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Basic Definitions: Atomic number, Mass number, Relative atomic mass, Atomic mass unit , Empirical formula and Molecular formula (Part 2)

Basic Definitions: Atomic number,  Mass number,  Relative atomic mass, Atomic mass unit ,  Empirical formula and Molecular formula (Part 2)
Question1.2: What are following terms:
a. Atomic number 
b. Mass number
c. Relative atomic mass
d. Atomic mass unit
e. Empirical formula
f. Molecular formula
Answer.

a. Atomic Number: 
The number of protons in the nucleus of an atom is called its atomic number. For example, there is only one proton in the nucleus of H-atom; so its atomic number is one. All the atoms of an element are same and have same number of protons and hence the same atomic number. Similarly, atomic number of He is 2 amu. 

b. Mass number: 
The total number of protons and neutrons in the nucleus of an atom is known as its mass number. For example, mass number of C-12 is 12 (as it has six protons and six neutrons).Similarly, mass number of N-14 is 14 (because it has seven protons and seven neutrons) No. of neutrons = mass number – atomic number 

Example 1.2: 
Atomic number of an element is 17 and mass number is 35. How many protons and neutrons are in the nucleus of an atom of this element? 
Solution: 
Number of protons = atomic number = 17 Number of neutrons = mass number – atomic number = 35-17 = 18 

c.Relative Atomic Mass:
Relative atomic mass may be defined as “the mass of an atom of an element relative to the mass of an atom of C-12 is called its relative atomic mass”.

d. Atomic mass unit: 
One atomic mass unit (amu) may be defined as a mass exactly equal to one-twelfth the mass of one C-12 atom. Mass of one C-12 atom = 12 amu (1/12) × Mass of one C-12 atom = 1 amu 1. 

Empirical Formula: 
The chemical formula that gives the simplest whole-number ratio of atoms of each element is called empirical formula. For example in hydrogen peroxide the simplest ratio of hydrogen to oxygen is 1: 1. So the empirical formula of hydrogen peroxide is written as HO. The simplest ratio between C, H and O atoms in glucose is 1 : 2 : 1. So, empirical formula of glucose is CH2O.

Molecular Formula: 
A molecular formula gives the actual number atoms of each element present in a compound. For example there are actually two H atoms and two O atoms in each molecule of hydrogen peroxide. So molecular formula of hydrogen peroxide is H2O2. The actual number of C, H and O atoms in a glucose molecule are 6 ,12 and 6, respectively. So, the molecular formula of glucose is C6H12O6.


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