The octet rule states that elements will gain or lose electrons in order to have a full outer shell of eight electrons.
#bb"Molecule" color(white)(........) bb"No. of electrons"#
#underline(color(white)(....................)bb"around central atom")#
#color(white)(....)"CH"_4 color(white)(......................) 4 × 2 = 8#
#color(white)(....)"PCl"_5 color(white)(....................) 5 × 2 = color(blue)10#
#color(white)(....)"NH"_3 color(white)(................) (3 × 2) + 2 = 8#
#color(white)(....)"H"_2"O" color(white)(................) (2 × 2) + 4 = 8#
In #"CH"_4# and #"PCl"_5# there are no lone pair of electrons on central atom. Whereas in #"NH"_3# and #"H"_2"O"# there are #1# and #2# lone pair of electrons respectively on central atom.
