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Atomic Structure

Fundamental particles

Atoms of the same element always have the same number of protons.
Isotopes are atoms of the same element but with different number of neutrons.
This gives rise to different mass numbers.
Relative abundance is the amount of each isotope as the percentage for that…

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Arrangement of Electrons

Energy Levels or Shells
The simplest model of electrons has them orbiting in shells around the nucleus. Each successive shell is
further from the nucleus and has a greater energy.
Sub Shells and Orbitals
This model can be further refined by the concept of sub shells and…

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placed in the 2s.
Beryllium (Z = 4) It has four electrons. The fourth can also go in 2s (but remember about the opposing
Boron (Z = 5) It has five electrons. The fifth one must go in 2p. There are three 2p orbitals available and

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We might expect [Ar]4s23d4 but instead we get [Ar]4s13d5
This is because of the special stability of the halffilled set of 3d orbitals. An electron promoted from the 4s
to the 3d. Although this requires energy, this is paid back by the extra stability.
Manganese (Z = 25) The gap…

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The electron to be removed from the group III element has to come from a porbital. This is higher in
energy than the sorbital of the same main energy level. So removal of this electron is easier and so the
ionisation is less.
The dip between Group V and VI…

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Shapes of Molecules
The shape of a molecule is determined by the number of electron pairs in its outside shell and whether these
electron pairs are bonding or nonbonding.
The Valence Shell Electron Pair Repulsion (VSEPR) theory is used to explain this.
Think only about the central atom of a…

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What about if I can't decide which shape?
There are some molecules or ions whose shapes cannot be predicted by this simple method. If this is the
case for a question that is set in an examination, credit will be given for you providing a SENSIBLE answer
based on the…

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Amount of Substance
Different atoms have different masses.
1g of carbon has far fewer atoms than 1g of hydrogen atoms.
Chemists need a method of quantifying atoms.
We use a quantity called the amount of substance which is measured in moles.
One mole contains 6.02 x 1023 particles.
The relative…

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1. P2O5
2. NO2
3. CuSO9H10

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Covalent Bonding
The atoms are held together by one or more shared pairs of electrons. The electrons come from the highest
energy level of each atom. One pair of electrons makes a single bond, two pairs make a double bond (such
as in alkenes or carbon dioxide etc), three…


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