Electron Weirdness: Metals, Orbitals, and Valence Electrons
Take a look at the Periodic Table. Notice the two periods below the table: where do they fit?
These two periods are metals, like most of the other elements on the Periodic Table.
These two “orphaned”
transitional metal elements
actually fit between elements 57
and 72 (the Lanthanides –
period 6) and elements 89 and
104 (the Actinides – period 7).
Their awkward position below
the rest of the elements of the
Periodic Table brings us to an
important rule about electrons
filling shells. For any period
(row) the total number of
valence electrons – those
electrons at the highest energy state and involved with making bonds to other atoms - that can
fill any shell, with the exception of the first period (only 2), is 8 electrons. We can see that
periods 2 and 3 have 8 elements in their rows so they obviously have 8 electrons to fill each shell,
but periods 4 and 5 have 18 elements in their rows and periods 6 and 7 have 32 elements in their
rows (when we fit in the Lanthanides and Actinides)! How can there be only 8 valence electrons
in those longer periods? Metals have a unique property of “hiding” excess electrons below their
valence electrons. The excess electrons go into orbitals (sort of shells within shells) that are not
involved in bonding with other elements the way valence electrons are (although there are some
rare exceptions!). In fact all electrons fit into specific orbitals within their respective shells but
we won’t go any further into detail for now. What we do need to know is that when we add
electrons into the outermost shell for periods 4 through 7 after the first 2 electrons (valence
electrons) the next 10 electrons in periods 4 and 5 are hidden below the valence electrons and are
not involved in bonding. And in periods 6 and 7 after the first 2 (also valence electrons) the next
18 electrons are hidden below the valence electrons and are not involved in bonding. In each of
these periods after the hidden electrons are filled we continue filling in up to 8 valence electrons
in any outer shell for any element. These hidden electrons in metals form a kind of sea of
electrons that flow and conduct electricity and give metals their strength as well. In fact metals
often bend before they break due to their excess electrons.
Let’s find the valence electrons for a couple of metals.
Example: Find Silver on the Periodic Table (Ag) – It is in Period 5. It has 2 electrons in its first
shell, 8 in its second shell, 8 in its third shell, 18 in its fourth shell (with 10 hidden), and 2 valence
electrons in its fifth shell with 9 electrons hidden.
Example: Find Gold on the Periodic Table (Au) – It is in Period 6. It has 2 electrons in its first
shell, 8 in its second shell, 8 in its third shell, 18 in its fourth shell (with 10 hidden), also 18
electrons in its fifth shell (with 10 hidden), and finally, 2 valence electrons in its sixth shell with
22 hidden – don’t forget the Lanthanides series!
Try these problems For the atoms below:
1. Determine how many electrons are in each shell for the following atoms.
2. Indicate how many valence electrons the element has.
Scandium, Titanium, Iron, Arsenic, Bromine, Cesium, Tungsten, Iodine, Uranium