An atom is a basic building block of matter around us. Each atom is consists of the fundamental particles: negatively charged electron, positively charged proton and neutral neuron.
Shells
Electrons revolve around the positively charged nucleus (made of neutrons and protons) in orbits called shells. The shell closest to nucleus is called the ‘K shell’ (also called ‘1 shell), followed by ‘L shell’ (or ‘2 shell’), then ‘M shell’ (or ‘3 shell’) and so on. Each shell can hold up to 2n2 electrons, where n is the shell number. The K shell can hold up to 2 electrons, the L shell can hold up to 8 electrons, the M shell can occupy up to 18 electrons.
Subshells
Each shell is composed of one or more subshells. The first K shell has one subshell, called ‘1s’; the L shell has two subshells, called ‘2s’ and ‘2p’; the third shell has ‘3s’, ‘3p’, and ‘3d’; and so on. A subshell is the set of states defined by azimuthal quantum number, l, within a shell. The values l = 0, 1, 2, 3 correspond s, p, d and f subshells, respectively. The maximum number of electrons which can occupy a subshell is given by 2(2l + 1). This gives two electrons in an s subshell, six electrons in a p subshell, ten electrons in a d subshell and fourteen electrons in an f subshell.
The table below shows the number of electrons a shell and a subshell can hold:
Header image source: https://www.tulane.edu/~sanelson/images/electshells.gif
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