft, low melting & boiling points, FORM IONIC BONDS.

With water - hydrogen gas & (soluble) metal hydroxide made.

Example 2Na (s) + 2H₂O (l) - 2NaOH (aq) + H₂ (g)

Other reactions - reacts with non-metals like chlorine. Example 2Na (s) + Cl₂ (g) - 2NaCl (s)

Halogens - Want to gain an electron. Reactivity decreases going down the group. As the atoms get bigger, the -ve electron joining the outer shell is attracted less to the +ve nucleus so is harder to gain. This is because:

• The distance between them gets bigger;
• There are more shells to 'shield' the +ve charge of the nucleus.

Halogen properties: low melting and boiling points, poor conductors, diatomic molecules (covalent bonds), FORM IONIC & COVALENT BONDS.

All react with metals. Gain 1 electron and a 1^- charge. Ionic salts (metal halides) are formed e.g. sodium chloride.

Reactions with hydrogen. Example F₂ (g) + H₂ (g) - 2HF (g)

Displacement reactions. More reactive halogens displace less reactive ones.

Example 2KBr + Cl₂ - 2KCl + Br₂

Transition metals

• Typical metallic structure;
• Good conductors, tough, hard, strong & malleable (easy to bend);
• (Except mercury) high melting & boiling points;
• [Higher] properties due to a inner shell filled up between Groups 2 & 3. This explains the colourful compounds & catalytic capabilities;
• Not very reactive with oxygen or water (corrode slowly);
• Useful alloys e.g. steel & brass;
• Can have many different ions e.g. iron (II) & iron (III)

Strong or Weak?

Acids are proton donors - they form H⁺ ions when dissolved in water. The ions become hydrated (surrounded by water molecules).

Hydroxides form OH^- ions in water. They accept the H⁺ ions from acids to make water (neutralisation) - proton acceptors.

An acid is strong or weak depending on how much it dissociates. pH measures the concentration of Hydrogen ions in solution. You can also test strength by observing the rate of reaction with a reactive metal.

Titrations

Titration finds out the volumes of acid/alkali needed to completely neutralise a solution. The end point is when the solution is comlpetely neutralised and this is shown using indicator.