Periodic Table and electron configurations.svg

To understand what this means in terms of an element's electron configuration, let's consider the Group 1A metals. If we write the electron configuration for the Group 1A metal from each row of the Periodic Table, we have:

row 2 Li: 1s22s1
row 3 Na: 1s22s22p63s1
row 4 K: 1s22s22p63s23p64s1
row 5 Rb: 1s22s22p63s23p64s23d104p65s1
row 6 Cs: 1s22s22p63s23p64s23d104p65s24d105p66s1
row 7 Fr: 1s22s22p63s23p64s23d104p65s24d105p66s24f145d106p67s1

Do you see any pattern? For Group 1A metals, it seems that element's row corresponds to the energy level of that element's valence electron. Lithium (Li), for instance, is found in row 2 of the Periodic Table, and its valence electron is found in the n = 2 energy level. Cesium (Cs) is found in row 6 of the Periodic Table, and its valence electron is found in the n = 6 energy level. Let's see if this same pattern holds for Group 2A metals:

row 2 Be: 1s22s2
row 3 Mg: 1s22s22p63s2
row 4 Ca: 1s22s22p63s23p64s2
row 5 Sr: 1s22s22p63s23p64s23d104p65s2
row 6 Ba: 1s22s22p63s23p64s23d104p65s24d105p66s2
row 7 Ra: 1s22s22p63s23p64s23d104p65s24d105p66s24f145d106p67s2

For Group 2A metals, the same rule applies! Magnesium (Mg) is found in row 3 of the Periodic Table, and its valence electrons are found in the n = 3 energy level. Similarly, Radium (Ra) is found in row 7 of the Periodic Table, and its valence electrons are found in the n = 7 energy level.

So far so good – but does the same pattern apply to the Group 3A–8A elements (also known as Groups 13–18). Let's find out by writing the electron configuration for the Group 3A element in each row.

row 2 B: 1s22s22p1
row 3 Al: 1s22s22p63s23p1
row 4 Ga: 1s22s22p63s23p64s23d104p1
row 5 In: 1s22s22p63s23p64s23d104p65s24d105p1
row 6 Tl: 1s22s22p63s23p64s23d104p65s24d105p66s25d106p1

Even though the valence electrons in Group 3A elements are found in both s and p orbitals, it turns out that an element's row still corresponds to the energy level of that element's valence electrons. For example, Gallium (Ga) is found in row 4 of the Periodic Table, and its valence electrons are found in the n = 4 energy level. Likewise, Thallium (Tl) is found in row 6 of the Periodic Table, and its valence electrons are found in the n = 6 energy level.

It really does seem as if we can predict the energy level of an element's valence electrons using the row number for that element in the Periodic Table. Let's try one last example, though, just to be sure by writing the electron configuration for the Group 7A element in each row.

row 2 F: 1s22s22p5
row 3 Cl: 1s22s22p63s23p5
row 4 Br: 1s22s22p63s23p64s23d104p5
row 5 I: 1s22s22p63s23p64s23d104p65s24d105p5
row 6 At: 1s22s22p63s23p64s23d104p65s24d105p66s25d106p5