4. Structure Of Atoms
The distribution of electrons in different orbits or shells is governed by a scheme known as Bohr bury scheme
. The arrangement of electrons in various energy levels of an atom is known as the electronic configuration of the atom. According to this scheme.
The electrons are arranged around the nucleus in different energy levels or energy shells. The electrons first occupy the shell with the lowest energy i.e., closest to the nucleus.
The first or the innermost energy shell (K or n = 1) can take only two electrons.
The second shell (L or n = 2) can contain upto 8 electrons.
From third shell (M or n = 3) onwards, the shells become bigger. The third shell can accommodate as many as 18 electrons. In general, the maximum number of electrons that can be present in any shell is 2n2
where n is the number of energy shell. Thus, the first orbit (n = 1, known as K shell) can contain
2 × 12
= 2 electrons, the second orbit (n = 2, known as L shell) can contain 2 × 22
= 8 electrons. Maximum No. of electrons in different orbits
|Orbit ||Values of n ||Maximum no. of electrons in the orbit |
|K ||1 ||2 × 12 = 2 |
|L ||2 ||2 × 2 2 = 8 |
|M ||3 ||2 × 32 = 18 |
|N ||4 ||2 × 42 = 32 |
The outermost shell of an atom cannot have more than 8 electrons and the shell next to the outermost shell cannot have more than 18 electrons.