# Concepts of elements atoms and molecules

Atom is the smallest constituent unit of the matter that has properties of a chemical element. Every matter which can be solid, liquid,gas, and plasma is composed of neutral or ionized atoms. Atoms are very small in size around 100 pm (a ten-billionth of a meter), there is many ways to define their size that give different but close values.

Every atom is composed of a nucleus and electrons bound to the nucleus. The nucleus is made of protons and neutrons having same counting(number of neutrons = number of neutrons). Protons and neutrons are called nucleons. About 99.94% of an atom's mass is in the nucleus. The protons got positive electric charge and the electrons got negative electric charge whereas neutrons have no electric charge. In case where(number of protons = number of electrons) Than atom is electrically neutral. The carge is depends number of electrons and protons, if electraons are more than protons then it will have negative carge or if protons are more than electraons then it will have positive charge, it is called an ion.

### Atomic Mass:

One atomic mass unit is defined as a mass exactly equal to one-twelth the mass of ane carbon-12 atom and 1 amu = 1.66056x10-24g
Mass of an atom of hydrogen = 1.6736x10-24g, Thus in term of amu , the mass of hydrogen = 1.0080 amu
Similarly,the mass of oxygen -16 atom would be 15.995amu.

'amu' has been replaced by 'u' which is known as unified mass

### Average Atomic Mass:

Isotobe     Relative Abundance(%)    Atomic mass(amu)
12-atom of carbon    98.892    12
13-atom of carbon    1.108    13.00335
14-atom of atom    2x10-10    14.00317

The average atomic mass= (0.98892)(12u)+(0.01108)(13.00335u)+(2x10-12)(14.00317u)=12.011u

### Molecular Mass:

Example,
molecular mass of mathane which contains can be obtained as follows:
Molecular mass of methane,
(CH4)=(12.011u)+4(1.008u) =16.043u
Similarly, molecular mass of water (H2O)
=2 x atomic mass of hyderogen +1 x atomic mass of Oxygen
=2(1.008u)+16.00u
=18.02u

### Formula Mass:

The formula mass of sodium chloride=
atomic mass of sodium + atomic mass of chlorine
23.0u + 35.5 u = 58.5 u