CHEMISTRY

Chemistry



Chemistry

Periodicity

Chemistry is made intricate and beautiful by the fact that each element reacts in a different way. Periodicity is the essence behind the Periodic Table. As the name suggests, the Periodic Table is built up of repeating patterns that conform to certain rules.

For example elements

Group I (the alkali Metals) all has similar chemistry.

Atomic radii increases across Period 3

Ionisation Energy,

We know an even more direct measure of the atoms energy levels than that of the size of the atom. (How are size and energy related?) The Ionization Energy (or Ionization Potential) can be thought of as the negative of the energy of the highest occupied atomic orbital. The IP should increase as Z2 within a shell, if electron-electron repulsion were completely ignored 

The IP is really the Heat (Enthalpy change) of the following reaction

X(g) = X(g)+ + e-                    DH = IP

Electro-negativity

Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons.

The Pauling scale is the most commonly used. Fluorine (the most electronegative element) is assigned a value of 4.0, and values range down to caesium and francium which are the least electronegative at 0.7.

Electron Affinity

onization energies measure the tendency of a neutral atom to resist the loss of electrons. It takes a considerable amount of energy, for example, to remove an electron from a neutral fluorine atom to form apositively charged ion.

F(g)  F+(g) + e-

Ho = 1681.0 kJ/mol

The electron affinity of an element is the energy given off when a neutral atom in the gas phase gains an extra electron to form a negatively charged ion. A fluorine atom in the gas phase, for example, gives off energy when it gains an electron to form a fluoride ion.

F(g) + e-  F-(g)

Ho = -328.0 kJ/mol

Electron affinities are more difficult to measure than ionization energies and are usually known to fewer significant figures. The electron affinities of the main group elements are shown in the figure below.

Atomic Radius

The atomic radius is a term used to describe the size of the atom, but there is no standard definition for this value. Atomic radius may refer to the ionic radius, covalent radius, metallic radius, or van der Waals radius. In all cases, the size of the atom is dependent on how far out the electrons extend. The atomic radius for an element tends to increase as one goes down an element group. The electrons become more tightly packed as you move across the periodic table, so while there are more electrons for elements of increasing atomic number, the atomic radius actually may decrease.

Measures Of Atomic Radius

Unlike a ball, an atom doesn't have a fixed radius. The radius of an atom can only be found by measuring the distance between the nuclei of two touching atoms, and then halving that distance. As you can see from the diagrams, the same atom could be found to have a different radius depending on what was around it.

The left hand diagram shows bonded atoms. The atoms are pulled closely together and so the measured radius is less than if they are just ...