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http://www.chem.queensu.ca/chembook/articles/Elements.htm
Important Elements
 Carbon has many different properties. It can be hard diamond or soft
graphite. All living things are carbon based. Carbon forms more compounds
than any other element.
 Hydrogen is the lightest element and has the simplest atoms. Water
contains hydrogen. Hydrogen is included in nuclear fusion reactions, which
are the energy in the sun. This keeps us warm.
 Oxygen makes up 20 percent of the air we breathe. Oxygen is also in water.
It makes up a large portion of many minerals and rocks.
 Nitrogen makes up about 80 percent of the air we breathe, although our
bodies do not readily absorb nitrogen. Farmers use fertilizers that contain
nitrogen compounds.
 Calcium is important for building strong bones and teeth. It helps control
muscle growth and the electrical impulses in the brain.
Valence Electrons
 Valence Electrons are the outermost electrons of an atom. This is important
in deciding how the atom reacts chemically with other atoms.

 We determine the number of valence electrons that an element has by
looking at the periodic table group that the element is in.
 We can shorten electron configurations to concentrate more on the valence
electrons by writing the Noble-gas notation.
 Instead of 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d9 for Silver, Ag
one could write [Kr] 5s2 4d9
 This way we write out the valance electrons, the important electrons,
but we acknowledge that the innermost electrons are still there.
 The atomic radius of an element is half of the distance between the centers
of two atoms of that element that are just touching each other. Generally,
the atomic radius decreases across a period from left to right and increases
down a group.
 Ionization energy is the energy required to completely remove a valence
electron from its atom when forming an ionic bond.