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# Structure of Atom

## 07 Atomic Number and Mass Number

#### ATOMIC NUMBER

• The modern periodic table is arranged in such a way that all the elements have an increasing atomic number, and subsequently, increasing mass number.
• The number of protons and neutrons are associated with the atomic number and mass number of the atom. Atomic number of an element is the number of protons in one atom of an element. The atomic number of an element is denoted by the letter Z.
• All the atoms of a particular element have the same number of protons, and hence the same atomic number.
• Atoms of different elements have different atomic numbers.
• For example, all atoms of Carbon have the atomic number of 6, whereas all atoms of Oxygen have 8 protons in their nucleus.

#### MASS NUMBER

• Mass number is defined by the total number of protons and neutrons present in one atom of an element.
• For example, an atom of Carbon has 6 protons and 6 neutrons.
• Thus, its mass number is 12.
• While the number of protons remains the same in all atoms of an element, the number of neutrons can vary.
• Thus, atoms of the same element can have different mass numbers, and these are called isotopes.
• The weight of an electron is almost negligible.
• Thus, the atomic mass of an atom is almost the same as its mass number.
• It is represented as:

Mass number = No. of protons + No. of neutrons.

• The mass number of an element is denoted by the letter A. The mass number of an atom also gives us the atomic mass.

Mass number = Atomic mass

• Relationship Between Mass Number and Atomic Number

Mass number = No. of protons + No. of neutrons.
Mass number = Atomic number + No. of neutrons.

#### ARRANGEMENT OF ELECTRONS IN THE ATOMS

 Electron shell Maximum capacity K shell 2 electrons L shell 8 electrons M shell 18electrons N shell 32 electrons

Electronic Configurations of First Twenty Elements:

 Element Symbol Atomic Number Electronic configuration (or Electron distribution)K L M N 1. Hydrogen H 1 1 2. Helium He 2 2 3. Lithium Li 3 2, 1 4. Beryllium Be 4 2, 2 5. Boron B 5 2, 3 6. Carbon C 6 2, 4 7. Nitrogen N 7 2, 5 8. Oxygen O 8 2, 6 9. Fluorine F 9 2, 7 10. Neon Ne 10 2, 8 11. Sodium Na 11 2, 8, 1 12. Magnesium Mg 12 2, 8, 2 13. Aluminium Al 13 2, 8, 3 14. Silicon Si 14 2, 8, 4 15. Phosphorus P 15 2, 8, 5 16. Sulphur S 16 2, 8, 6 17. Chlorine Cl 17 2, 8, 7 18. Argon Ar 18 2, 8, 8 19. Potassium K 19 2, 8, 8, 1 20. Calcium Ca 20 2, 8, 8, 2