Chemical Bonding
I. Chemical bonds
1. A chemical bond is the force that holds atoms together in compounds.
2. There are two main types of bonds: ionic and covalent.
II. Ionic bond- the chemical bond resulting from the attraction of positive and negative
ions for each other.
1. Typically, a bond between a metal and a non-metal is ionic.
2. The metal loses electrons and becomes a cation and the non-metal gains
electrons and becomes an anion.
3. For example: Na and Cl
III. Covalent bond - the chemical bond resulting from two atoms sharing electrons.
1. Typically, a bond between non-metals is covalent.
2. Covalent bonds can be polar or non-polar.
a. A non-polar bond consists of a pair of electrons that are being shared
equally by both elements (the difference in electronegativity between
the two elements is between 0.0 and 0.4).
For example: H and H
b. A polar bond consists of a pair of electrons that are being shared
unequally between two elements (the difference in electronegativity
between the two elements is between 0.5 and 1.8) Consequently, the
atom that has the electrons closer to itself (the more electronegative)
develops a partially negative charge (δ-), while the other atom
develops a partially positive charge (δ+). This separation of charges in
a polar bond is called a dipole. For covalent compounds, the larger the
electronegativity difference (ΔEN), the larger the dipole and the more
polar the bond is.
For example: H and Cl
3. Electronegativity (EN) – refers to the tendency of an atom to attract electrons
to itself when it is chemically combined with another atom. For representative
elements, electronegativity increases from left to right across a period and from
bottom to top within a group.
IV. Lewis dot notation for representative elements:
1. Lewis dot notation is used to represent an element and its valence electrons.
2. Write the element’s symbol and place dots around the symbol to represent the
valence electrons.
3. The dots are placed on the four sides of the element (top, bottom, right and left
sides). Do not place two dots on the same side until each side has one dot.
Examples:
V. Lewis structures for ions –
1. First, write the Lewis dot notation for the element and then add or remove
electrons as needed.
2. Example: write the Lewis structures for the ions produced by a) Na b) Al c) O
a. For Na, the one valence electron is lost; the resulting ion has a charge of +1.
b. For Al, the three valence electrons are lost; the resulting ion has a charge of +3.
c. For O, since the atom contains 6 valence electrons, 2 electrons are gained to
complete the octet; the resulting ion has a charge of -2.
VI. Lewis structures of ionic compounds: For ionic compounds, the Lewis structures
are simply produced by combining the Lewis structures of the individual ions.
Representative metals lose their valence electrons and non-metals gain as many electrons
as is necessary to become isoelectronic with the noble gas in the same row.