Atomic weight or relative average Atomic mass:

Atom: The smallest particle of an element which cannot exist in free form is called an atom.

Atoms have a tendency to exist as clusters (group of atoms or molecules) of same or different atoms. They take part in a chemical reaction without undergoing any decomposition. 

An atom is very small (atomic size of the order 10^–6 cm) and its real mass is very low.

The smallest and lightest atom is of hydrogen. The mass of the hydrogen atom is 1.008 a.m.u.

Weights of atoms of other elements are in the range of 2 to 260 a.m.u.

Atomic weight: The atomic weight of an element is a number which indicates how many times heavier is an average atom of that element as compared with 1/12  of the mass of an atom of carbon (12) (C^12).

* Atomic weights of elements are a absolute quantity and absolute quantity having an unit average relative weights.

* Both atomic weight and molecular weight are unit less quantities. These quantities are also regarded as relative atomic mass and relative molecular mass.

Average (relative) atomic mass: 

Most often the atomic weight of elements are fractional. It is due to the existence of isotopes of the elements which are atoms with different masses. 

Since an element is a mixture of all of its isotopes, the atomic weight of an element is the average of weights of all its isotopes. So, atomic weight have non –integral (fractional) values.

"Atomic weight (Average atomic weight or relative atomic mass) can be define as average weight of an elements with respect to all its isotopes and their relative abundance.

Calculation of average (relative) atomic mass: 

(A) If an elements exists in two isotopes having atomic masses ‘M1’ and ‘M2’ in the ratio  X: Y, Then   

                         or

(B) If an elements exists in two isotopes having atomic masses ‘M1’ and ‘M2’  and their abundant  percentage (%)  are  X % and Y % then 

(C) the calculated value may be or may not be a whole number.

Exercise: (1) Find the average atomic weight of mixture of containing 25% by mol Cl^35 and 75% by mol Cl^37.

Ans key: Cl^35 = 75%, Cl^37 = 35%

Exercise: (2) Boron have two isotopes B^11 and B^A and their relative abundant %  are 80% and 20% respectively and average atomic weight of Boron is 10.8 find the value of  “A”

Ans key: A = 10

Exercise: (3) Silver contains two isotopes Ag^107 and Ag^109 and average atomic weight of silver is 108.5 find percentage abundant of  isotopes of silver.

Ans key: Ag^107 = 25%, Ag^109 = 75%

Exercise: (4) Calculate the % abundance of Ag^109  it is know that silver contains two isotopes Ag^107 and Ag^109 and average atomic weight of silver is 108.5.

 Ans key: Ag^107 = 25%, Ag^109 = 75%

Exercise: (5) An element (X) have three isotopes X^20, X^21 and X^22. The percentage abundance of X^20 is 90% and it's average atomic mass is of element is 20.18. the percentage of abundance of X^21 should be.

Ans key: X^21 = 2%, X^22 = 8%

Exercise: (5) Calculate the average atomic weight of an element (X) containing three isotopes X56, X57 and X59 and natural occurrence of these isotopes are 90%, 8% and 2% respectively.

Other topic:

(1) What is the % composition of each element in (Mg(OH)2) magnesium hydroxide?

(2) Haemoglobin contains 0.33% of Iron by weight. The molecular weight of it is approx. 67200. The numbers of iron atoms (Atomic wt of Fe=56 u) present in one molecule of Haemoglobin are?

(3) The hydrated salt, Na2SO4.nH2O undergoes 55.9% loss in weight on heating and becomes anhydrous. The value of n will be?

(4) Air contain 20 % O2 by volume. How many cm³ of air will be required for oxidation of 100 cm³(ml) of acetylene?