Humboldt State University ® Department of Chemistry

Richard A. Paselk

Science 331
Fall 2004 Lecture/Activity Office: SA560a
Notes: 1 November Phone: x 5719
Home: 822-1116
e-mail: rap1

What is the basis of the periodicity of properties?

Demo spectroscopes and gratings - rainbows vs. line spectra.

In any atom under Earth surface conditions the electrons will be in the lowest possible energy state (as close as possible to the nucleus). An atom with all of its electrons in this lowest energy state is said to be in its ground state.

In atoms, electrons are held in shells. Shells correspond to average distances of the electron from the nucleus.

Just for fun and your greater wisdom - The modern atom.


Lewis Structures

Lewis structures are a very simple model for representing some chemical properties of atoms. Lewis structures are generally only used for Representative elements ("A" Group elements). For Lewis structures we assume

  1. all atoms above B (C and beyond, Z>5) want eight electrons in their outermost shells.
  2. We represent the nucleus and all inner electrons as the symbol which is referred to in Lewis structures as the "kernal" of the atom.
  3. Outer, valence, electrons are then represented as dots for individual electrons.
  4. Bonding pairs of electrons may be also represented as lines to simplify structure representations for molecules.

Lewis Structures for Atoms: Here we just show the inner "kernel" where the symbol stands for the nucleus and all inner shell electrons. Only good for Representative elements. For ions the charge is always shown. Thus for metal ions such as calcium the Lewis Structure simply becomes the symbol for the ion. For negative ions such as we see for oxygen (2-) we enclose the ion and its electrons in brackets to indicate that the electrons are all "owned" by the oxygen - it does not share.

Note that for elemental ions:


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© R A Paselk

Last modified 1 November 2004