Valence electrons are the electrons in the highest occupied principal energy level of an atom.

Elements are organized by period and group, with the period corresponding to the principal energy level and the group relating to the extent the subshells are filled. The properties of an atom relate directly to the number of electrons in various orbitals. Valence electrons, the electrons in the outermost or valence shell, are important as they provide insight into an element’s chemical properties and are the ones gained, lost, or shared during a chemical reaction. In general, atoms are most stable and least reactive when their outermost electron shell is full. Inner-shell electrons are not involved directly in the element’s reactivity or in the formation of compounds.

Remember, elements are placed in order on the periodic table based on their atomic number. In a neutral atom, the number of electrons will equal the number of protons. In addition, the position of an element in the periodic table—its column, or group, and row, or period—provides useful information about how those electrons are arranged.

Considering the first three rows of the table, each row corresponds to the filling of a different electron shell: helium and hydrogen place their electrons in the 1n shell, while second-row elements like Li start filling the 2n shell, and third-row elements like Na continue with the 3n shell. Similarly, an element’s column number gives information about its number of valence electrons and reactivity. In general, the number of valence electrons is the same within a column and increases from left to right within a row. Group 1 elements have just one valence electron while group 18 elements have eight (except for helium which has only two electrons total). Thus, group number is a good predictor of how reactive each element will be.

The periodic table can be broken up into different blocks based on which orbitals their valence electrons occupy. These are named after the orbitals, so there is an s-block, p-block, d-block and f-block.

Practice Questions

Khan Academy

MCAT Official Prep (AAMC)

Chemistry Question Pack Passage 19 Question 106

Key Points

• Valence electrons are the outermost electron in an electron configuration.

• Valence electrons govern many chemical properties, reactivity, and bonding

• The group numbers (columns) of the periodic table indicate the total number of outer electrons in the valence shell

• The periods (rows) of the periodic table indicate the number of shells that surround a nucleus.

Key Terms

Inner-shell electrons: Those electrons that are not in the outer shell and are not involved in the reactivity of the element.

Valence electrons: The electrons in the highest occupied principal energy level of an atom.