group 17

• The group 17 elements are called halogens.
• The halogens have uses in water purification and as bleaches agents (chlorine), as flame-retardants and re-extinguishers (bromine) and as antiseptic and disinfectant agents (iodine).
• All halogens have distinct colours which get darker going down the group.
• Volatility refers to how easily a substance can evaporate.
• The melting & boiling points of the Group 17 elements increase going down the group which indicates that the elements become less volatile.
• Going down the group, the boiling point of the elements increases which means that the volatility of the halogens decreases.
• Uorine is the most volatile and iodine the least volatile.
• 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 it.
• A covalent bond is formed by the orbital overlap of two atoms and the attraction of electrons towards the nuclei; the bigger the atom, the weaker the covalent bond.
• The bond strength of the halogen molecules therefore decreases going down the group.
• The bond enthalpies decrease indicating that the bond strengths decrease going down the group.
• Bond enthalpy is the heat needed to break one mole of a covalent bond; the higher the bond enthalpy, the stronger the bond.
• An exception to this is uorine which has a smaller bond enthalpy than chlorine and bromine.
• Fluorine is so small that when two atoms of uorine 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.
• The more electrons there are in a molecule, the greater the instantaneous dipole-induced dipole forces.
• The oxidising power of the halogens decreases going down the group (the halogens get less reactive).
• As you go down the group, it becomes more difficult to separate the molecules and the melting and boiling points increase.
• This can be explained by looking at their electronegativities: 2.2+ - 2.
• Halogens become reduced as they gain an extra electron from the metal atom (the oxidation number of the halogen decreases).
• Halogens are therefore oxidising agents: Halogens oxidise the metal by removing an electron from the metal (the oxidation number of the metal increases).
• Lone pairs on fluorine get so close to each other in a fluorine molecule that they cause repulsion which decreases the bond strength.
• Halogens are non-metals and exist as diatomic molecules at room temperature.
• Halogens have weak van der Waals’ forces between the diatomic molecules caused by instantaneous dipole-induced dipole forces.
• As it becomes more difficult to separate the molecules, the volatility of the halogens decreases going down the group.
• When a molecule gets close to another non-polar molecule, it can induce a dipole as the cloud of electrons repel the electrons in the neighbouring molecule to the other side.
• Halogens react with metals by accepting an electron from the metal atom to become an ion with 1 − charge.
• The larger the molecule, the stronger the van der Waals’ forces between molecules.
• A sudden distribution of electrons in a nonpolar molecule can cause an instantaneous dipole.
• Instantaneous induced – induced dipole forces are a type of van der Waals’ forces.
• Halide ions can be identified in an unknown solution by dissolving the solution in nitric acid and then adding a silver nitrate solution followed by ammonia solution.
• The halide ions themselves get oxidised and lose electrons.
• The reducing power of the halide ions increases going down the group.
• As a result of this, the outermost electrons are held less tightly to the positively charged nucleus.
• A silver halide precipitate is formed upon addition of silver nitrate solution to halide ion solution.
• The reactivity of halogens is also shown by their displacement reactions with other halide ions in solutions.
• Dilute followed by concentrated ammonia is added to the silver halide solution to identify the halide ion.
• If the precipitate dissolves in dilute ammonia the unknown halide is chloride.