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  Chemistry (NTSE/Olympiad)  

3. Atoms & Molecules

Atomic mass

Atoms are extremely small ; the heaviest atoms have masses of about 10–22g. Even an ultramicrobalance cannot measure the mass of a single atom. However, relative masses of atoms of different elements can be determined. At first, the mass of the lightest atom, hydrogen.
In 1961, International Union of chemists selected the most stable isotope of carbon (C - 12 isotope) as standard for comparison the atomic masses of various elements. Atomic mass of an element tells us the number of times an atom of the element is heavier than of the mass of an atom of carbon - 12.
Atomic mass of an element may be defined as the average relative mass of an atom of the element as compared with mass of an atom of carbon (C - 12 isotope) taken as 12 amu.

Example. The atomic mass of magnesium is 24 u which indicates that one atom of magnesium is 24 times heavier than 1/12 of a carbon 12 atom.
  Element Symbol Atomic mass
1 Hydrogen H 1 u
2 Carbon C 12 u
3 Nitrogen N 14 u
4 Oxygen O 16 u
5 Sodium Na 23 u
6 Magnesium Mg 24 u
7 Aluminium Al 27 u
8 Phosphorus P 31 u
9 Sulphur S 32 u
10 Chlorine Cl 35.5 u
11 Potassium K 39 u
12 Calcium Ca 40 u
13 Iron Fe 56 u
14 Copper Cu 63.5 u

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