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OCR CHEMISTRY A

Notes for Topic 1

1.1.1 Atoms
1.2.1 Electron Structure
1.3.1 Periodicity




























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1.1.1 ATOMS

a) Protons, neutrons and electrons

Atoms are made up of three fundamental particles: protons, neutrons and electrons .

Protons and neutrons are found in the nucleus and are collectively called nucleons .
Electrons orbit the nucleus in a similar way to that in which planets orbit a sun.…

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Not all atoms of the same element have equal numbers of neutrons this may vary
slightly. The sum of the number of protons and neutrons in the nucleus of an atom is
called its mass number . It is represented by the symbol A.

The mass number is the sum…

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b) Relative atomic mass


The mass of an atom is measured in atomic mass units, where one unit is 12th of the
mass of one atom of carbon12.

The relative isotopic mass of an isotope is the ratio of the mass of one atom of that
th
isotope to 1/12…

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The relative atomic mass can be calculated by the formula:
(perentage abundance of each isotope x mass of each isotope)
100

eg Neon consists of 90% neon20 and 10% neon22. What is its relative atomic mass?

ram = (90 x 20 + 10 x 22)/100 = 20.2

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1.2.1 ELECTRON STRUCTURE

a) Energy levels

Electrons do not orbit the nucleus randomly they occupy certain fixed energy levels.
Each atom has its own unique set of energy levels, which are difficult to calculate but
which depend on the number of protons and electrons in the atom.

Energy levels in…

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An sorbital in the first energy level is a 1s orbital.
An sorbital in the second energy level is a 2s orbital, etc



porbitals: these are shaped like a 3D figure of eight. They exist in groups of three:







Every energy level except the first level contains three porbitals. Each…

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Since the different sublevels have different energies, and the energies of the different
levels get closer together with increasing energy level number, the high energy
sublevels of some energy levels soon overlap with the low energy sublevels of
higher energy levels, resulting in a more complex energy level diagram:







Starting…

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It is thus possible to calculate the maximum possible number of electrons in each
sublevel, and thus in each energy level:

Energy Level Number of electrons in each sublevel Max. no of electrons
1 2 x 1s 2
2 2 x 2s, 6 x 2p 8
3 2 x 3s,…

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The electronic configuration of the first 18 elements using the arrow in box method is
as follows:

1s 2s 2p 3s 3p
H

He

Li

Be

B

C

N

O

F

Ne

Na

Mg

Al

Si

P

S

Cl

Ar


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