•Nature’s chemical language
•Elements, Atoms, Molecules, Chemical Bonds
•Water’s life supporting properties
•Chemical reaction

Life requires about 25 chemical elements
• Living organisms are composed of matter.
Matter: Anything that occupies space and has mass (air, water, etc.). Matter is found on the
Earth in three physical states: Solid, Liquid, Gas
• Matters are composed of elements.
Element: A substance that cannot be broken down to other substances by ordinary chemical means

Elements can combine to form compounds
• Chemical elements combine in fixed ratios to form compounds
• The smallest particle of an element is an atom
• Different elements have different types of atoms
• An atom is made up of protons and neutrons located in a central nucleus
• The nucleus is surrounded by electrons (The electrons orbit the nucleus) – The number of protons, the atomic number , determines which element it is
–
An atom’s mass number is the sum of the number of protons and neutrons

• Electrons are arranged in shells
– The outermost shell determines the chemical properties of an atom
– In most atoms, a full outer shell holds eight electrons

• Chemical reactions enable atoms to give up or acquire electrons in order to complete their outer shells
– These interactions usually result in atoms staying close together
– The atoms are held together by chemical bonds:
Ionic Bonds
Covalent Bonds
Hydrogen Bonds

Covalent bonds, the sharing of electrons, join atoms into molecules

THE PROPERTIES OF WATER
Water is a polar molecule
• Atoms in a covalently bonded molecule may share electrons equally, creating a nonpolar molecule
• If electrons are shared unequally, a polar molecule is created

Water’s polarity leads to hydrogen bonding and other unusual properties
• The charged regions on water molecules are attracted to the oppositely charged regions on nearby molecules
– This attraction forms weak bonds called hydrogen bonds
Hydrogen bond

• Water molecules stick together as a result of hydrogen bonding
Hydrogen bonds make liquid water cohesive
Water’s hydrogen bonds moderate temperature

• Like no other common substance, water exists in nature in all three physical states:
– as a solid
– as a liquid
– as a gas
Ice is less dense than liquid water

• A solution is a liquid consisting of two or more substances evenly mixed
– The dissolving agent is called the solvent
– The dissolved substance is called the solute
• aqueous solutions

The chemistry of life is sensitive to acidic and basic conditions
• Acid
– A chemical compound that donates H + ions to solutions
• Base
– A compound that accepts H + ions and removes them from solution
• Acidity is measured on the pH scale:
– 0-7 is acidic
– 8-14 is basic
– Pure water and solutions that are neither basic nor acidic are neutral, with a pH of 7

• Cells are kept close to pH 7 by buffers
• Buffers are substances that resist pH change
– They accept H + ions when they are in excess
– They donate H + ions when they are depleted

Acid precipitation threatens the environment
• Some ecosystems are threatened by acid precipitation
• Acid precipitation is formed when air pollutants from burning fossil fuels combine with water vapor in the air to form sulfuric and nitric acids

• Cells constantly rearrange molecules by breaking existing chemical bonds and forming new ones
– Such changes in the chemical composition of matter are called chemical reactions
–On the left side of the equation are the reactants, the starting materials
–On the right side of the equation are the products, the end materials