Electron Arrangement in Atoms

• Electrons are arranged in shells or energy levels around the nucleus of an atom.
• This arrangement of electrons around the nucleus of an atom is called electronic configuration.
• Shells are numbered starting from the shell which is nearest to the nucleus of the atom.
• Hence the shell which is furthest away from nucleus is called the outermost shell and the electrons in the shell are referred to as the outermost electrons.
• The shell nearest to nucleus is called the first shell and this is also the lowest energy level.
• Each shell can hold a fixed number of electrons. The formula helps us to find the number of electrons in each shell. And the formula is 2n² where ‘n’ is the shell number.

For the first 20 elements (hydrogen to calcium)
» The first shell (lowest energy level) can hold maximum of 2 electrons.
» The second shell can hold a maximum of 8 electrons.
» The third shell can hold a maximum of 8 electrons.
» The remaining electrons are placed in the fourth shell.
» The energy levels must be filled in order of increasing energy. The first level is filled first before going to the second level and subsequent higher levels.