Welcome to Chem Zipper.com......: Atomic mass:

## Thursday, August 25, 2022

### Atomic mass:

(1) Atomic mass:
Atomic mass represent the mass of single isotopes or single atoms.

"Atomic mass and atomic weight do not mean the same:

Atomic mass is equal to the mass of total number of protons , neutrons  and electrons ( mass of electrons are negligible hence ignore)

Atomic mass = total mass of nucleons (total number of nucleons × mass of ons nucleons.

Atomic mass is independent of the atomic masses of isotopes.

Calculated  value of the atomic mass is whole number

Unit of atomic mass: the unit of atomic mass is atomic mass unit (amu) or unified atomic mass (u) and amu also known as Dalton(Da)

It is representation of unit of atomic weight is a.m.u. and  it is the smallest unit of mass and is define as;

We know that one mole (6.022x10^23) atoms C^12 atoms has  12 gm mass.

Hence mass of one C^12 atom is = 12/N_A=  12/6.022x 10^23 gm =1.99 x 10^-23 gm.

The real mass of one atom of C^12 has been determined as 1.9924 × 10^-23 gram.

Conclusively we can summarise as

1 amu is = mass of one nucleons (protons or  nutrons)

1 amu is = 1.67×10^27 kg

1 amu is = mass of one proton or nutron ( 1.67×10^27 kg )

(3) Gram-atomic weight: When atomic weights of elements are expressed in grams, they are called gram atomic weights.

Thus, the weight of an element in grams which is equal to its atomic weight is known as the gram atomic weight (GAW) of that element.

For example, atomic weight of sodium is 23 and its gram atomic weight is 23 grams. Similarly, the gram atomic weights of hydrogen, sulphur, nitrogen are 1.008 gram, 32 gram and 14 gram respectively.

Thus

1 GAW P = 31.0 gram phosphorus,

1 GAW K = 39.0 gram potassium,

1 GAW Ca = 40.0 gram calcium.

(4) Gram atom or mole atom: The atomic weight of an element taken in grams is one gram-atom or mole atom of that element.

For instance:

one gram atom of oxygen represents 16 gram oxygen.

One gram atom of sulphur represents 32 gram sulphur.

Number of gram atoms of an element "One gram atomic weight of every element contains 6.023 × 10^23 atoms of the element.”

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