Biology 10, Las Positas College
Chemistry

I-Atoms
II-Chemical bonds
III-Water
IV-pH

INTRODUCTION
Definition: Chemistry = the science dealing with the composition & properties of substances & the reactions by which substances are produced from or converted into other substances

I-ATOMS

Atom = smallest particle of an element which has all the properties of that element.
Consists of electrons, neutrons, protons.
Weight of proton + neutron = atomic mass or weight. Electron mass negligible.

Isotopes-forms of given element which have same number of protons, and therefore, electrons, but different numbers of protons -> different weights.

II-CHEMICAL BONDS
A. Electrons & Energy Levels

Electrons stay in defined spaces around nucleus, called shells.

B. Electrons and the Bonding Behavior of Atoms

Atoms with vacancies in their outer shell will try to fill them or will lose extra ones.
If shell almost full, will take or share electron(s) from another atom or share electrons

Octet rule: outer shell tends to hold 8 electrons (unless 1st shell which only holds 2)

2 or more atoms bonded together form a molecule.

C. Chemical symbols & formulas of compounds.

4 most common elements of human body
Know: elements: C = carbon, H = hydrogen, O = oxygen, N = nitrogen
molecules: H2O = water, C6H12O6 = sugar, CO2 = carbon dioxide, O2 = oxygen

Elements and number of atoms of each element constant in sum of substrates & sum of products.
Ex: 6 CO2 + 6 H2O -> C6H12O6+ 6 O2

Totals - 6 C + 12 H + 18 O = 6 C + 12 H + 18 O.

Types of bonds

Ionic bonds = lose or take electrons.
Covalent bonds = share bonds
Hydrogen bonds = weak bonds involving hydrogen

Borrowing, losing or sharing electrons among or between atoms -> chemical bonds. Chemical bonds -> molecules.
Molecule = 2 or more atoms chemically combined. Atoms are attracted to each other because atoms have ideal number or electrons in outer shell and will combine with other atoms to steal or share electrons to fill outer shell.

A-Ionic = electrons completely transferred from one atom to another. Those atoms that lost electrons become positively charged and those that gained electrons take on a negative charge. Opposite charges attract each other and form a chemical bond.

Ion = an atom, or atoms, that have lost or gained 1 or more electrons.

B-Covalent = electrons shared. An atom strongly attracts one or more electrons of a neighboring atom, but not strongly enough to pull them completely from their shell.

C-Hydrogen bonding = an atom or molecule interacts weakly with a neighboring hydrogen atom that is already taking part in another covalent bond. May form between two or more molecules or between different parts of the same molecule. Important for stabilizing many molecular structures.

III-Properties of Water

A. Introduction

Water is one of the most common & important molecules on Earth.
Present in all cells-plant & animal, common solvent in industry, covers most of the earth.
Is useful because of some of its properties - its polarity, -> its ability to absorb or release great amounts of heat without changing state (gas <-> liquid <-> solid) - moderates temperature, its ability to stick to itself = cohesiveness, and its ability to stick to other things and form solutions = solvent.

B. Polarity of water.

Polar molecules = molecules that have opposite partial charges at different ends of the molecule. Atoms of different elements do not exert the same pull on shared electrons. The more attractive atom pulls the electrons to it -> slightly negative portion of molecule. The other, smaller, atom has slightly less pull on electrons -> slightly positive portion of molecule.

Molecules with a charge, i.e., ions or other polar molecules readily dissolve in H2O = hydrophilic (hydro = water, philic = love).
Molecules with no or symmetrical charge will not dissolve in H2O = hydrophobic (hydro = water, phobic = fear).

C. Solution = a homogeneous mixture of one or more substances (solutes) dispersed in a sufficient quantity of dissolving medium (solvent).

Ex: H2O + NaCl

D. Water's cohesion.

Water sticks to itself because of its hydrogen bonds, -> enough surface tension to support water striders.

E. Temperature-stabilizing effects of water.

Hydrogen bonds in water absorb a lot of energy so temperature of water does not change as much per unit energy input as do other liquids. This property helps stabilize the temperature of water and keep it in a liquid state.

A great deal of energy is required for vaporization. When this occurs, evaporation cools water and helps maintain constant temperature.
When water freezes (big energy difference between liquid and frozen 32oF water), it forms lattice which is less dense than water, floats -> insulating layer on water, helps it keep from freezing.

IV-pH

pH refers to the relative concentration of H+ ions in a solution.
pH range = 0 (acid) - 14 (base)

H2O has pH of 7, is neutral, has equal numbers of H+ and OH-.
H2O <--> H+ and OH-, [H+] = 10-7 = pH 7
H+ comes from acidic chemicals -> pH <7.

Other chemical groups bind H+ -> basic condition -> pH > 7.

Acid = any substance that dissociates to form H+ ions when it is dissolved in water.
Base = any substance that combines with H+ when it is dissolved in water.