Chemistry OCR AS Mod 2

When working out an element from ionisation energy, find a large jump. The bigger value of the two is a new shell CLOSER to the nucleus. Thus you can find how many electrons the element has in its outer shell which is its group number.

ACROSS A PERIOD

Ionisation energy INCREASES because:

• nuclear charge INCREASES - more protons in the nucleus
• atomic radius DECREASES - increased nuclear charge so more attraction
• shielding REMAINS THE SAME - since the same period mean its in the same shell

Therefore MORE ENERGY is required to loose an electron

DOWN A GROUP

Ionisation Energy DECREASES because:

• Atomic radius INCREASES - another shell is added
• shielding INCREASES - more shells between the outer electrons and the nucleus
• Which both outweigh th INCREASE in nuclear charge

Therefore LESS ENERGY is required to loose an electron

DEFINITIONS

A Covalent Bond is a shared pair of electrons

A Dative Covalent Bond (or coordinate bond) is a shared pair of electrons which has been provided by one of the bonding atoms only.

Electronegativity is the ability of an atom to attract the bonding electrons in a covalent bond

A Lone Pair is an outershell pair of electrons that are not involved in chemical bonding.

EXPANSION OF THE OCTECT

Group 5 atoms can form 3 or 5 covalent bonds

Group 6 elements can form 2,4 or 6 covalent bonds

Group 7 elements can form 1,3,5 or 7 covalent bonds

Electronegativity is the ability of an atom to attract the bonding electrons in a covalent bond.

Fluroine is the most electronegative element

In a covalent bond the electrons are shared equally

The GREATER the SIMILARITY in the electronegativities of the bonding atoms the GREATER the COVALENT character of the bond

A POLAR COVALENT BOND is a covalent bond which has a permanent dipole and whoose electrons are unequally shared.

A polar molecule has an overall DIPOLE when any dipoles across the bonds are taken into account

PERMANENT DIPOLE is small charge difference across a bond that results from a difference in the elctronegativities of the bonded atoms

HCl has a polar bond and is a polar molecule since it is NON SYMMETRICAL and there is a CHARGE DIFFERENCE over the entire molecule

An IONIC BOND is the electrostatic attraction between oppositely charged ions

In an electrons are tranferred from the metal ion to the non metal ion.

• Oppositely charged ions are formed which are bonded together by electrostatic forces
• METAL ION = POSITIVE (cation)
• NON METAL ION = NEGATIVE (anion)

A giant ionic lattice is a 3D structure of oppositely charged ions held together by strong ionic bonds

Atoms of metals in group  1-3 LOOSE electrons to form positive ions with the electronic configuration of the previous noble gas in the periodic table

Atoms of non metals in groups 1-7 GAIN electrons to form negative ions with the electronic configuration of the next noble gas in the periodic table

Atoms of Be, B, C and Si do not normally form ions as TOO MUCH ENERGY IS REQUIRED to tranfer outer shell electrons

Charge on group 1 metal = +1

Charge on group 2 metal = +2

Charge on Aluminium = +3

Charge on hydrogen = +1

Charge on a group 6 non metal = -2

Charge of a group 7 non metal = -1

Charge on a Hydroxide ion = OH-

Charge on a sulfate ion =SO4 ^2-

Charge on a Nitrate ion =NO3-

Charge of a carbonate ion =CO3 ^2-

Charge on Phosphate ion=PO4 ^3-

Charge on a Nitrogen ion=N^3-

Charge on a Hydride ion= H-

Charge on a ammonium cation= NH4+

3 electron pairs = Triagonal Planar (120 degree angle)

4 electron pairs = Tetrahedral (109.5)

5 electron pairs = Triagonal Bipyramid 

6 electron pairs = Octahedral

1 Lone pair = Pyramidal (107)

2 Lone Pairs = Non linear (104.5)

Van Der Waal forces.

Oscillating electrons in one atom/molecule causes an instantaneous dipole resulting in an individual dipole in a neighbouring molecule.

Weak intermolecular attraction between small temporary dipoles

Going down group 7 = volatility decreases = more electrons = stronger val der waal forces

The electrostatic attraction between d+ hydrogen atoms and a lone pair on a d- oxygen atom occurs in ice. The structure of ice is an open network of water molecules with each oxygen molecule covalentl bonded to two hydrogen atoms