Class 9, Science, Chapter-4, Lecture-1, Electronic Configuration (Notes)
Bohr & Bury Electronic Configuration Rules:
- The maximum number of electrons present in a shell is given by ${\rm{2}}{{\rm{n}}^{\rm{2}}}$, where is the orbit number.
- The maximum number of electrons that can be accommodated in the outermost orbit is 8.
- The maximum number of electrons that can be accommodated in the penultimate orbit is 18.
- Electrons are not accommodated in a given shell, unless the inner shells are filled.
VALENCY:
The number of electrons that can be lost or gained or shared to get stable configurations is called the valency of atom.
Valence Electrons:
The electrons present in the outermost shell of an atom are termed as the Valence Electrons.
Relationship between Valence Electrons and Valency:
Number of Valence Electron |
Valency |
---|---|
Less than or equal to 4 |
Equal to the number of valence electrons |
Greater than 4 |
${\rm{8 - number~of~valence~electrons}}$ |
Atomic Number:
The number of protons present in the nucleus of an atom of an element is termed as its atomic number.
OR
The number of electrons present in the extra nuclear part of a neutral atom of an element is termed as its atomic number.
- It is denoted by Z
Mass Number:
The sum of the total number of protons and neutrons present in the nucleus of an atom of an element is termed as its mass number.
OR
The number of nucleons present in the nucleus of an element is termed as its mass number.
- It is denoted by A
Atomic Notation:
If X is the symbol of an element, it is denoted as ${}_{{\rm{Atomic Number}}}^{{\rm{Mass Number}}}{\rm{X}}$
Isotopes:
Atoms of an element, having the same atomic number but different mass numbers are called the isotopes of the element.
Isotopes of Hydrogen:
Isotopes |
Protium |
Deuterium |
Tritium |
---|---|---|---|
Protons |
1 |
1 |
1 |
Neutrons |
0 |
1 |
2 |
Atomic Number |
1 |
1 |
1 |
Mass Number |
1 |
2 |
3 |
Isobar:
Atoms of different elements with different atomic numbers, which have the same mass number, are termed as isobars.
Calcium and argon are isobars.
Justification:
Calcium (Ca) has atomic number 20, Argon (Ar) has atomic number 18, but both have mass number 40.