The boiling points of the halogens are: Explain the trend in these boiling points.

The trend in these boiling points increases on going down Group 17. This is because on going down group 17 in the Periodic Table, the number of electrons increases, thereby increasing the Instantaneous dipole-induced dipole forces of the respective halogens down the group. The Instantaneous dipole-induced dipole forces (id-id forces) between the atoms are increased with an increasing number of electrons. So, more energy is needed to change the liquid into vapour and the boiling point is higher in the case of Iodine compared to the other halogens which are above Iodine in group 17. Instantaneous dipole-induce dipole forces (id-id forces) : The electron charge clouds in a non-polar molecule (or atom) are constantly moving. It often happens that more of the charge cloud is on one side of the molecule than the other. This means that one end of the molecule has, for a short time, more negative charge than the other. A temporary dipole is set up. This dipole can set up (induce) a dipole on neighbouring molecules. As a result of this, there are forces of attraction between the δ + end of the dipole in one molecule and the δ - end of the dipole in a neighbouring molecule. These dipoles are always temporary because the electrons clouds are always moving. Instantaneous dipole-induced dipole forces are sometimes called temporary dipole-induced dipole forces. These forces are also called London dispersion forces. It is the weakest intermolecular attractive force. Instantaneous dipole—induced dipole forces increase with: • increasing the number of electrons (and protons) in the molecule. • increasing the number of contact points between the molecules: contact points are places where the molecules come close together.

The boiling points of the hydrogen halides are shown in the table. Hydrogen halide HF HCl HBr HI Boiling point / °C +20 –85 –67 –35 Explain the trend in boiling points from HCl to HI .

The trend in boiling points increases from HCl to HI. This is because on going down group 17 in the Periodic Table, the number of electrons increases, thereby increasing the Instantaneous dipole-induced dipole forces of the respective halogens down the group. The Instantaneous dipole-induced dipole forces (id-id forces) between the respective molecules are increased with an increasing number of electrons of the atoms constituting the molecule. So, more energy is needed to change the liquid into vapour and the boiling point is higher in the case of HI compared to the HCl and HBr. Instantaneous dipole-induce dipole forces (id-id forces) : The electron charge clouds in a non-polar molecule (or atom) are constantly moving. It often happens that more of the charge cloud is on one side of the molecule than the other. This means that one end of the molecule has, for a short time, more negative charge than the other. A temporary dipole is set up. This dipole can set up (induce) a dipole on neighbouring molecules. As a result of this, there are forces of attraction between the δ + end of the dipole in one molecule and the δ - end of the dipole in a neighbouring molecule. These dipoles are always temporary because the electrons clouds are always moving. Instantaneous dipole-induced dipole forces are sometimes called temporary dipole-induced dipole forces. These forces are also called London dispersion forces. It is the weakest intermolecular attractive force. Instantaneous dipole-induced dipole forces (id-id forces) increase with: • increasing the number of electrons (and protons) in the molecule. • increasing the number of contact points between the molecules: contact points are places where the molecules come close together.

The boiling points of the hydrogen halides are shown in the table. Hydrogen halide HF HCl HBr HI Boiling point / °C +20 –85 –67 –35 Explain why the boiling point of HF is so much higher than the boiling point of HCl.

The rise in boiling point from HCl to HI is due to the increasing number of electrons in the halogen atoms as we go down the group 17 of the Periodic Table. This leads to increased instantaneous dipole-induced dipole forces(id-id forces) as the molecules get bigger. If hydrogen fluoride only had instantaneous dipole- induced dipole forces between its molecules, we would expect its boiling point to be lower than the boiling point of HCl (i.e lesser than -85 ° C). However, the boiling point of hydrogen fluoride is +20 °C, as given in the table, which is much higher than any other hydrogen halide. This is because of the stronger intermolecular forces of hydrogen bonding between the HF molecules. Hydrogen bonding is the strongest form of intermolecular bonding. It is a type of permanent dipole- permanent dipole bonding. For hydrogen bonding to occur between two molecules we need: one molecule to have a hydrogen atom covalently bonded to F, O, or N (the three most electronegative atoms) a second molecule to have an F, O, or N atom with an available lone pair of electrons. Instantaneous dipole-induce dipole forces (id-id forces) : The electron charge clouds in a non-polar molecule (or atom) are constantly moving. It often happens that more of the charge cloud is on one side of the molecule than the other. This means that one end of the molecule has, for a short time, more negative charge than the other. A temporary dipole is set up. This dipole can set up (induce) a dipole on neighbouring molecules. As a result of this, there are forces of attraction between the δ + end of the dipole in one molecule and the δ - end of the dipole in a neighbouring molecule. These dipoles are always temporary because the electrons clouds are always moving. Instantaneous dipole-induced dipole forces are sometimes called temporary dipole-induced dipole forces. These forces are also called London dispersion forces. It is the weakest intermolecular attractive force. Instantaneous dipole-induced dipole forces (id-id forces) increase with: • increasing the number of electrons (and protons) in the molecule. • increasing the number of contact points between the molecules: contact points are places where the molecules come close together.

The table lists the boiling points of some Group 15 hydrides. Explain the trend in the boiling points from phosphine to stibine.

The trend in boiling points increases from phosphine to stibine. This is because on going down group 15 in the Periodic Table, the number of electrons increases, thereby increasing the Instantaneous dipole-induced dipole forces of the respective elements down the group. The Instantaneous dipole-induced dipole forces (id-id forces) between the respective molecules are increased with an increasing number of electrons of the atoms constituting the molecule. So, more energy is needed to change the liquid into vapour and the boiling point is higher on going down the table from phosphine to stibine. Instantaneous dipole-induce dipole forces (id-id forces) : The electron charge clouds in a non-polar molecule (or atom) are constantly moving. It often happens that more of the charge cloud is on one side of the molecule than the other. This means that one end of the molecule has, for a short time, more negative charge than the other. A temporary dipole is set up. This dipole can set up (induce) a dipole on neighbouring molecules. As a result of this, there are forces of attraction between the δ + end of the dipole in one molecule and the δ - end of the dipole in a neighbouring molecule. These dipoles are always temporary because the electrons clouds are always moving. Instantaneous dipole-induced dipole forces are sometimes called temporary dipole-induced dipole forces. These forces are also called London dispersion forces. It is the weakest intermolecular attractive force. Instantaneous dipole-induced dipole forces (id-id forces) increase with: • increasing the number of electrons (and protons) in the molecule. • increasing the number of contact points between the molecules: contact points are places where the molecules come close together.