1. Chapter 19 – Chemical Bonds

2. 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

3. Chemical Bonds
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.

4. 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.

5. Chemical Bonds What is the exception?
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.
He - it has 2 valence electrons

6. Chemical Bonds
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.

7. Compounds
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.

8. Chemical Formulas
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

9. Chemical Formulas
4H2O
Coefficients are normal size and in front of formulas.
Subscripts are small and after/below elements in the compound

10. Chemical Formulas
4H2O
This formula stands for 4 molecules of water, each molecule of water has 2 atoms of hydrogen and 1 atom of oxygen.

11. Ions
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.

12. Ions
Metals reach a full valence shell by losing electrons. Ex: Magnesium has 2 valence electrons in its 3rd energy level.
P – 6
N - 8 Mg

13. Ions
It is easier for magnesium to get rid of 2 valence electrons than to gain 6. The 3rd energy level is no longer occupied. The 2nd energy level has 8 electrons that are now valence electrons.
P – 6
N - 8
P – 6
N - 8
P – 6
N - 8 Mg

14. Ions 10 12 The magnesium atom is now stable… How many electrons does
it have now?
How many protons does 10 12

15. Ions
With 12 protons and only 10 electrons the magnesium ion has a charge of +2. charge = # p+ - # e- Ions that are formed by losing electrons have a positive charge and are called cations.
P – 6
N - 8 Mg

16. Ions
Nonmetals can reach a full valence shell by gaining electrons. Ex: Nitrogen has 5 valence electrons.
P – 6
N - 8 N

17. Ions
It is easier for nitrogen to get find 3 more valence electrons than to get rid of 5. The 2nd energy level now has 8 valence electrons.
P – 6
N - 8 N

18. Ions
The nitrogen atom is now stable… How many electrons does it have now? How many protons does
P – 6
N - 8 N 10 7

19. Ions
With 10 electrons and only 7 protons the nitrogen ion has a charge of -3. charge = # p+ - # e- Ions that are formed by gaining electrons have a negative charge and are called anions.
P – 6
N - 8 N

20. Ions
Metals lose electrons to form positively charged ions called cations.
Nonmetals gain electrons to form negatively charged ions called anions.
Cation Anion

21. Ions Cation +1 Anion -1 Anion -2
What type of ion does lithium form? What type of ion does chlorine form? what type of ion does oxygen form?
Cation +1
Anion -1
Anion -2

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