Across Groups
As you travel down the groups on the periodic table, the energy required to remove an electron decreases. This is due to the increased distance from the electron to the nucleus, resulting in a decreased attraction between the two, and also due to the increased number of shells in between the two, resulting in more shielding, further weakening the attraction.
Successive Ionisation Enthalpies
Ionisation enthalpies increase after successive electrons are removed, due to the higher number of protons, giving a stronger attraction, therefore requiring more energy to remove.
Also there are sharp jumps as you move onto the next shell. This is due to the next shell being full of electrons further increasing the strength of the attraction between the electrons and the nucleus.
Oxidation and Reduction
The reduction of a substance is the loss of an electron or the decrease of an oxidation state, and the oxidation of a substance is the loss of an electron or the increase of the oxidation state. When these both happen simultaneously in a reaction, it is a redox reaction.
Displacement reactions occur when a more reactive halogen, like CL2, is passed into a solution of a less reactive halide ion such as KI. This results in a redox reaction in which the chlorine displaces the Iodine, becoming reduced, and the Iodine becomes oxidised.
Oxidation states
The oxidation states of atoms are always zero, and the combined oxidation states within a compound must always add up to zero.
There are some elements that can exist in more than one oxidation state, and so they are given a systematic name, with roman numerals telling you what oxidation state it is, for example Iron(II)Oxide and Iron(III)Oxide.
Half-equations
The transfer of electrons from one atom to another in a reaction can be summarised in two separate half equations.
For example 2Na + Cl2 2NaCl
Can be written as 2Na 2Na + 2e
And Cl2 2Cl + 2e.
In these types of reactions, the atoms that cause the other to be oxidised, meaning they themselves become reduced, are said to be called the oxidising agent. The same is said for the atoms that cause the other to be reduced, meaning they themselves become oxidised, which are called reducing agents.
Halogens
Physical Properties
Chemical Properties
Electrolysis
When a current is passed through NaCl, the Cl- ions become oxidised producing Chlorine gas at the anode.
2Cl Cl2 + 2e-
Storage and Transport
Due to its reactivity Fluorine cannot be stored, and so it is mad ein situ by electrolysing hydrogen fluoride. Chlorine also cannot be stored to its high toxicity, and so it is transported by rail or road as a liquid. Bromine is transported in lead-lined steel tanks, through a planned route to minimise accidents.
Electron Structure
Electrons exist in shells, and are designated in increasing energy levels. Each of these shells can further be divided into sub-shells labelled s, p, d and f. Each of these sub-shells contains a specific number of orbitals, which can each hold 2 electrons.
s,p and d blocks
s-block elements have 1 or 2 electrons on their outermost shell, which is on an s-orbital.
p-block elements have 3, 4, 5, 6, 7 or 8 electrons on their outer shell, which is on a p-orbital.
d-block elements are the transition metals, with a gradual increase in the number of electrons filling their outer shell, which is a d-orbital.
f-block elements are the further transition metals placed below the transition metals, and have their outer shells gradually filling up, which are all on their f-orbitals.
Intermolecular Bonds
The bonds between molecules are called intermolecular bonds.
Polar Molecules
The electronegativity of an atom of an element is the degree to which it attracts electrons. When there is a notable difference in electronegativity between two atoms bonded together, the electrons will be more attracted to the atom with a higher electronegativity. This will result in a difference in electron densities making one end negative and one end positive. When a molecule has this they are called dipoles. This can only happen when one end is clearly negative and another positive. Some molecules may have their electronegative molecules evenly spread out making the molecule overall a neutral molecule. A polar molecule is said to have a permanent dipole.
Instantaneous and induced dipoles
If a molecule does not have a permanent dipole, the electron density of the molecule may be unevenly distributed at any one time. This will result in an instantaneous dipole, and this polarity of the molecule could induce polar bond in other molecules resulting in an induced dipole.
Bonds
Halogenoalkanes
These are alkanes with one or more of the hydrogen atoms being substituted by a halogens.
Physical properties
The more halogen atoms in the Halogenoalkanes, and the heavier the halogens bonded to them, the higher the boiling point of the halogenoalkane. This is due to the increase in number of electrons due to the number, or size of the halogens bonding to the alkenes, therefore increasing the number of electrons, and increasing the strength of the instantaneous dipoles. Also the bond enthalpies decrease as you go down the group. This is due to the increasing size of the halogen, meaning that there’s less attraction to the nucleus, and more shielding.
Homolytic fission
This forms radicals. It must occur when the element is in a gaseous state, and in the presence of UV radiation.
R-Cl R + Cl