An atom consists of a small positively charged nucleus surrounded by negatively charged electrons. We organize the electrons in a logical manner. As the atomic number increases, electrons are added to the subshells according to their energy. Lower energy subshells fill before higher energy subshells The order of filling is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p. The periodic table can be used to help you remember the order. Give the ground-state electron configuration for silicon (Si). Express the complete electron configuration using superscripts where appropriate. For example, the configuration for Li would be entered as 1s^22s^1.