Atoms, Bonds and Groups OCR Chemisty notes

Atoms, Bonds and Groups for OCR AS Level

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Module 2: Electrons, Bonding and Structure
Spread 1: Evidence For Shells:
Key Definitions:
First Ionisation Energy: The energy required to remove one electron from each atom, in one
mole of gaseous atoms, to form one mole of gaseous 1+ ions.
Factors Affecting Ionisation Energy:
Atomic Radius: A larger atomic radius is the result of a larger number of shells. A larger atomic
radius weaker nuclear attraction lower ionisation energy.
Nuclear Charge: A larger nuclear charge is the result of a larger number of protons. A larger
nuclear charge stronger nuclear attraction higher ionisation energy.
Electron Shielding: A larger shielding effect is the result of an increased number of inner
shells. More electron shielding weaker nuclear attraction lower ionisation energy.
Successive Ionisation Energies:
1st Ionisation energy:
Li(g) Li+(g) + e-
2nd Ionisation energy:
Li+(g) Li2+(g) + e-
3rd Ionisation energy:
Li2+(g) Li3+(g) + e-

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The graph above helps provide evidence for shells, due to successive ionisation energies.
The big jump in ionisation energies from the 6th to 7th electron, indicates that the last two
electrons have been removed from a different shell, that is closer to the nucleus. Thus, this element
has six outer shell electrons and must be in group 6.…read more

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Spread 2: Shells and Orbitals:
Key Definitions:
Orbital: A region within an atom that can hold up to two electrons, with opposite spins.
N Shell Electrons
1 1st Shell 2
2 2nd Shell 8
3 3rd Shell 18
4 4th Shell 32
Top Tip: Use the formula (2n²) work out the number of electrons in a shell. (n) being
the shell number.
Each shell is made up of atomic orbitals. Each atomic orbital can hold up to 2 electrons.…read more

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From the second shell onwards, each shell contains three p-orbitals. This gives 3x2 = 6
The shape of a p-orbital is a dumb-bell.
D-Orbitals and F-Orbitals:
From the third shell onwards, each shell contains five d-orbitals. This gives 5x2 = 10 electrons.
From the fourth shell onwards, each shell contains seven f-orbitals. This gives 7x2 = 14
electrons.…read more

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Spread 3&4: Sub-shells and Energy Levels:
Electrons always fill orbitals from lowest energy level to highest energy level.
Electrons are always removed from the highest energy level first.
The 4s orbital has a slightly lower energy level than the 3d orbital, so is filled first by electrons.
The 4s and 3d orbitals are so close together that, after the 4s- orbital has been filled, it is
actually at a slightly higher energy level than the 3d level.…read more

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By using this pattern in the periodic table, we can easily find the electron configuration of an
Oxygen is the 4th element in the 2p block. Therefore, the electron configuration would be
If an element has it's highest energy electron in a p subshell, it can be referred to as a p-block
element.…read more

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Spread 5: Chemical Bonding:
The noble gases make up group zero/eight pf the periodic table. They are extremely stable and
this is why they are so unreactive.
The tendency to acquire a noble gas electron configuration, is called the octet rule. Elements
try to achieve this through sharing or transferring electrons to form a chemical bond, with other
Ionic Bonding:
Ionic bonding occurs in compounds consisting of a metal and a non-metal.…read more

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Metallic bonding occurs in compounds consisting of two metals.
Electrons are shared between all the atoms.
Positive metal ions are held in fixed lattice by delocalised electrons.…read more

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Ammonium: NH4+ Hydroxide: OH- Carbonate: CO32- Phosphate: PO43-
Nitrate: NO3- Sulfate: SO42-
Nitrite: NO2- Sulfite: S032-
HCO3-…read more

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Spread 10: Shapes of Molecules and Ions:
Electron-Pair Repulsion Theory:
Each electron pair repels other electron pairs.
The electron pairs push one another as far apart as possible.
Lone pairs repel slightly more strongly than normal bonding electron pairs.
Molecule BF3 CH4 SF6
Number of bonding pairs 3 4 6
around central atom
Bond angle 120 109.5 90
Name of shape Trigonal Tetrahedral Octahedral
Assuming that there are no lone pairs:
3 bonding pairs 120 Trigonal Planar
4 bonding pairs 109.…read more


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