Chapter 5: Electrons and bonding

• Shells are regarded as energy levels • The energy increases as shell number increases • The shell number of energy level is given the principal quantum number n Shells are made up of atomic orbitals.

An atomic orbital is a region around the nucleus that can hold up to two electrons with opposite spins.

General formula = 2n^2 n=1: 2 n=2: 8 n=3: 18 n=4: 32

• Shape of a sphere • Each shell from n=1 contains 1 s-orbital (therefore 1 s subshell can hold a maximum of 2 electrons) • The greater the shell number n, the greater the radius of its s-orbital

• Shape of a dumb-bell • Each shell from n=2 has 3 p-orbitals (therefore a p subshell can hold up to 6 electrons) • The greater the shell number n, the further the p-orbital is from the nucleus

From shell n=3 Has 5 orbitals Therefore can hold up to 10 electrons in one subshell

From shell n=4 Has 7 orbitals Can therefore hold up to 14 electrons in each subshell

Electrons fill orbitals in order of increasing energy? 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 4d, 4f

Orbitals fill in order of increasing energy. 4s subshell has a lower energy level than the 3d subshell, and therefore 4s is filled first (and also emptied first) Electrons pair with opposite spins. This is because electrons are negatively charged and so repel one another. Electrons have a property caled spin - either up or down. Two electrons must have opposite spins, as it helps to counteract the repulsion between the negative charges of the two electrons. Orbitals with the same energy are occupied singly first. Within a subshell, the orbitals have the same energy. One electron occupies each orbital before pairing starts. This prevents any repulsion between paired electrons until there is no further orbital available at the same energy level.

Bonding: the electrostatic force of attraction that keeps two or more atoms or ions together in a chmeical substance. Structure: is how the atoms or ions are arranged in a three-dimensional space.

The electrostatic attraction between positive and negative ions.

A giant ionic crystal lattice: Oppositely charged ions held in a regular 3-dimensional lattice by electrostatic attraction, in all directions.

Very high melting and boiling points as there is a strong electrostatic attraction between the ions. High temperatures are needed to provide the large quantity of energy to overcome this attraction. Solubility: Many ionic substances dissolve in polar solvents like water. Polar water molecules break down the lattice and surround each ion in solution. Electrical conductivity: In a solid state the ions are in a fixed position and so there are no mobile carries of charge, but when liquid or dissolved in water, the ions can be mobile charge carries as the solid ionic lattice breaks down, therefore conduciting electricity. Strength: ionic compounds are brittle - any dislocation leads to the layers moving and similar ions being adjacent. The repulsion splits the crystal.

Ionic charge: the greater the charge, the stronger the electrostatic force, so the higher the melting and boiling point. Ionic radius: the smaller the ionic radius, the stronger the electrostatic attraction, so the higher the melting and boiling point. (The same factors affect solubility of an ionic compound)

The ionic lattice must be broken down Water molecules must attract and surround the ions

In a compound made of ions with large charges, the ionic attraction may be too strong for water to be able to break down the lattice structure.

In a solid state: the ions are in a fixed position in the giant ionic lattice there are no mobile carriers of charge therefore the ionic compound does not conduct electricity In a liquid or aqueous (dissolved in water) state: The solid ionic lattice breaks down The ions are now able to move as mobile charge carries The ionic compound is a conductor of electricity in liquid and aqueous states

A covalent bond is the strong electrostatic force of attraction between a shared pair of electrons and the positive nuclei of the bonded atoms.

In non metals In non metallic compounds (e.g. H2O) in polyatomic ions (e.g. NH4+)

Attraction is localised as opposed to ionic bonds, which are in all directions. Localised: confined to a specific region between 2 atoms i.e. solely between the shared pair of electrons and the nuclei of the bonding atoms

Carbon: 4 bonds Nitrogen: 3 bonds Oxygen: 2 bonds Hydrogen: 1 bond

A covalent bond is the overlap of atomic orbitals, each containing one electron, to give a shared pair of electrons. The shared pair of electrons is attracted to the nuclei of both bonded atoms The bonded atoms often have outer shells with the same electron structure as the nearest noble gas

Dative covalent or coordinate bond: A shared pair of electrons in which the bonded pair has been provided by one of the bonded atoms only.

Measurement of covalent bond strength. The larger the value, the stronger the bond.