Chemistry = The arrangement of electrons in atoms

Each electron in an atom is in an energy level. Energy levels can be represented as shells, with electrons in the lowest energy level closest to the nucleus. We can draw them as circles on a diagram, with electrons represented by crosses.

The lowest energy level or first shell can hold 2 electrons, and the second energy level can hold 8. Electrons occupy the lowest possile energy levels, so the electronic structure of neon with 10 electrons is 2,8. Sodium with 11 electroms has an electronic structure of 2,8,1.

Elements in the same group of the periodic table have the same number of electrons in their highest energy level. All the elements in Group 1 have one electron in their highest energy level, showing that after each noble gas the next energy level begins to fill.