Thursday, January 29, 2009

igcse chemistry notes - atomic structure

The structure of an atom
Particle inside the atom


Atomic number, mass number and relative atomic mass

Atomic number is equal to the number of protons or electrons in an atom.

Mass number is equal to the number of neutrons plus the number of protons in an atom.

Relative atomic mass is the average mass of atom of an element compared to a standard.


Electronic structures of the first twenty elements in the periodic table
Put electrons in shells so that the first is filled before the second etc and
the first shell has a maximum of 2 electrons
the second shell has a maximum of 8 electrons
and the third shell has a maximum of 8 electrons

Electronic structure of elements
1 Hydrogen 1

11 Sodium 2,8,1

2 Helium 2
12 Magnesium 2,8,2
3 Lithium 2,1
13 Aluminium 2,8,3
4 Beryllium 2,2
14 Silicon 2,8,4
5 Boron 2,3
15 Phosphorous 2,8,5
6 Carbon 2,4
16 Sulphur 2,8,6
7 Nitrogen 2,5
17 Chlorine 2,8,7
8 Oxygen 2,6
18 Argon 2,8,8
9 Fluorine 2,7

19 Potassium 2,8,8,1

10 Neon 2,8

20 Calcium 2,8,8,2
make up your own atom


Isotopes are atoms of the same element which have different numbers of neutrons in their nuclei. As these are the same element the atoms all have the same number of protons. For example hydrogen has 3 isotopes. Each atom has 1 proton but a different number of neutrons.
Relative atomic mass of an element

Use relative mass of isotopes and their relative abundance. E.g. Chlorine has two
isotopes with mass numbers 35 and 37.

35 37

75% is Cl,25% is Cl

17 17
Let there be 100 atoms
Total mass of 100 atoms = (75 * 35) + (25 * 37) = 3550
Average mass of an atom (relative atomic mass of chlorine) = Total mass /Number of atoms =3550/100
so relative atomic mass of chlorine = 35.5

home work
draw the electron arrangement of


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