Chemical Bonds Chemical Bonds Ch 20Ch 20

Chemical Bonds Chemical Bonds Ch 20Ch 20

What makes matter different!What makes matter different!

Sect 1 Stability in Bonding

• When elements combine to form a compound, the compound has properties very different from those of the elements that make it.

• Ex. NaCl

Formulas• A compound ALWAYS has the same chemical

formula. Ex. H2O

• This tells the types and numbers of atoms making up the simplest unit of the compound.

• This means the same elements in the same proportions

• The same elements can combine in different ratios to form different compounds.

Ex. H2O vs. H2O2

The goal and purpose of bonding is stability

• Using electron dot diagrams and Bohr models, we are able to see how valence electrons are arranged.

• What is the goal # for outer level electrons?

• When outer levels are not at this number, they try to act in a way to fill that level.

Stability is Reached• When atoms gain, lose or share

electrons, an attraction forms between the atoms that pulls them together to form a compound.

• This attraction is called a chemical bond.

Section 2 Types of Bonds

Chemical Bonds hold compounds together!

In this section, we will discuss Ionic and Covalent bonds.

Gain or Loss of Electrons!

• Chemical bonds between atoms involve the electrons.

• Bonded atoms typically have a stable electron configuration.

• They stabilize by forming ionic or covalent bonds.

Ionic Bonds• Ionic bonds are formed by the

transfer of electrons• Ionic bonds form networks instead

on “molecules” . In NaCl, the ratio is one (1) Na+ ion to one (1) Cl- in a “formula unit”.

• The total charge is net zero (0).Ex. MgCl2, NaCl, KI,

Ionic Bonds• Ionic bonds form due to the

attractive forces between the oppositely charged ions.

• Ionic compounds are often crystalline solids with very high melting points.

• They will also conduct electricity when dissolved in water.

Covalent Bonds• Atoms that are joined by covalent

bonds, share electrons• The number of covalent bonds an

atom can form is dependant upon the number of electrons available for sharing.

• Covalent bonds:Single – 1 pair of shared electronsDouble – 2 pair of shared electronsTriple – 3 pair of shared electrons

Covalent Bonds• Unequal sharing of electrons

sometimes occurs in covalent bonds.• The strength of attraction of each

atom to its electrons is related to the:1. size of the atom2. charge of the nucleus3. the total number of electrons

the atom contains

Covalent Bonds• You may think of this as an “unequal

sharing of electrons”• This unequal sharing has a

consequence of on part of the atom having a slight negative (-) charge and another part of the atom has a slight positive (+) charge.

• This is called a polar molecule. Ex.H2O

Assignment• Complete questions 1-5, Section 2

review on page 614.