Biology 107 Chemical Context of Life August 30, 2002

Chemical Context of Life Student Objectives:As a result of this lecture and the assigned reading, you should understand the following: 1.The six (6) major elements in living systems are: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur (CHNOPS). 2.The number of protons in a nucleus determines the atomic number, and it is different for each element. The total number of protons and neutrons is the atomic mass number. 3.It is mainly the electrons and the different energy levels of electrons in electron shells that determine how an atom behaves when it encounters other atoms. The number of electrons in the outermost shell determine the chemical properties of an atom.

Chemical Context of Life 4.When electrons move from higher electron shells to lower shells they give off energy. Conversely, energy must be supplied to move electrons from lower to higher shells (e.g., photosynthesis, where light energy raises electrons to higher shells). 5.Atoms with filled outer shells tend to be relatively inactive (inert), while atoms whose outer shell is not full tend to interact with other atoms by participating in chemical reactions that result in the filling or emptying of a shell. Atoms may share or transfer electrons of the outer shell. These interactions typically result in atoms staying close together, held together by attractions called chemical bonds. 6.Chemical bonds may be ionic or covalent (polar or non-polar). 7.Atoms and molecules may also interact through intermolecular forces that are not chemical bonds (e.g., hydrogen bonds).

The number of electrons in the outermost shell determine the chemical properties of an atom

When electrons move from higher electron shells to lower shells they give off energy

Atoms participate in chemical reactions by sharing or transferring electrons of the outer shell to result in the filling or emptying of a shell

Ionic chemical bonds result from the transfer of electrons - Video

Covalent chemical bonds result from the sharing of electrons

Covalent chemical bonds result from the sharing of electrons - Video

Covalent chemical bonds which share electrons equally are non-polar

Covalent chemical bonds which share electrons unequally are polar

Covalent bonding - Video