Valence Electrons & Ionic Compounds (College Board AP® Chemistry): Revision Note

Valence electrons

• Valence electrons are the electrons present in the outermost shell of an atom

• Valence electrons dictate an atom's reactivity and ability to form bonds

• The number of valence electrons can be obtained from the electron configuration of an atom

The beryllium atom

Beryllium electron configuration and valence electrons

• The interactions between the valence electrons and nuclei of elements determine if two elements are likely to form a chemical bond

• There are two types of chemical bonds depending on the elements that take part in the bonding process: ionic bond and covalent bond

  • An ionic bond occurs between a metal and a nonmetal

  • A covalent bond occurs between two nonmetals

Ion Charges & Valence Electrons

Valence electrons and the periodic table

• Elements in the same group (columns) of the periodic table have the same number of electrons in their valence shell

• The valence electrons determine the chemical behavior

• As you move across a period from left to right, without counting the transition metals, the number of valence electrons increases by one

Formation of ions

• Ions are atoms that have lost or gained electrons

• Atoms form ions to achieve the electron configuration of their closest noble gas

• Noble gasses have a stable electron configuration because of their full outer shell

• Atoms lose electrons to form cations

  • Since a cation has more protons than electrons, it has a positive charge

  • Cations are formed by metals, which have few valence electrons

Sodium atom becoming a sodium ion

Formation of positively charged sodium ion - sodium loses 1 electron to become a sodium ion

• Atoms gain electrons to form anions

  • Since an anion has less protons than electrons, it has a negative charge

  • Anions are formed by non-metals which are elements that lack a few electrons

• The electrostatic attraction between a positive ion (cation) and a negative ion (anion) is defined as an ionic bond

Chlorine atom becoming a chloride ion

Formation of negatively charged chloride ion - it gains 1 electron to become a chloride ion

Common ions and the periodic table

• The periodic table can help you to identify the typical charges of cations or ions

• Elements in Group 1 tend to lose their lonely electron from their valence shell, therefore, the common charge for all the elements is 1+

  • E.g. sodium (Na) tends to form Na⁺ cation

• Elements in Group 2 tend to lose two electrons from their valence shell, therefore, the common charge for all the elements is 2+

  • E.g. magnesium (Mg) tends to form Mg²⁺ cation

• Elements in the Group 3 (skipping the transition metals) tend to lose three electrons from their valence shell, therefore, the common charge for all the elements is 3+

  • E.g. aluminum (Al) tends to form Al³⁺ cation

• Elements in the Group 5 (skipping the transition metals) tend to gain three valence electrons, therefore, the common charge for all the elements is 3-

  • E.g. nitrogen (N) tends to form N^3- anion

• Elements in the Group 6 (skipping the transition metals) tend to gain two valence electrons, therefore, the common charge for all the elements is 2-

  • E.g. oxygen (O) tends to form O^2- anion

• Elements in the Group 7 (skipping the transition metals) tend to gain one valence electron, therefore, the common charge for all the elements is 1-

  • E.g. fluorine (F) tends to form F^- anion

• Atoms in Group 4 tend not form ions, therefore it is not possible to form ionic compounds with those elements