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

1. CHAPTER 2The Chemical Basis of Life By Dr. Par Mohammadian • Nature’s chemical language • Elements, Atoms, Molecules, Chemical Bonds • Water’s life supporting properties • Chemical reaction

2. 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

4. Elements can combine to form compounds • The smallest particle of an element is an atom • Different elements have different types of atoms • Chemical elements combine in fixed ratios to form compounds • 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

6. Electron arrangement determines the chemical properties of an atom • 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

7. Chemical Bonds • 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

8. Ionic bonds: Sharing of electrons Covalent bonds, the sharing of electrons, join atoms into molecules

9. 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

10. 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

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

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

13. Water as the Solvent of Life • The dissolving agent is called the solvent • The dissolved substance is called the solute • A solution is a liquid consisting of two or more substances evenly mixed • aqueous solutions

14. 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

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

16. 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

17. Chemical Reactions • Such changes in the chemical composition of matter are called chemical reactions • Cells constantly rearrange molecules by breaking existing chemical bonds and forming new ones • 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