1. Chapter 13: Atomic Structure and Chemical Bonds
Part 1: Why do atoms combine?
Part 2: How Do Elements Bond?

2. GPS Standards S8CS5.b – Understand that different models
can be used to represent the same thing.
S8P1 – Examine the scientific view of the
natural world.
a. Atoms versus Molecules
b. Elements vs. Compounds vs. Mixtures
e. Chemical vs. Physical Changes
f. Periodic Table

3. Why do atoms combine? 1
Element Structure
Each element has a different atomic structure consisting of a specific number of protons, neutrons, and electrons.
The number of protons and electrons is always the same for a neutral atom of a given element.

4. Why do atoms combine? 1
Element Structure
This neutral lithium atom has three positively charged protons, three negatively charged electrons, and four neutral neutrons.

5. Electron Arrangement— Electron Energy
Why do atoms combine? 1
Electron Arrangement— Electron Energy
The different areas for an electron in an atom are called energy levels.

6. Electron Arrangement— Electron Energy
Why do atoms combine? 1
Electron Arrangement— Electron Energy
This shows a model of what these energy levels might look like. Each level represents a different amount of energy.

7. Why do atoms combine? 1
Number of Electrons
The farther an energy level is from the nucleus, the more electrons it can hold.
1st floor/energy level = (2 electrons) 2nd floor/energy level = (8 electrons) 3rd floor/energy level = (8 electrons) 4th floor/energy level = (18 electrons)

8. Why do atoms combine? 1
Energy Steps
Electrons in the level closest to the nucleus have the lowest amount of energy and are said to be in energy level one.
Electrons farthest from the nucleus have the highest amount of energy and are the easiest to remove.

9. Why do atoms combine? 1
Energy Steps
The closer a negatively charged electron is to the positively charged nucleus, the more strongly it is attracted to the nucleus. Therefore, removing electrons that are close to the nucleus takes more energy than removing those that are farther away from the nucleus.

10. Periodic Table and Energy Levels
Why do atoms combine? 1
Periodic Table and Energy Levels
You can determine the number of electrons in an atom by looking at the atomic number written above each element symbol.

11. Periodic Table and Energy Levels
Why do atoms combine? 1
Periodic Table and Energy Levels
You can determine the number of energy levels in an atom by looking at the row or period that it is found on.

12. Electron Configurations
Why do atoms combine? 1
Electron Configurations
If you look at the periodic table shown, you can see that the elements are arranged in a specific order.
Fig. 5, p. 467

13. Electron Configurations
Why do atoms combine? 1
Electron Configurations
The number of electrons in a neutral atom of the element increases by one from left to right across a group.
Fig. 5, p. 467

14. Electron Configurations
Why do atoms combine? 1
Electron Configurations
Atoms with a complete outer energy level are stable (group 18 – The Noble Gases)
Example: helium, neon, and argon are all stable.

15. Why do atoms combine? 1
Electron Dot Diagrams
If you want to see how atoms of one element will react, it is handy to have an easier way to represent the atoms and the electrons in their outer energy levels.
You can do this with electron dot diagrams.
An electric dot diagram is the symbol for the element surrounded by as many dots as there are electrons in its outer energy level.

16. Dot Diagrams - How to Write Them
Why do atoms combine? 1
Dot Diagrams - How to Write Them
The dots are written in pairs on four sides of the element symbol.
Start by writing one dot on the top of the element symbol, then work your way around adding dots to the right, bottom, and left.

17. 1
Dot Diagrams - How to Write Them
Add a fifth dot to the top to make a pair. Continue in this manner until you reach eight dots to complete the level.

18. GPS Standards S8CS5.b – Understand that different models
can be used to represent the same thing.
S8P1 – Examine the scientific view of the
natural world.
a. Atoms versus Molecules
b. Elements vs. Compounds vs. Mixtures
e. Chemical vs. Physical Changes
f. Periodic Table

19. A chemical bond is the force that holds two atoms together.
How Elements Bond
Chemical Bonding
A chemical bond is the force that holds two atoms together.
Atoms bond with other atoms in such a way that each atom becomes more stable. That is, their outer energy levels will resemble those of the noble gases.

20. Ionic Bonds—Loss and Gain
How Elements Bond 2
Ionic Bonds—Loss and Gain
Atoms form bonds with other atoms using the electrons in their outer energy levels (called valence electrons).
They have three ways to do this—by losing/ gaining electrons (ionic), by pooling electrons (metallic), or by sharing electrons (covalent) with another element.

21. Forming Salt (Ionic Bonding)
How Elements Bond 2
Forming Salt (Ionic Bonding)
Sodium has only one electron in its outer level. Removing this electron empties this level and leaves the completed level below.
By removing one electron, sodium’s electron configuration becomes the same as that of the stable noble gas neon.

22. Forming Salt (Ionic Bonding)
How Elements Bond 2
Forming Salt (Ionic Bonding)
By losing an electron, the balance of electric charges changes creating an ion (electrically charged atom).
Sodium becomes a positively charged ion because there is now one fewer electron than there are protons in the nucleus.

23. Forming Salt (Ionic Bonding)
How Elements Bond 2
Forming Salt (Ionic Bonding)
Chlorine forms bonds in a way that is the opposite of sodium—it gains an electron.
When chlorine accepts an electron, its electron configuration becomes the same as that of the noble gas argon. Chlorine becomes a negatively charged ion.
Click image to view movie.

