Valence Electrons & Ionic Compounds (College Board AP® Chemistry)
Study Guide
Written by: Martín
Reviewed by: Stewart Hird
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 elements 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) tend 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) tend 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) tend 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) tend to form N3- 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) tend to form O2- 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) tend to form F- anion
Atoms in Group 4 tend not form ions, therefore it is not possible to form ionic compounds with those elements
Examiner Tips and Tricks
From Group 1 to Group 3, the positive charge is equal to: charge = number of group. From Group 5 to Group 7, the negative charge is equal to: charge = number of group - 8. This trick works only if the groups of the transition metals are not numbered
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