AS AQA Chemistry

Revision for some of the topics in Unit 1 for AQA AS Level Chemistry

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  • Created on: 28-11-09 12:50
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Atomic Structure
Fundamental particles
Isotopes
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 element occurring on the Earth
Mass Spectrometer

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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 orbitals.
Sub shells are known by letters s, p, d, and f. The s sub shell can contain 2 electrons, p 6, d 10 and f 14.…read more

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Beryllium (Z = 4) It has four electrons. The fourth can also go in 2s (but remember about the opposing
spins)
1s22s2
Boron (Z = 5) It has five electrons. The fifth one must go in 2p. There are three 2p orbitals available and
these are identified by their direction (px, py, pz). It doesn't matter which of these is chosen when there is
only one electron in one of these orbitals.
1s22s22p1
Carbon (Z = 6) It has six electrons.…read more

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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.…read more

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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
The electron to be removed from the group VI element comes from a porbital that has two electrons.
These are repelled slightly more than the electrons in a porbital with only one electron.…read more

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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 molecule eg the C of CH4.
Spread the electron pairs of its outside shell around the surface of a sphere. They should be spread out as
far as possible.…read more

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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 principles discussed above. Remember that you need to know the shapes of some molecules
and deduce good estimates of the shapes of others.…read more

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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 atomic mass is the mass of one mole of that element's atoms.…read more

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Answers
1. P2O5
2. NO2
3.…read more

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Bonding
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 pairs make a triple bond (as in alkynes or in a nitrogen
molecule)
Ionic Bonding
Atoms lose or gain electrons to attain a complete outer shell of electrons. The charged atoms are called
ions.…read more

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