Chapter 19 – Chemical Bonds

Review: How do you find the: # of Protons?

# of Neutrons?

# of Electrons?

Practice: How many protons, neutrons, and electrons does Calcium have?

= Atomic number

= Atomic mass # - atomic number (protons)

= # of protons

p= 20, e= 20, n= 20

Chemical bonds are the forces that hold atoms together in compounds.

Remember, a compound is a substance with atoms of more than one element combined in fixed proportions.

Chemical Bonds

Atoms form bonds when the resulting compound is more stable than the separate atoms.

Remember, the number of valence electrons determines an elements stability.

8 is the most stable number of valence electrons.

Chemical Bonds

The noble gases already have the most stable number of valence electrons, so they do not react with any elements.• Noble Gases have 8 valence electrons.

What is the exception?

Other elements need to gain, lose, or share electrons to reach a stable 8 valence electrons.

Chemical Bonds

He - it has 2 valence electrons

When an atom gives, takes, or shares electrons with another atom the two atoms become attracted to each other.

This attraction is the chemical bond- the force that holds the atoms together in a compound.

Chemical Bonds

Compounds are formed when they are more stable than the separate atoms.

Compounds can have properties that are very different from the separate atoms.

Remember NaCl? Explosive and poisonous by themselves and yummy table salt when combined.

Compounds

Compounds have chemical formulas that describe the proportions of the different elements in the compound.

Subscript numbers – how many atoms of that element are in one unit of the compound

Coefficients – how many units of the compound there are

Chemical Formulas

4H2OCoefficients are normal size and in front of formulas.

Subscripts are small and after/below elements in the compound

Chemical Formulas

4H2OThis formula stands for 4 molecules of water, each molecule of water has 2 atoms of hydrogen and 1 atom of oxygen.

Chemical Formulas

Some elements become stable by gaining or losing electrons.

These atoms no longer have equal numbers of protons and electrons, so they are charged.

Ions are atoms that have become charged by gaining or losing electrons.

Ions

Metals reach a full valence shell by losing electrons.

Ex: Magnesium has 2 valenceelectrons in its 3rd energy level.

Ions

P – 6N - 8P – 6N - 8P – 6N - 8Mg

It is easier for magnesium to get rid of 2 valence electrons than to gain 6.

The 3rd energy level is nolonger occupied.

The 2nd energy level has 8electrons that are now valence electrons.

Ions

P – 6N - 8P – 6N - 8P – 6N - 8Mg

The magnesium atom is now stable…

How many electrons doesit have now?

How many protons doesit have now?

Ions

10

12

With 12 protons and only 10 electrons the magnesium ion has a charge of +2.

charge = # p+ - # e-

Ions that are formed bylosing electrons have a positive charge and are called cations.

Ions

P – 6N - 8P – 6N - 8Mg

Nonmetals can reach a full valence shell by gaining electrons.

Ex: Nitrogen has 5 valenceelectrons.

Ions

P – 6N - 8P – 6N - 8N

It is easier for nitrogen to get find 3 more valence electrons than to get rid of 5.

The 2nd energy level now has 8valence electrons.

Ions

P – 6N - 8P – 6N - 8N

The nitrogen atom is now stable…

How many electrons doesit have now?

How many protons doesit have now?

Ions

P – 6N - 8P – 6N - 8N

10

7

With 10 electrons and only 7 protons the nitrogen ion has a charge of -3.

charge = # p+ - # e-

Ions that are formed bygaining electrons have a negative charge and are called anions.

Ions

P – 6N - 8P – 6N - 8N

Metals lose electrons to form positively charged ions called cations.

Nonmetals gain electrons to form negatively charged ions called anions.

Cation Anion

Ions

What type of ion does lithium form?

What type of ion does chlorine form?

what type of ion does oxygen form?

Ions

Anion -1

Cation +1

Anion -2

Elements that are in the same group form ions with the same charge.

Ions