Atoms, Molecules And Life

Chapter 2: Atoms, Molecules and Life
Lecture Outline
The Structure of Atoms
1. An atom is the smallest unit of matter that is unique to a particular element.
2. An element is a substance that can neither be broken down nor converted to different substances by ordinary chemical reactions.
3. A molecule is a bonded unit of two or more (same or different) atoms.
4. Compounds are substances inwhich two or more elements are combined in fixed proportions. i.e. water
5. A mixture is composed of two or more compounds, in variable proportions.
6. Atomic Building Blocks
a. Protons (p+) are part of the atomic nucleus and have a positive charge.
b. Neutrons are also a part of the nucleus; they are neutral.
c. Electrons (e-) have a negative charge. Their quantity is equal to that of theprotons. They move around the nucleus.
7. Atomic Number and Mass Number
a. Atomic number = the number of protons in the nucleus of an element.
b. Protons plus neutrons = atomic mass.
 
Isotopes: Variant Forms of Atoms
1. Atoms with the same number of protons (for example, carbon with six) but a different number of neutrons (carbon can have six, seven, or eight) are called isotopes.(e.g. 12C, 13C,14C).
2. Some radioactive isotopes are unstable and tend to decay into more stable atoms.
a. They can be used to date rocks and fossils, or to diagnose disease.
Electrons and Energy Levels
1. Electrons are attracted to protons but are repelled by other electrons.
Orbitals (energy levels) permit electrons to stay as close to the nucleus and as far from each other as possible.
Orbitals canbe thought of as occupying shells around the nucleus.
a. The shell closest to the nucleus has one orbital holding a maximum of two electrons.
b. The next shell can have 4 orbitals with two electrons each for a total of 8 electrons.
2. The basic principles of atomic reactivity are:
a. An atom is stable (will not react with other atoms) when its outermost electron shell is completely full orcompletely empty.
b. An atom is reactive when its outermost electron shell is on partially filled.
 
Chemical Bonds: Joining Atoms to Make Molecules
A. What Is a Chemical Bond?
1. A chemical bond is a union between atoms formed when they give up, gain, or share electrons.
2. Whether one atom will bond with another depends on the number and arrangement of its electrons.
B. Ionic Bonding
1. Whenan atom loses or gains one or more electrons, it becomes positively or negatively charged. This is called an ion.
2. In an ionic bond, (+) and (-) ions are linked by mutual attraction of opposite charges. Ionic bonds between two atoms with large differences in electronegativity. e.g. NaCl, KCl.
C. Covalent Bonding
1. A covalent bond holds together two atoms that share one or more pairs ofelectrons.
2. In a nonpolar covalent bond, atoms share electrons equally.
Most nonpolar covalent bonds occur in diatomic molecules.
e.g. H2, Cl2, O2, and N2.
3. In a polar covalent bond, because atoms share the electron unequally, there is slight difference in charge between the two poles of the bond; HCl and water are examples.
D. Hydrogen Bonding
1. In a hydrogen bond, an atom or a moleculeinteracts weakly with a hydrogen atom already taking part in a polar covalent bond.
2. These bonds impart structure to liquid water and stabilize nucleic acids and other molecules.
 
Water and Life
A. Water has outstanding solvent properties.
1. Water is a great solvent because ions and polar molecules (solutes) dissolve in it.
B. The polarity of the water molecule influences the behavior ofother substances.
1. Because of the electron arrangements in the water molecule, a polarity results that allows water to form hydrogen bonds with itself and other polar substances.
2. Polar substances are hydrophilic (water-loving); nonpolar ones are hydrophobic (water-dreading) and are repelled by water.
C. Water has temperature-stabilizing effects.
1. Water tends to stabilize temperature...