Chemical Bonds. The Elements 90 naturally occurring elements Most are not found as pure elements The majority of elements are found combined with other.

Chemical Bonds

The Elements

• 90 naturally occurring elements• Most are not found as pure elements• The majority of elements are found combined

with other elements to form compounds. • Gold, silver and platinum are rare examples of

metals found in elemental form (precious metals)

Classifying

• Since the 90 elements can form thousands of different compounds, classification systems have been developed.

• The classification of compounds is based on their properties to help our understanding of compounds.

• Example: melting point, boiling point, hardness, etc.

Electron Configuration

a) Atomic number = number of electrons

b) Electrons vary in the amount of energy they possess, and they occur at certain energy levels or electron shells.

c) Electron configuration determines how an atom behaves when it encounters other atoms

Electron Dot Structure

Symbols of atoms with dots to represent the valence-shell electrons

1 2 13 14 15 16 17 18H He: Li Be B C N O : F :Ne :

Na Mg Al Si P S :Cl :Ar :

Chemical Bonds

Octet Rule- rule that states that atoms tend to gain, lose, or share electrons so that each atom has full outermost energy level which is typically 8 electrons.

There are two ways to try to do this1. Ionic bonds – 2. Covalent bonds –

Octet Rule = atoms tend to gain, lose or share electrons so as to have 8 electrons

C would like to N would like toO would like to

Gain 4 electronsGain 3 electronsGain 2 electrons

Properties of Ionic and Covalent Componds

Chemical Bonds

• forces that attract atoms to each other to form compounds

• involves the interactions of valence electrons between atoms

• usually the bond forms a compound that is more stable than the atoms individually.

Ionic Bond

bond formed between two ions by the transfer of electrons

Covalent Bond

bond formed by the sharing of electrons

Formation of Ions from Metals

Ionic compounds result when metals react with nonmetals

Metals lose electrons to match the number of valence electrons of their nearest noble gas

Positive ions (cations) form when the number of electrons are less than the number of protons

Group 1 metals ion 1+

Group 2 metals ion 2+

• Group 13 metals ion 3+

Formation of Sodium Ion

Sodium atom Sodium ion

Na – e Na +

2-8-1 2-8 ( = Ne)

11 p+ 11 p+

11 e- 10 e-

0 1+

Ions from Nonmetal Ions

In ionic compounds, nonmetals in 15, 16, and

17 gain electrons from metals (anions)

Nonmetal add electrons to achieve the octet

arrangement

Nonmetal ionic charge:

3-, 2-, or 1-

Fluoride Ion

unpaired electron octet 1 -

: F + e -> : F :

2-7 2-8 (= Ne)

9 p+ 9 p+

9 e- 10 e- 0 1 -

ionic charge

Ionic Bond

• Between atoms of metals and nonmetals (usually) with very different electronegativity

• Electronegativity difference e.d. > 1.7• Bond formed by transfer of electrons• Examples; NaCl, CaCl2, K2O

Some characteristics of an Ionic Bond

1. Crystalline at room temperatures2. Have higher melting points and boiling points

compared to covalent compounds 3. Conduct electrical current in molten or

solution state but not in the solid state4. Polar bonds

Covalent Bond

• Between nonmetallic elements of similar electronegativity.

• Formed by sharing electron pairs

• Examples; O2, CO2, C2H6, H2O, SiC

Some characteristics of a Covalent Bond

1. Covalent bonds have definite and predicable shapes.

2. Very strong3. Low melting and boiling points

Covalent Bonds can have multiple bonds, so you should be familiar with the following…

• Single Covalent Bond- chemical bond resulting from sharing of an electron pair between two atoms.

Double Covalent Bond- chemical bond resulting from sharing of two electron pairs between two atoms.

Triple Covalent Bond-chemical bond resulting from sharing of three electron pairs between two atoms.

Non-polar covalent Bonds

when electrons are shared equally

Polar Covalent Bonds

when electrons are shared but shared

unequally

•water is a polar molecule because oxygen is more electronegative than hydrogen, and therefore electrons are pulled closer to oxygen.

Review of Bonds

Electronegativity (EN)

• a measure of an atoms ability to attract electrons in a chemical bond.

• a property of an atom involved in a bond

Using EN to predict the bond type

When two atoms form a bond the difference in electronegativity (ΔEN) can help to determine the bond type.

| | | 0 0.5 1.7 3.3

Examples

• HCl3.16- 2.2= 0.96 Polar Covalent• CrO3.44- 1.66= 1.78 Ionic• Br2

2.96-2.96= 0 Covalent