Video Transcript
For which of the following elements
is the second electron shell the valence shell? (A) Hydrogen, (B) helium, (C)
aluminum, (D) chlorine, (E) carbon.
There are several layers of
organization to the periodic table of elements. For example, the A group numbers
indicate the number of valence electrons of elements in the same column. And the period or horizontal row
number represents the number of electron shells in elements in that row. Excluding the transition and inner
transition elements, the period number also identifies the valence electron
shell.
Finally, the subshell blocks can
help us to identify which subshells the valence electrons are found in. For example, sodium is found in
group 1A and period three. Because sodium is found in period
three, sodium atoms have electrons found in three electron shells. The third shell is the outermost
electron shell that contains electrons and is therefore the valence shell. And inside this valence shell, we
find one valence electron which corresponds to the A group number of the group that
sodium is in.
To answer this question, we need to
determine which of the elements listed in the answer choices has electrons in two
electron shells, with its valence electrons in the second electron shell. The elements hydrogen and helium
are in period one. So they only have electrons in the
first electron shell. Therefore, answer choices (A) and
(B) cannot be correct.
The elements aluminum and chlorine
are located in period three. So these two elements have
electrons found in three electron shells. And their valence electrons are
located in the third electron shell, not the second shell. Therefore, answer choices (C) and
(D) are also incorrect.
Finally, the element carbon is
located in period two. Therefore, in carbon atoms,
electrons are found in two electron shells. And the outermost or valence
electron shell in a carbon atom is the second shell. In which of the following elements
is the second electron shell the valence shell? The correct answer is carbon or
answer choice (E).
