Chemistry of Life A Brief … Overview

Matter Matter occupies space and has weight. It can exist as a solid, liquid, or gas. It may be possible to break some kinds of matter down into other kinds of matter with different properties. For example, water (H2O) can be broken down into hydrogen and oxygen. Hydrogen and oxygen in the above example cannot be broken down any further because they are elements.

Elements Elements cannot be broken down into substances with different properties. For example, water (H2O) is not an element because it can be broken down into hydrogen (H) and oxygen (O). The smallest particle of an element is an atom. Elements are substances made up of only one kind of atom. There are 92 naturally occurring elements. Matter is therefore composed of 92 different kinds of elements. The following elements make up 98% of the body weight of organisms: Carbon, Hydrogen, Oxygen, Nitrogen, Sulfur, Phosphorus.

Atoms

Characteristics of Subatomic Particles The mass of subatomic particles is too small to be conveniently measured in grams so atomic mass units (amu) are used instead. Atomic mass units are also called daltons. One amu (or dalton) is approximately 1.7 X g. Protons and neutrons have a mass of approximately 1 amu. The mass of an electron is much less. The total mass of an atom is due mostly to the mass of protons and neutrons.

Atomic Weight The atomic weight is calculated as the sum of the mass of protons, electrons, and neutrons. The mass of electrons is small enough that we can generally disregard it in our calculations. Most of an atom is empty space.

Atomic Number The atomic number is the number of protons. All atoms of an element have the same atomic number. The number of protons contributes to the physical properties of an element. Atoms are neutral, therefore the number of electrons is equal to the number of protons.

Atomic Symbols

Isotopes Isotopes are atoms that have the same number of protons and differ only in the number of neutrons. Three different forms of hydrogen are shown below Most isotopes are stable but radioactive isotopes are unstable and break down into more stable forms by emitting radiation. Radiation can be detected, so radioactive isotopes are useful as labels in scientific research and medical diagnostic procedures.

Energy Energy is the ability to do work. For example, it takes energy to move matter. Potential energy is stored energy. Electrons at greater distances from the nucleus contain more stored energy. Example - Many of the chemical reactions that are associated with energy (ex: photosynthesis) involve electrons moving to higher or lower energy levels.

Distribution of Electrons Electron Shells Orbitals The Outer Shell Chemical bonds form when atoms react to fill their outer shells with electrons. A compound is two or more elements joined together by chemical bonds.

Periodic Table of the Elements

Ionic Bonding Ionic bonding is the transfer of electrons from one atom to another. Atoms that have lost or gained electrons are called ions. The gain or loss of electrons results in ions having a positive or negative charge.

Activity Draw a calcium atom. Draw a chlorine atom. Use circles to represent electrons and tell how many protons and neutrons are in the nucleus. Draw calcium chloride.

Covalent Bonds Covalent bonds form when atoms share electrons. Hydrogen atoms contain one electron and one proton. In the diagram, two hydrogen atoms are bonded by a single covalent bond. The two atoms each share a pair of electrons.

Molecules CH 4

Double and Triple Bonds

A Shorthand Method for Drawing Covalent Bonds

Polar Covalent Bonds When the atoms share the electrons equally, the molecule is nonpolar. Polar molecules are formed when two different size atoms are covalently bonded. The molecule is polar because the electrons are not shared equally between the two atoms; the electrons spend more time with the larger atom.

Cont’d

Hydrogen Bonds

Activity Draw two atoms that are bonded by a single covalent bond. Draw 2 atoms bonded by a double covalent bond and 2 bonded by a triple covalent bond. Finally, draw two atoms that are bonded by a polar covalent bond. Use any hypothetical atoms. After your diagrams are complete, identify the atom. Use the periodic table if necessary.

Chemical Equations Not Balanced Balanced

Important Concepts Regarding Covalent Bonds Energy and Covalent Bonds Energy is required to form a covalent bond and energy is released when a Covalent bond is broken. Covalent bonds can therefore be used to store energy.

Oxidation-Reduction Reactions Oxidation is the loss of elecrons or the loss of hydrogen atoms from an atom or molecule. As a result, energy is released. Reduction is the gain of electrons or the gain of hydrogen atoms. This process stores energy. Oxidation and reduction occur together. When a atom or molecule is oxidized, another must be reduced. Redox Reaction

Water Life evolved in water. Living things are % water. In nature, water is a solvent for many kinds of chemical reactions. Water is a polar molecule because the hydrogen atoms are much smaller than the oxygen atoms and also because the molecule is arranged so that the two hydrogen atoms are on one side while the oxygen is on the other.

Cont’d Ions Separate (Dissolve) in Water

Cont’d Water molecules adhere to polar molecules.

Cont’d The temperature of water changes more slowly than that of other liquids. It takes 1 calorie of heat to raise the temperature of 1 gram of water 1° C. To raise the temperature of an equal volume of air 1° C takes calories. Water requires 3000 times more energy than air. This property protects organisms from rapid temperature changes.

Cont’d Water resists changing to a solid or a gas. Ice is less dense than water. Ionization of Water

Acids and Bases HCl  H+ + Cl- NaOH  Na+ + OH- H2O  H+ + OH-

pH The measure of the strength of an acid or base is called the pH. It is a measure of the concentration of hydrogen ions (H+). The pH ranges from 0 to 14. An acid has a pH less than 7. A base has a pH greater than 7. A pH of 7 is neutral.