melting and boiling points




Characteristics of periods

The first period starts with hydrogen (H) and ends with helium (He). It has just two elements H (Z=1) and He (Z = 2).

H has one electron in the first-shell. He has 2 electrons in the first-shell.

As we have seen in the chapter on the structure of atoms, the first-shell can hold only 2 electrons.

Thus the first period is complete.