Bonding

Why do atoms bond together?

Some atoms are very reluctant to combine with other atoms and exist in the air around us as single atoms. These are the Noble Gases and have very stable electron arrangements and are shown in the diagrams below.

Element (atomic number) and electron arrangement.

All other atoms therefore, bond to become electronically more stable, that is to become like Noble Gases in electron arrangement. Atoms can do this in two ways …

By sharing electrons to form molecules with covalent bonds, the bond usually formed between two non- metallic elements OR by one atom transferring electrons to another atom. The atom losing electrons forms a positive ion and is usually a metal. The atom gaining electrons forms a negative ion and is usually a non- metallic element.

Octet Rule: Atoms tend to lose, gain, or share electrons until they are surrounded by 8 valence electrons. (There are many exceptions to this rule, however, it provides a useful model for understanding the basis for chemical bonding).

Ionic Bonding Ionic bonds are formed by one atom transferring electrons to another atom to form ions. Ions are atoms, or groups of atoms, which have lost or gained electrons.

The atom losing electrons forms a positive ion (a cation) and is usually a metal. The overall charge on the ion is positive due to excess positive nuclear charge (protons do NOT change in chemical reactions).

The atom gaining electrons forms a negative ion (an anion) and is usually a non-metallic element. The overall charge on the ion is negative because of the gain, and therefore excess, of negative electrons.

Examples:

2Na + Cl2  2NaCl Mg + Cl2  MgCl2

Al + 3F  AlF3 4K + O2  2K2O

2Ca + O2  2CaO 4Al + 3O2  2Al2O3

D:Science/chemistry/bonding/bondreadingandquestions Ionic Bonds  Bonds between metal and non-metals  Fulfill Octet Rule – stability of molecule from a energy level with 8 electrons  Metals – lose electrons, gain positive charge  Charge gained by a metal = the number of electrons it has lost (ex: Mg2+)  Non-metals – gain electrons, gain negative charge  Called formula units instead of molecules

Properties of Ionic Compounds  Salts and metal oxides are typical ionic compounds.  This strong bonding force makes the structure hard (if brittle) and have high melting and boiling points. Unlike covalent molecules, ALL ionic compounds are crystalline solids at room temperature.  Many ionic compounds are soluble in water, but not all.  The solid crystals DO NOT conduct electricity because the ions are not free to move to carry an electric current. However, if the ionic compound is melted or dissolved in water, the liquid will now conduct electricity, as the ion particles are now free.

Covalent Bonding Covalent bonds are formed by atoms sharing electrons to form molecules. This type of bond usually formed between two non-metallic elements. The molecules might be that of an element ie one type of atom only OR from different elements chemically combined to form a compound. The covalent bonding is caused by the mutual electrical attraction between the two positive nuclei of the two atoms of the bond, and the electrons between them.

Covalent Bonds  Bonds between two non-metallic elements  Elements share electrons

Examples:

2H 2 + O2  2H 2O Cl + Cl  Cl 2

H2 + Cl2  2HCl H + H  H 2

3H2 + 2N  2NH 3 2H2 + C  CH 4

O 2 + C  CO 2 2C + 2H 2  C 2H 4

Properties of Covalent Molecules  Small covalent molecules have low melting and boiling points.  They are poor conductors of electricity because there are no free electrons or ions in any state to carry electric charge. Most small molecules will dissolve in a solvent to form a solution.

D:Science/chemistry/bonding/bondreadingandquestions Name ______Reading Review Section ______

Ionic and Covalent Bonds

1. What group of elements has very stable electron arrangements and is reluctant to react with other elements? Give 2 examples of these elements.

Group______Examples: ______

2. Why do other elements need to bond with one another?

3. Name two ways atoms can bond: a) b)

4. Explain how the two bonds differ: a)

b)

5. What kind of elements typically combine to form covalent bonds:

6. What kind of elements typically combine to form ionic bonds:

7. What is the Octet rule?

8. What is an ion?

9. What is the name for an atom that loses an electron? ______What type of element typically loses electrons? ______

D:Science/chemistry/bonding/bondreadingandquestions What charge is left on an ion that has lost an electron? ______

10. What is the name for an atom that gains an electron? ______What type of element typically gains electrons? ______What charge is left on an ion that has gained an electron? ______

Answer the following questions based on the Bonding Reading. Identify each item as either a covalent or an ionic bond.

H + H  H ______Mg + Cl2  MgCl2 ______

2C + 2H 2  C 2H 4 ______4K + O2  2K2O ______

2Ca + O2  2CaO ______4Al + 3O2  2Al2O3 ______

2H 2 + O2  2H 2O ______Cl + Cl  Cl 2 ______

H2 + Cl2  2HCl2 ______2Na + Cl2  2NaCl ______

3H2 + 2N  2NH 3 ______2H2 + C  CH 4 ______

O 2 + C  CO 2 ______Al + 3F  AlF3 ______

D:Science/chemistry/bonding/bondreadingandquestions