24. How Elements Bond 2

25. How Elements Bond 2
Ionic Bond Formation
The positive sodium ion and the negative chloride ion are strongly attracted to each other.
This attraction, which holds the ions close together, is a type of chemical bond called an ionic bond.

26. Bond Formation

27. How Elements Bond 2
Bond Formation
The compound sodium chloride, or table salt, is formed.
A compound is a pure substance containing two or more elements that are chemically bonded.

28. More Gaining and Losing
How Elements Bond 2
More Gaining and Losing
Elements can lose or gain more than one electron.
The element magnesium, Mg, in Group 2 has two electrons in its outer energy level.
Magnesium can lose these two electrons and achieve a completed energy level.

29. How Elements Bond 2
More Gains and Losses
Some atoms, such as oxygen, need to gain two electrons to achieve stability.
The two electrons released by one magnesium atom could be gained by a single atom of oxygen. When this happens, magnesium oxide (MgO) is formed.

30. Metallic Bonding—Pooling
How Elements Bond 2
Metallic Bonding—Pooling
In a metal, the electrons in the outer energy levels of the atoms are not held tightly to individual atoms.
Instead, they move freely among all the ions in the metal, forming a shared pool of electrons.

31. Metallic Bonding—Pooling
How Elements Bond 2
Metallic Bonding—Pooling
Metallic bonds form when metal atoms share their pooled electrons. This bonding affects the properties of metals.
Metallic bonding also is the reason that metals conduct electricity well.
The outer electrons in metal atoms readily move from one atom to the next to transmit current.

32. The alternative is sharing electrons.
How Elements Bond 2
Covalent Bonds—Sharing Electrons
Some atoms are unlikely to lose or gain electrons because the number of electrons in their outer levels makes this difficult.
The alternative is sharing electrons.

33. How Elements Bond 2
The Covalent Bond
The chemical bond that forms between nonmetal atoms when they share electrons is called a covalent bond.
Click image to view movie.

34. How Elements Bond 2
The Convalent Bond
Shared electrons are attracted to the nuclei of both atoms.
They move back and forth between the outer energy levels of each atom in the covalent bond. Each atom thus has a stable outer energy level some of the time.

35. The Covalent Bond 2
The neutral particle is formed when atoms share electrons is called a molecule.
You can see how molecules form by sharing electrons in this figure.

36. Polar and Nonpolar Molecules
How Elements Bond 2
Polar and Nonpolar Molecules
Some atoms have a greater attraction for electrons than others do.
This unequal sharing makes one side of the bond more negative than the other. Such bonds are called polar bonds.

37. Polar and Nonpolar Molecules
How Elements Bond 2
Polar and Nonpolar Molecules
A polar bond is a bond in which electrons are shared unevenly.

38. The Polar Water Molecule
How Elements Bond 2
The Polar Water Molecule
Water molecules form when hydrogen and oxygen share electrons.
The oxygen atom has a greater share of the electrons in each bond—the oxygen end of a water molecule has a slight negative charge and the hydrogen end has a slight positive charge.

39. The Polar Water Molecule
How Elements Bond 2
The Polar Water Molecule
Because of this, water is said to be polar—having two opposite ends or poles like a magnet.
Molecules that do not have these uneven charges are called nonpolar molecules.

40. Today’s Essential Questions:
What are compounds?
What are chemical formulas and how do we read them?

41. Compounds and Mixtures3
Compounds
The elements hydrogen and oxygen exist as separate, colorless gases.
However, these two elements can combine to form the compound water, which is different from the elements that make it up.
A compound is a substance whose smallest unit is made up of atoms of more than one element bonded together.

42. Elements versus Compounds

43. Compounds and Mixtures3
Compounds
Compounds often have properties that are different from the elements that make them up.
Water is distinctly different from the elements that make it up.
It is also different from another compound made from the same elements.
Hydrogen peroxide (H2O2) is a different combination of hydrogen and oxygen and has different properties from those of water.

44. Compounds and Mixtures3
Compounds (Formulas)
H2O is the chemical formula for water, and H2O2 is the formula for hydrogen peroxide.
The formula tells you which elements make up a compound as well as how many atoms of each element are present.

45. Compounds and Mixtures3
Compounds (Formulas)
The subscript number written below and to the right of each element’s symbol tells you how many atoms of that element exist in one unit of that compound.
For example, hydrogen peroxide (H2O2) has two atoms of hydrogen and two atoms of oxygen.
Water (H2O) is made up of two atoms of hydrogen and one atom of oxygen.

46. Compounds and Mixtures3
Compounds (Formulas)
Carbon dioxide, CO2 is made up of one atom of carbon and two atoms of oxygen.
Carbon and oxygen also can form the compound carbon monoxide, CO, which is a gas that is poisonous to all warm-blooded animals.
No subscript is used when only one atom of an element is present.

47. Compounds (Formulas) H2S = 2 hydrogen ; 1 sulfur
CCl4 = 1 carbon; 4 chlorines
CaCO3 = 1 calcium ; 1 carbon; 3 oxygen
HCl = 1 hydrogen; 1 chlorine