1. The Chemical Basis of Life
Chapter 2

2. Organisms are Composed of Elements
Matter is anything w/mass that takes up space
Matter is composed of Elements
Elements cannot be broken down into other substances by ordinary chemical means
Each element is composed of a unique type of atom
Elements combine to form molecules and compounds

3. Compounds
A compound is a substance made of two or more different elements combined in a fixed ratio
Ex’s. NaCl, H2O, CH4, C6H12O6,

4. Atoms Consist of Protons, Neutrons and Electrons
An atom is the smallest unit of an element that still retains the properties of an element
Atoms have smaller particles
Neutrons
No charge; in nucleus
1 atomic mass unit (amu) or 1 dalton
Protons
Positive charge; in nucleus
1 amu
Electrons
Negative charge , orbiting nucleus
Zero amu
Equal in number to protons in a neutral atom

5. Atomic Structure
Atoms of different elements contain different numbers of protons (compare hydrogen, helium and lithium)
Proton
Neutron
Electron
Hydrogen (H)
Helium (He)
Lithium (Li)

6. A lithium atom has 3 protons and 4 neutrons
A lithium atom has 3 protons and 4 neutrons. What is its atomic mass in daltons?
3 daltons
4 daltons
7 daltons
12 daltons
None of the above

7. Atomic Number and Atomic Mass
Atomic number = number of protons in nucleus
Mass number = mass of protons and neutrons
Atoms of the same element can have different mass numbers (isotopes)
Isotopes are atoms of the same element with different mass numbers due to varying #’s of neutrons
Unstable isotopes are called radioactive isotopes

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

9. Radioactive Isotopes Have Useful Applications
Treating some medical conditions, especially cancer
provide diagnostic information
Sterilization of medical equipment

10. The Formation of Molecules Depends on Chemical Bonding Between Atoms
Molecules are substances consisting of two or more elements held together by a covalent bond
Ex’s. CO2, H2O, C6H12O6,
The distribution of electrons in the valence shell determines the reactivity of an atom
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)
Unreactive elements have a full valence shell
Reactive elements lack a full valence shell and will gain, lose, or share electrons to achieve stability

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

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

13. Covalent Bonds Join Atoms Into Molecules
Covalent Bonds are formed when atoms share two/more valence shell electrons
Allows each atom to fill its valence shell at least part of the time
Two types of Covalent Bonds
When electrons are shared equally = non polar covalent bond (electrically balanced) (CO2 and O2 )
When electrons are not shared equally = polar covalent bond

14. Covalent Bonds Join Atoms Into Molecules
How are polar covalent bonds produced?
Electronegativity is the attraction for electrons being shared in a covalent bond
Atoms with different electronegativities produce polar molecules
Ex. H2O

15. Comparing the charge around Types of Chemical Bonds

16. 2. What type of chemical bond is holding a molecule of water together ?

17. Ionic Bonds are Attractions Between Ions
Ionic bonds are formed when oppositely charged ions attract
First an atom transfers a valence shell electron to another atom
When an atom loses one/more electrons ions are formed
Anions (– ions): have gained one or more electrons
Cations (+ ions): have lost one or more electrons – +
Sodium atom (Na)
Chlorine atom (Cl)
Sodium ion (Na+)
Chloride ion (Cl–)
Sodium chloride (NaCl)

18. Hydrogen Bonds +
A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom –
Hydrogen bond + + – – – + + + –
(a) The slightly positive ends (+) of the water molecules become aligned with the slightly negative ends (–) of other water molecules.

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

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

21. Capillary Action