1. The Chemical Basis of Life
Chapter 2

2. Organisms are Composed of Elements
are substances that cannot be broken down to other substances by ordinary chemical means
are composed of atoms

3. An Atom is the building block of an Element
Atoms have smaller particles
Neutrons
no charge; in nucleus
mass = 1 dalton
Protons
positive charge; in nucleus
1 dalton
Electrons
negative charge , orbiting nucleus
zero dalton
equal in number to protons of an atom

4. 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)

5. 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

6. 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

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

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

9. Organisms are Composed of Elements
can combine to form compounds

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

11. Organisms are Composed of Elements
can also combine to form molecules
A molecule is formed when two or more atoms are held together by covalent bonds
All compounds are molecules but not all molecules are compounds

12. Chemical Bonds
The number of electrons in the valence shell determine the reactivity of an atom
Octet rule: Except for the first shell (full with two electrons), atoms interact in order to have eight electrons in their valence shell
Inert elements have a full valence shell and will not react to form a molecule
Reactive elements lack a full valence shell and will react by gaining, losing, or sharing valence electrons (forming a molecule)

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

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

15. Covalent Bonds Join Atoms Into Molecules
Covalent Bonds are formed when atoms share two or more valence shell electrons
Two Types of Covalent Bonds
In non-polar covalent bonds valence electrons are equally shared (Ex’s. are CO2 and O2 )
In polar covalent bonds valence electrons are unequally shared (Ex’s. are H2O and )

16. Covalent Bonds Join Atoms Into Molecules
Why are electrons shared equally/unequally?
Electronegativity is the attraction for electrons being shared in a covalent bond
When atoms in a bond have a significant difference in electronegativities polar covalent bonds are produced

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

18. Ionic Bonds are Attractions Between Ions
Ionic bonds are formed when oppositely charged ions attract
A valence electron is lost from, or is added to another atom
When an atom loses one/more electrons a cation is formed and when it gains one/more electron an anion is formed. – +
Sodium atom (Na)
Chlorine atom (Cl)
Sodium ion (Na+)
Chloride ion (Cl–)
Sodium chloride (NaCl)

19. 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.

20. 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

21. 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

22. Capillary Action