AS OCR Chemistry Unit 1

AS Chemistry – Revision Notes

Unit 1 – Atomic Structure, Bonding And Periodicity

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First 693 words of the document: AS-Level Revision Notes
AS Chemistry ­ Revision Notes
Unit 1 ­ Atomic Structure, Bonding And Periodicity
Atomic Structure
1. All atoms have a mass number, A (the number of nucleons), and a proton number, Z (the number
of protons).
2. Isotopes have different numbers of neutrons, and have different physical properties but the same
chemical properties.
3. In a mass spectrometer:
a. Ionisation ­ The vaporised sample is passed through an electron beam, from an electron
gun, forming cations.
b. Acceleration ­ The cations are attracted to negatively charged plates, passing through a
small hole to focus the beam.
c. Deflection ­ The magnetic field of an electromagnet deflects the beam, so lighter ions are
deflected more. The ions passing out of the electromagnet will be of the same mass.
d. Detection ­ The ions reaching the detector are counted, to give the relative abundance of
each isotope.
the average mass of an atom of the element
4. Relative atomic mass (A r ) = 1
of the mass of a carbon - 12 atom
5. The average mass can be determined from a mass spectrum ­ the peaks show the relative
abundances of each isotope, which can be multiplied by the mass of each to give the average mass.
6. Electron shells are arranged into sub-orbitals (s, p, d and f), each of which can contain two
electrons spinning in opposite directions due to the Pauli exclusion principle.
7. When filling orbitals:
a. The lowest energy orbitals will be filled first (e.g. 1s before 2s).
b. Orbitals of the same type (e.g. p) will only pair up electrons after there is one electron in
each (e.g. you will get 2p 1 1 1 2 1 0
x 2p y 2p z rather than 2p x 2p y 2p z ) ­ the Aufau principle.
c. The 4s orbital will always fill before the 3d orbital.
d. When forming ions, electrons will be removed from the 4s orbital before the 3d orbital.
e. For chromium and copper, an electron will be taken from the 4s orbital and placed in the
3d orbital to make half-full or full 3d orbitals respectively.
8. The first ionisation energy is the energy required to remove one mole of electrons from one mole
of atoms in a gaseous state ­ X( g ) X + ( g ) + e - .
9. With successive ionisation energies, big increases in energy will occur between electron shells
because the shell closer to the nucleus will have a greater attraction for the electrons, and less
shielding from complete shells.
10. Ionisation energy trends can be explained by:
a. The distance from the nucleus.
b. The nuclear charge / attraction.
c. The amount of shielding.
11. Going down a group, the first ionisation energy will always decrease because the electron is
further from the nucleus, and has more shielding, even for the increase in nuclear charge.
12. Going across a period (Li to Ne):
a. Overall increase in ionisation energy due to the increase in nuclear charge for the same
distance from the nucleus.
b. Drop from Be to B due to shielding from full 2s orbital.
c. Drop from N to O due to electron repulsion when p electrons pair up (easier to remove).
1. An ionic bond is the attraction between ions, creating a giant ionic lattice. Ions are formed by
gaining or losing electrons to create a stable octet (the octet rule).
2. Most elements that become ions do so by becoming isoelectronic with a noble gas.
3. The strongest bonds (most reactive elements) are formed between the bottom of group I / II and
the top of group VI / VII.
4. Cations will become slightly smaller and anions will become slightly bigger than their neutral
atoms (e.g. Na+ is smaller than Na; Cl­ is bigger than Cl).
5. X-Ray diffraction can be used to show the structure of an ionic compound.
6. The coordination number tells you how many ions can surround each other ion:

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AS-Level Revision Notes
a. 6:6 coordination number if one ion is much smaller than the other (e.g. NaCl)
b. 8:8 coordination number if both ions are roughly the same size (e.g. CsCl)
7. A unit cell is the smallest unit of structure that has all the features of the lattice, and can build up
the lattice by simple repetition.
8. A covalent bond is the sharing of a pair of electrons.…read more

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AS-Level Revision Notes
d. Insoluble in water.
22. Properties of metallic structure:
a. High melting/boiling point due to strong attraction between electrons and cations.
b. High electrical and thermal conductivity.
c. High density, very strong, ductile and malleable.
23. In order to change state, a certain amount of energy is needed:
a. The energy needed to melt a solid at its melting point is the enthalpy of fusion.
b. The energy needed to vaporise a liquid at its boiling point is the enthalpy of vaporisation.…read more

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5. Electrical conductivity across period 3:
a. This increases from Na to Al, because the number of delocalised electrons able to carry
current increases, thus increasing the ability to conduct electricity.
b. The elements Si to Ar have close to zero electrical conductivity, because they bond
covalently, so the electrons are not free to conduct electricity.
6. Melting and boiling points across period 3:
a. These are linked to the bond strength and structure of the elements.
b.…read more

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AS-Level Revision Notes
d. The acid is added to remove carbonate ions which would form barium carbonate (also
2H + (aq) + CO 3 2-
(aq) CO 2 (g ) + H 2 O(l)
12. The atypical nature of beryllium:
a. The chlorides of other group II elements are ionic, but for beryllium, it forms a linear
covalent molecule. This is because of the high-density charge on the small beryllium ion,
which attracts the electron cloud between the two ions, creating a covalent bond.…read more


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