Elements on the periodic table are arranged
based on how many protons and electrons each
element has. Protons are positively charged and
are found the in the nucleus along with neutrons.
Electrons, which are much smaller than protons,
are negatively charged and are found in orbitals
('shells') around the nucleus.
The families of elements are arranged in
columns on the periodic table. Elements in the
same family have similar properties because they
have similar electron configurations. The atomic
number (Z) of an element tells us how many protons
(and electrons) an atom has. For example,
magnesium (Mg, Z=12) has 12 protons and 12
electrons and calcium (Ca, Z=20) has 20 protons
and 20 electrons. Even though Mg and Ca have
different atomic numbers, they are in the same
family because they both have the same electron
configuration - full s-orbitals.
What does that mean? Atoms have s, p, d, and
f-orbitals. Each s-orbital hold 2 electrons,
p-orbtials hold 6, d-orbitals hold 10, and
f-orbitals hold 14 electrons. They fill with
electrons in the order:
1s2s2p3s3p4s3d4p5s4d5p6s4f5d6p7s5f6d7p. Mg has 2
electrons in 1s, 2 electrons in 2s, 6 electrons in
2p, and 2 electrons in 3s, totaling 12 electrons.
Only the outermost (valence) orbital matters in
determining which family the element is in.
Magnesium's outermost orbital is the filled
3s-orbital and calcium's outermost orbital is the
filled 4s-orbital. All elements in that family
have filled s-orbitals.
The columns of the periodic table are arranged
by valence orbital. The first two columns are for
the s-orbitals. Columns 3-12 are known as the
d-block elements because their valence orbital is
the d-orbital. Columns 13-18 are the p-block
elements. Column 13 elements have one electron in
their outermost p-orbital while column 18, the
noble gases, have filled outermost p-orbitals. The
two rows at the very bottom of the periodic table
are the f-block elements.
Here is a really great periodic table from the
National Institute of Standards that shows all of
the electron configurations for all of the atoms
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