1. The Chemical Basis of Life
Chapter 2

2. Matter Consists of Elements
Elements cannot be broken down by ordinary chemical means
Atoms are unique building blocks for each element
Atomic structure
Compound
A substance consisting of two or more different elements combined in a fixed ratio
Ex. (C6H12O6)

3. Atomic Structure
There are three important subatomic particles in atoms
Neutrons
No charge in the nucleus
1 atomic mass unit (amu)
Protons
Positive charge, in the nucleus
1 amu
Electrons
Negative charge , orbit nucleus
Zero amu
Equal in number to protons in atom
Atoms of different elements contain different numbers of subatomic particles
Compare hydrogen, helium and lithium

4. Hydrogen (H) Helium (He) Lithium (Li) Proton Neutron Electron
Figure 2.2

5. Atomic Number and Atomic Mass
Atomic number = number of protons in nucleus
Mass number = mass of protons and neutrons
Mass numbers of atoms of an element: are not all identical
Isotopes: atoms of the same element that differ in the # of neutrons they contain

6. Isotopes Hydrogen (1H) Deuterium (2H) Tritium (3H) Proton Neutron
Electron
Hydrogen (1H)
Deuterium (2H)
Tritium (3H)

7. Radioactive Isotopes can Help or Harm Us

8. The Distribution of Electrons Determines an Atoms Chemical Properties
Electrons occupy up to seven electron shells (energy levels) around nucleus
Octet rule: Except for the first shell which is full with two electrons, atoms interact in order to have eight electrons in their outermost energy level (valence shell)
Stable elements have their valence shell fully occupied
Reactive elements lack a full valence shell and tend to gain, lose, or share electrons (form bonds) with other atoms to achieve stability

9. (a) Chemically inert elements
Valence shell complete 8e 2e 2e
Helium (He)
Neon (Ne)

10. (b) Chemically reactive elements
Valence shell incomplete 4e 1e 2e
Hydrogen (H)
Carbon © 1e 6e 8e 2e 2e
Oxygen (O)
Sodium (Na)

11. Covalent Bonds join atoms into molecules through electron sharing
Three Important Types of Chemical Bonds
Covalent, Ionic and Hydrogen
Covalent Bonds
Formed when atoms share two or more valence shell electrons
Molecule = two or more atoms held together by covalent bonds
Allows each atom to fill its valence shell at least part of the time
Sharing of electrons can be equal or unequal
Equal sharing produces electrically balanced, nonpolar molecules
Ex. CO2
Unequal sharing by atoms with different electron-attracting abilities produces polar molecules
Ex. H2O
Atoms with six or seven valence shell electrons are electronegative, e.g., oxygen

12. Ionic bonds are attractions between ions of opposite charge
Ions are formed by: transfer of valence shell electrons between atoms
Anions (– charge): have gained one or more electrons
Cations (+ charge): have lost one or more electrons
Attraction of opposite charges: results in an ionic bond – +
Sodium atom (Na)
Chlorine atom (Cl)
Sodium ion (Na+)
Chloride ion (Cl–)
Sodium chloride (NaCl)

13. Hydrogen Bonds +
Attractive force between electropositive hydrogen of one molecule and an electronegative atom of another molecule –
Hydrogen bond + + – – – + + + –
(a) The slightly positive ends (+) of the water molecules become aligned with the slightly negative ends (–) of other water molecules.

14. Properties of Water Charged Exists as Gas, Liquid and Solid
Universal solvent
Adhesion and Cohesion (via H-bonds)
Surface tension
Ionizes into acid and base

15. Properties of Water Cohesion Adhesion Water binding to water
Water binding to anything that has a charge
Remember water has a charge and thus will bind to anything with a charge

16. Capillary Action