In a molecule of water, the difference in electronegativity between the oxygen and hydrogen atoms leads to unequal sharing of electrons.
In more extreme cases, when an atom of high electronegativity is paired with an atom of low electronegativity, the difference in electronegativity may be so great that the electronegative atom "steals" the electron from its less electronegative partner.
This creates an electrically charged atom, which you may recall is known as an ion.
The atom with the extra electron contains more electrons than protons, which gives it a negative charge.
The atom that lost the electron has a positive charge because it now has more protons than electrons.
The two ions form an ionic bond, a chemical bond in which two ions with opposite electrical charges associate with each other because of the differences in charge.
Some atoms may gain or lose more than one electron when they form an ionic bond.
For instance, the calcium ion, which is often used a single (Ca) or a double (Ca++) positively charged ion.
Note: In standard notation, this can be represented as Ca^{+} or Ca^{2+}, with Ca^{2+} corresponding to the common Ca^{++} form.
Key Concepts
Electronegativity difference drives unequal electron sharing in bonds (e.g., in water: O vs H).
Ion formation occurs when electrons are transferred, creating cations and anions.
Anions are negatively charged ions (gain electrons).
Cations are positively charged ions (lose electrons).
Ionic bonds are the electrostatic attractions between oppositely charged ions.
Ionic bonding can involve the loss or gain of more than one electron.
Terms and Definitions
Electronegativity: the tendency of an atom to attract electrons in a chemical bond.
Ion: a charged species formed when electrons are transferred between atoms.
Anion: negatively charged ion formed after gaining electrons.
Cation: positively charged ion formed after losing electrons.
Ionic bond: a chemical bond formed by the electrostatic attraction between oppositely charged ions.
Example Clarifications
Calcium ion discussion:
The text mentions a single (Ca) or double (Ca++) positively charged ion.
In standard notation, this corresponds to Ca^{+} or Ca^{2+}, with Ca^{2+} reflecting the more common doubly-ionized state (Ca^{++}).