Chemistry Review

Chapter Outline
• What are atoms?
• How do atoms interact to form molecules?
• Why is water so important to life?

What are Atoms?
• All matter is composed of very small particles called atoms
• Atoms themselves are composed of smaller, subatomic particles called protons, neutrons and electrons

Name of
Particle
Proton
What are Atoms?
Location of
Particles
Charge of
Particle
Nucleus Positive
Mass of
Particle
1 amu
Neutron
Electron
Nucleus
Orbits around nucleus
Neutral 1 amu
Negative 1/2000 amu

What are Atoms?
• Bohr Model of the Sodium Atom

What are Atoms?
• There are approximately 100 different types of atoms
• These correspond to the 100 elements present on the Periodic Table of the
Elements
• Elements on the Periodic Table are organized by atomic number, atomic mass, and similar properties

What are Atoms?
• Atomic Number =
# of protons =
# of electrons
Atomic Mass =
# of protons +
# of neutrons

What are Atoms?
• Using the Periodic Table
• # protons= atomic number
• # electrons= atomic number
• # neutrons= atomic mass – atomic number

What are Atoms?
• The protons and neutrons are in the central nucleus
• Electrons arrange themselves into distinct orbitals around the nucleus
• The first orbital nearest the nucleus holds a maximum of 2 electrons
• The remaining outer orbitals hold a maximum of 8 electrons each

• For example, sodium is atomic number 11 and atomic mass 23
• #protons= 11
• #electrons= 11
• #neutrons= 23-11 = 12
• The sodium atom has 11 electrons: 2, 8 and 1 electron in the outer orbital

What are Atoms?
• For example, chlorine is atomic number 17 and atomic mass 35
• #protons= 17
• #electrons= 17
• #neutrons= 35 – 17 = 18
• The chlorine atom has 17 electrons: 2, 8, and 7 electrons in the outer orbital

How do atoms interact to form molecules?
• The Octet Rule states that atoms are most stable if the outer orbital is either full (8 electrons) or empty (0 electrons)
• Atoms will lose or gain electrons in order to fill the orbital and achieve stability
• When atoms lose or gain electrons, they become charged atoms called ions

How do atoms interact to form molecules?
• For example, sodium has one outer electron and will lose the one electron in order to become stable → +1 sodium ion
• For example, chlorine has seven outer electrons and will gain one electron in order to become stable → -1 chlorine ion

How do atoms interact to form molecules?
• Ions of opposite charge attract to form ionic compounds
• +1 Na ion + -1 Cl ion → NaCl compound
• The bond that keeps the ions together is called an ionic bond

What are Atoms?
• Outer orbitals of Na and Cl ions

How do atoms interact to form molecules?
• Metallic and non-metallic elements on the periodic table form ionic compounds by losing/gaining electrons
• Non-metallic and non-metallic elements form covalent compounds by sharing electrons

How do atoms interact to form molecules?
• For example, H atom has one outer electron: it needs one more electron to become stable
• C atom has four outer electrons: it needs four more electrons to become stable
• Four H atoms will share their electrons with one C atom so the C atom will be stable

How do atoms interact to form molecules?
• One C atom will share its four electrons with four H atoms in order for the H atoms to become stable
• The bond that forms between the atoms is called a covalent bond : the equal sharing of electrons between atoms

How do atoms interact to form molecules?
• Outer orbitals of H and C atoms

How do atoms interact to form molecules?
• Non-metallic and non-metallic atoms do not always share their electrons equally
• For example, H and O are non-metallic elements
• When H and O atoms share electrons, the
O atom has a stronger pull on the H atom’s electrons → the O atom becomes slightly negative, the H atom slightly positive

How do atoms interact to form molecules?
• This unequal sharing of electrons creates a polar covalent bond between H and O atoms
• The molecule as a whole has no net charge, but there are distinct poles of charge
• The molecule is a polar covalent compound

Why is water so important to life?
• Water is a polar covalent molecule held together by polar covalent bonds between
H and O atoms

Why is water so important to life?
• Because water is a polar covalent molecule, the slightly positive H regions of one water molecule will attract the slightly negative O regions of another water molecule
• This electrical attraction between the water molecules is called hydrogen bonding

Why is water so important to life?

Why is water so important to life?
• The extraordinary properties of water are due to water’s intramolecular polar covalent bonds and intermolecular hydrogen bonds

Why is water so important to life?
• Water is a good solvent : it interacts with many other molecules that are ionic or polar covalent
• Water molecules are cohesive (stick together) due to hydrogen bonding between water molecules

Why is water so important to life?
• Because of hydrogen bonding between water molecules, it takes a large amount of added energy to change water from solid →liquid→gas ( energy of vaporization )
• Because of hydrogen bonding between water molecules, one must remove a large amount of energy in order to change water from gas →liquid→solid ( energy of fusion )

Why is water so important to life?
• Water can dissociate (separate) into +1 H atoms and -1 OH ions in a solution
• The amount of +1 H ions vary within and between biological and chemical systems
• Systems that have an excess of +1 H ions are described as acidic
• Systems that have an excess of -1 OH ions are described as basic
• Systems that have equal amounts of +1 H ions and -1 OH ions are described as neutral

Why is water so important to life?
• The pH scale measures the degree of acidity/ alkalinity in a system
• Acidic < pH 7
• Neutral = pH 7
• Basic > pH 7

• A chemical that ends to maintain a solution at a constant pH is called a buffer
• When the solution becomes too basic, buffers release +1 H ions into the solution to decrease pH
• When the solution becomes too acidic, buffers collect and remove +1 H ions from the solution in order to increase pH