Chapter Two: The Chemical Basis of Life

Questions:

1. What are the differences between an ionic bond, and a covalent bond?
2. What makes a covalent bond polar? 
3. What are some of the benefits of radioactive isotopes?

Answers: 

1. In an ionic bond, the atoms are bound together by the attraction between oppositely-charged ions. For example, sodium and chloride form an ionic bond, to make NaCl, or table salt. In a covalent bond, the atoms are bound by shared electrons. 
2. A molecule is nonpolar when its covalently bonded atoms equally share electrons.  If a molecule is polar, electrons are not shared equally because there are differences in electronegativity.  An example of a polar molecule is water.  The H atoms of one water molecule may be attracted to O or N atoms. 
3. Radioisotopes can be used as radioactive tracers that follow molecules as they undergo changes in an organism.  They can also be used in the radiation treatment of cancer as well as in carbon dating.  

Important Facts:

1. The pH (potential of hydrogen) scale describes how acidic or basic a solution is.  It ranges from 0 to 14.  The lower a solution is on the scale, the more acidic it is.  Meaning that the higher a solution is, the more basic it is.  Every unit of the pH scale depicts a tenfold change in the concentration of hydrogen in a solution.
2. The different isotopes of an element possess different amount of neutrons, but still have the same amount of protons and electrons.  Some isotopes are stable because their nuclei don't have the tendency to lose neutrons.  Some isotopes, however, are radioactive or unstable.  This means that the isotope in question has a nucleus that decays spontaneously, giving off particles and energy.  Radioisotopes give off radiation that can be harmful to the health of an organism. 
3. Electron arrangement determines the properties of an atom.  The farther away an electron is from the nucleus, the greater its energy.  The number of electrons in the outermost shell determines the chemical properties.  The first electron shell can hold two electrons, while the second and third shell can have up to eight electrons.  Atoms whose outer electron shells are not full tend to participate in chemical reactions.
4. Hydrogen bonds are very important in life.  They make water molecules cohesive.  The cohesiveness of those molecules create surface tension and allow water to move from plant roots to leaves.  Cohesion creates surface tension.  Hydrogen bonds also cause water to be adhesive, meaning they cling to other substances.  
5. Hydrophilic substances are water-soluble.  Ionic compounds and polar molecules are hydrophilic.  Hydrophobic substances are not water soluble.  These substances include nonpolar molecules such as oils.    

Diagram: This shows the structure of an atom.  As you can see, the protons and neutrons make up the nucleus, while electrons surround them.  

Key Terms:

Matter- Anything that occupies space and has mass, composes living organisms.

Element- A substance that cannot be broken down to other substances by chemical reactions. (gold, copper,silver, iron)
Compound- A substance consisting of two or more different elements combined in a fixed ratio.
Trace Elements- The 25 elements essential to life.
Atom- The smallest unit of matter that still retains the properties of an element.
Proton- Subatomic particle that has a single positive charge.
Neutron- Subatomic particle that is electrically neutral.
Electron- Subatomic particle that has a single negative charge.
Cohesion- the tendency of molecules to stick together.
Adhesion- the clinging of one substance to another.
Electronegativity- An atom's attraction for shared electrons.
Solution- A liquid consisting of a uniform mixture of two or more substances.
Aqueous Solution- A solution in which water is the solvent.

Relevant Video: 

http://www.youtube.com/watch?v=K5Ks2X5TphI

Summary: 


This chapter focuses on the composition of matter and then delves  into the life-supporting properties of water.  It begins by discussing the 25 elements essential to the survival of living organisms.  The four most important ones are nitrogen, oxygen, carbon, and hydrogen.  The chapter goes on to explain how elements bond to form compounds.  There are two different types of chemical bonds, ionic bonds and covalent bonds.  It also explains what makes a molecule polar and what makes hydrogen bonds so important.  It also discusses the pH scale and the nature of solutions, solutes, and solvents.