Transcript electron shells

Biology 107
Chemical Context of Life
August 29, 2003
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. Isotopes differ in the number
of neutrons.
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
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
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
Atoms participate in chemical reactions by sharing
or transferring electrons of the outer shell to result
in the filling or emptying of a shell
Covalent chemical bonds result from the
sharing of electrons
Covalent chemical bonds result from the
sharing of electrons
Covalent chemical bonds which share
electrons equally are non-polar
Covalent chemical bonds which share
electrons unequally are polar
A molecule’s biological function is related to
its shape.