OCR AS Chemistry (F321.2) - Electrons, Bonding and Structure Revision Notes

These revision notes are for Electrons, Bonding and Structure - the second module of F321 (Atoms, bonds and groups) from OCR AS Chemistry A.

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OCR AS Chemistry Notes
IONISATION ENERGIES
FIRST IONISATION ENERGY
Energy required to remove one electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous
1+ ions
o E.g. N a (g) Na+(g) + 1e-
o H i 1) = + 494 KJ /mol (endothermic)
SECOND IONISATION ENERGY
Energy required to remove one electron from each 1+ ion in 1 mole of gaseous ions to form 1 mole of gaseous 2+ ions
o Na+ (g) Na2+(g) + 1e-
o H i 2) = + 4564 KJ /mol (endothermic)
Notice: 2nd is higher than 1st
Ionisation energy depends on:
1. The ion
o More positive charge
more difficult to remove electron
2. Distance between nucleus and electron
o 2nd energy is higher than 1st because the 1st electron is further away from nucleus than second one
attraction is less and less energy is needed to remove electron
3. Shielding effect
o Shielding electrons are complete shells of electrons between the nucleus and the electron to be removed
o The inner shells repel the outer electrons, so the more inner shells
stronger shielding and less energy needed to remove electron
ELECTRON ENERGY LEVELS
Energy levels = electronic shells
Energy levels given numbers (=principle quantum numbers)
o E.g. n=1 / n=2 etc
Provide 2 pieces of information:
o Total number of electrons in atom (number of separate ionisation energies recorded
o Number of electron shells present in atom
Example for Be (Beryllium):
IONISATION KJmol-1
ENERGY
1st 900 2 electrons easy to
remove
2nd 1758
3rd 14905 2 electrons hard to
remove
4th 21060
Conclusion: Be has 2 shells
o Energy level n=1 contains low energy electrons (take more energy to remove)

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Energy level n=2 has higher energy electrons (further from nucleus less energy to remove)
Electrons represented as arrows
When energy levels are being filled, electrons pair up
o Electrons in each pair spin in opposite directions
o The spin generates magnetic field
o As they spin in opposite directions, a magnetic attraction develops
o This overcomes the electrostatic repulsion between 2 (negative) electrons
EVIDENCE FOR SUB-SHELLS
Main energy levels divided into sub-levels
Principal quantum number Number of
(shell) sub-shells
1 1 (s)
2 2 (s,p)…read more

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For n=2 (max 8 electrons) 2s2 + 2p6 (2px + 2py + 2pz)
o (1 orbital + 3 orbitals = 4 orbitals)
For n=3 (max 18 electrons) 3s2 + 3p6 + 3d10
o (1 orbital + 3 orbitals + 5 orbitals = 9 orbitals)
For n=4 (max 32 electrons) 4s2 + 4p6 + 3d10 + 4f14
o (1 orbital + 3 orbitals + 5 orbitals + 7 orbitals)
OCR AS Chemistry Notes
WORKING OUT ELECTRONIC STRUCTURE OF ATOMS
RULES
1) ELECTRONS FILL UP THE LOWEST…read more

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NEON:
1s2 2s2 2p6 2p
2s
1s
=Noble gas core [Ne]
ALUMINIUM:
1s2 2s2 2p6 3s2 3p1 3p
=[Ne] 3s2 3p1 3s
2p
2s
1s
ARGON:
1s2 2s2 2p6 3s2 3p6
[Ne] 3s2 3p6
=Noble gas core [Ar]
IRON:
1s2 2s2 2p6 3s2 3p6 4s2 3d6
[Ar] 4s2 3d6
KRYPTON:
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6
[Ar] 4s2 3d10 4p6
=Noble gas core [Kr]
EXCEPTIONS
CHROMIUM:
1s2 2s2 2p6 3s2 3p6 4s1 3d5
4s
3d
etc
COPPER:
1s2 2s2 2p6 3s2 3p6 4s1…read more

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PERIODIC TABLE + STRUCTURE OF ATOMS
BLOCKS
Periodic table is divided into blocks
o s-block (groups 1+2)
Outermost electron is placed on
s-orbital
o p-block (groups 3-8)
Outermost electron is placed on
p-orbital
o d-block (transition metals)
Outermost electron is placed on
d-orbital
o f-block (inner transition metals: lanthanides + actinides)
Outermost electron is placed on f-orbital
IONISATION ENERGIES AND TRENDS
As you go down the group ionisation energy decreases
o Higher shielding effect (more complete inner shells)
o Distance from nucleus increases
This overcomes…read more

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Going across the period from group 5 to group 6
NITROGEN: 2p
2s
1s
OXYGEN: 2p
2s
o Group 6 lose the first electron from a paired orbital
The repulsion between those paired electrons gives the electron more energy so it is easier to
remove
EXPLAINING IONISATION ENERGY GROUPS
Example: 8OXYGEN = 1s2 2s2 2p4 2p
2s
1s…read more

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The first 4 electrons are the easiest to remove (similar ionisation energy)
o But the first one is much easier to remove than the remaining 3
OCR AS Chemistry
IONIC BONDING
PROPERTIES OF IONIC COMPOUNDS
Conduct when molten or dissolved
Crystalline structures
Very high melting point
o Require a lot of energy to overcome the strong electrostatic forces between oppositely charged ions
o A lot of these forces in a giant lattice
Soluble in polar solvents such as water
o Energy is released when water…read more

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They achieve noble gas configuration
GIANT IONIC STRUCTURE (LATTICE)
They do not form molecules as electrostatic force acts in all directions
Exist as giant ionic structures
Different types of lattice
Cubic lattice
Each Na+ surrounded by 6 Cl- ions
Each Cl- surrounded by 6 Na+
NaCl lattice has 6:6 coordination
Na+ and Cl- have a coordination number of 6
Cubic lattice
Cl- and Cs+ coordination is also 8
Lattice has 8:8 coordination
WHAT FACTORS AFFECT WHETHER AN ATOM FORMS AN ANION OR CATION?
Metals form…read more

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When 2 p orbitals overlap, a pi () orbital bond forms
A and orbital make up a double covalent bond
Triple Covalent Bond
3 pairs of electrons shared
Coordinative (Dative Covalent) Bonding
Some atoms in molecules have pairs of electrons on the outer shell not involved in bonding
o These are lone pairs
If a molecule has a lone pair it can be used to make a bond with an atom/ion or another molecule
o Such bonds have both electrons which come from the same…read more

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Hydrogen ­ its outer shell can hold max 2 electrons
GIANT COVALENT STRUCTURES
Elements which can form 4 covalent bonds (e.g. C or Si) often form giant covalent structure
o E.g.…read more

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