Structure of the Periodic Table (College Board AP® Chemistry): Revision Note

Structure of the Periodic Table

• The periodic table is a tabular arrangement of chemical elements organized based on their increasing atomic number, from 1 to 118

• The periodic table is organized into horizontal rows called periods and vertical columns called groups

Periods, groups and blocks

The periodic table

• The period number equals the number of the highest occupied shell

  • So, the valence electrons of every period 2 element are in the second shell

• Groups are columns of elements that share similar chemical properties because they have the same number of valence electrons

• The layout follows patterns of recurring properties, and these patterns are explained by ground-state electron configurations

  • Elements in the same group have the same number of valence electrons in the same type of subshell

  • Each period ends with a noble gas, whose outer s and p subshells are completely filled (only the 1s subshell for helium)

The structure of the beryllium atom

• The periodic table is divided into four blocks: the s-block, p-block, d-block and f-block

  • s-block elements: the highest-energy electrons fill an s subshell

  • p-block elements: the highest-energy electrons fill a p subshell

  • d-block elements: the highest-energy electrons fill a d subshell (3d, 4d, 5d or 6d)

  • f-block elements: the highest-energy electrons fill a 4f subshell (lanthanides) or a 5f subshell (actinides)

• The following table shows the subshell that fills in each block of the periodic table, period by period:

Electron configurations

• The arrangement of electrons in the energy shells and subshells of an atom is described by its electron configuration

• The electron configuration of an element can be predicted by using the periodic table

• For example, the electron configuration of germanium can be worked out from its position in the periodic table

Electron configuration of the germanium atom

• Germanium is in group 14 and period 4

• For groups 13–18, the last digit of the group number gives the number of valence electrons, so germanium has 4 valence electrons

• Period 4 shows that these valence electrons are in the fourth shell

• Germanium is the second element in the p-block, so two of its valence electrons are in the 4p subshell and the other two are in the 4s subshell

• This gives (full inner shells) + 4s² 4p², so the full ground-state electron configuration of germanium is:

1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p²