Covalency and the Halogens

covalent bonds

2 **non metal** elements that both **need an extra electron to get a full outer shell**, so they **share** a pair of electrons they form strong covalent bonds, covalently bonded substances may consist of small molecules

simple molecular substances

(ammonia, chlorine, water, methane), small molecules in which the atoms in the molecule are joined by strong covalent bonds, while between the molecules there are only weak inter-molecular forces which are easily broken

giant covalent structures

billions/trillions of atoms arranged in a regular lattice, very strong because all of the atoms are joined by covalent bonds silicone dioxide, diamond, graphite

how many electrons in its outer shell does oxygen share

4

how many electrons in its outer shell does hydrogen share

2

how many electrons in its outer shell does nitrogen share

6

how many electrons in its outer shell does hydrogen chloride share

2

how many electrons in its outer shell does water share

4

how many electrons in its outer shell does ammonia share

6

substances that consist of small molecules are usually…

gases or liquids with relatively low melting and boiling points, with weak inter-molecular forces between the molecules, they don’t conduct electricity because the covalent molecules don’t have an overall charge

when the substance boils..

the weak inter-molecular forces between the molecules are broken, not the covalent bonds

intermolecular forces increase with..

the size of the molecules, meaning larger molecules have higher melting and boiling points than smaller ones

Halogens

* elements in group 7
* chlorine/bromine/iodine
* almost all toxic in some way
* all have similar reactions because they all have seven electrons in their outer shell
* non-metals
* consist of molecules made of pairs of atoms = diatomic and forming a covalent bond from sharing electrons (F2, Cl2, Br2, I2)
* can also form covalent bonds with other non-metals, such as carbon or hydrogen

(simple molecular structures)

Fluorine

toxic, very reactive pale yellow, gas at room temperature , F2

Chlorine

poisonous, yellow-green gas, Cl2

Bromine

poisonous, fuming red-brown liquid at room temperature, Br2

Iodine

Iodine = grey solid with poisonous purple fumes, also antiseptic, I2

Astatine

Astatine = rarest element In the world, never viewed, At2

displacement

a more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt

as you go down the group

1. melting and boiling point increase
2. reactivity decreases = as you go down the outermost shell gets further away from the positive nucleus, so the attractive force needed to pull an electron from another atom gets weaker, and if a halogen can't attract an extra electron to complete it's outer shell and is unreactive
3. higher up group = more reactive = displaces less reactive elements further down group

chlorine + potassium bromide

= 2KCl + Br2 = colourless solution to orange

chlorine + potassium iodide 

2KCl + I2  = colourless solution to Brown

bromine + potassium chloride

doesn’t displace

bromine + potassium iodide

2KBr + I2 = colourless solution to brown

iodine + potassium chloride

doesn’t displace

iodine + potassium bromide

doesn’t displace