We’ve already seen that elements are arranged on the periodic table from left to
right in order of increasing atomic number (except, of course, for the f area
elements, which are alone at the bottom). We’ve also noted that the periodic
table can be divided into four regions: the s, p, d, and f areas. By arranging
elements in both of these ways, two important themes emerge.
Elements in the same period (horizontal row) have electrons in the same
energy shells.
- Elements in the same group (vertical column) generally have similar
chemical and physical properties.
Let’s look at these ideas a little more closely, one at a time.
The first period on the table consists of just hydrogen and helium. Both of these
elements have electrons in the 1st energy shell. Since the 1st energy shell
consists of one 1s orbital, it can hold only 2 electrons, so the third element,
lithium (Li) has an electron in the 2nd energy shell. So do Be, B, C, N, O, F, and
Ne. These elements make up the 2nd period, and their 2nd energy shells are
filled. Third-period elements from sodium (Na) to argon (Ar) fill up the 3rd
energy shell. Now, you may ask: What about elements in the d area, such as iron
(Fe)? Doesn’t iron have valence electrons in the 3d subshell of the third energy
shell? Well, it does, but iron also has 2 electrons in the 4s subshell. It’s an
element in the 4th period, and it has electrons in its 4th energy shell.
Chemical Families
Valence electrons are the most important electrons in an atom because they can
participate in chemical bonds. Since chemical reactions involve bond breaking
and bond making, the behavior of valence electrons is responsible for all the
chemical reactions you see, from the souring of milk to the burning of rocket
fuel. Thus, it makes sense that if two elements have atoms with the same number