11.1 physical properties of group 17 elements

What is the colour of fluorine

Pale yellow gas

What is the colour of chlorine

pale green gas

What is the colour of bromine

Orange, brown liquid

What is the colour of iodine

Grey solid

Purple vapour

Describe the trend in volatility of chlorine, bromine and iodine

Going down the group, the boiling point of the elements increases, therefore the volatility of halogens decreases

Describe and explain the trend in the bond strength of the halogen molecules

• Halogens are diatomic molecules in which covalent bonds are formed by overlapping their orbitals

• In a covalent bond, the bonding pair of electrons is attracted to the nuclei on either side and it is this attraction that holds the molecule together

• Going down the group, the atomic size of the halogens increases

• The bonding pair of electrons get further away from the halogen nucleus and are therefore less strongly attracted towards in

Describe the trend in group 7 bond enthalpies

Bond enthalpy is the heat needed to break one mole of a covalent bond

• The higher the bond enthalpy, the stronger the bond

• Exception: fluorine has a smaller bond enthalpy than chlorine and brominei

Why is fluorine an exception in bond enthalpies

• Fluorine is so small that when 2 atoms of fluorine get together their lone pairs get so close that they cause significant repulsion counteracting the attracting between the bonding pair of electrons and two nuclei

Interpret the volatility of elements in terms of instantaneous dipole-induced dipole forces

• The halogens are simple molecular structures with weak van der Waals’ forces between the diatomic molecules caused by instantaneous dipole-induced dipole forces

• The more electrons there are in a molecule, the greater the id-id forces

• Therefore, the larger the molecule the stronger the van der Waals’ forces between molecules

• Therefore as you go down the group, it gets more difficult to separate the molecules and the melting and boiling points increase

• As it gets more difficult to separate the molecules, the volatility of the halogens decreases going down the group

Halogens are… agents

Oxidising

How does electronegativity affect a halogen’s oxidising power?

• Going down the group, atomic radii decreases

• Therefore halogen’s ability to accept an electron decreases as incoming electron experiences shielding

• Therefore, oxidising power decreases

Describe the reaction between fluorine and hydrogen gas

Reacts explosively, even in cool or dark conditions

Describe the reaction between chlorine and hydrogen gas

Reacts explosively in sunlight

Describe the reaction between bromine and hydrogen gas

Reacts slowly on heating

Describe the reaction between iodine and hydrogen gas

Forms an eq mixture on heating

Explain the trend in thermal stability of hydrogen halides down the group

• decreases

• Atomic radius of halogens increases

• Overlap of outer shell with hydrogen atom gives longer bond length

• The longer the bond, the weaker, and the less energy required to break it.