Video Transcript
Which of the following pairs of
elements have a valence of one? (A) Eight O and 13 Al. (B) Seven N and six C. (C) 20 Ca and three Li. (D) Three Li and nine F. (E) 15 P and 16 S.
The valence of an element is the
number of electrons an atom gains, loses, or shares when forming a bond during a
chemical reaction. A valence of one means that atoms
of that element have a tendency to lose, gain, or share one electron. Atoms have this tendency to gain or
lose electrons to attain a stable electronic configuration. So let’s take a look at the
elements we were given in our answer choices.
This question gave us the atomic
number for each element, which is equal to the number of protons and for a neutral
atom is also equal to the number of electrons. The first element in answer choice
(A) is oxygen, which has an atomic number of eight. This means that atoms of oxygen
have eight electrons. The first two are in the K-shell
and the remaining six are in the L-shell. We want to know how many electrons
an atom of oxygen will lose or gain to attain a stable electronic configuration,
which is two electrons in the K-shell and eight in other shells.
To attain the stable electronic
configuration, atoms of oxygen can either gain two electrons to fill the L-shell
with eight electrons or lose the six electrons in the L-shell, leaving two electrons
in the K-shell. It is far more likely an oxygen
atom will gain a small number of electrons than lose a larger number. So atoms of oxygen tend to gain two
electrons, meaning oxygen has a valence of two. As oxygen has a valence of two and
not one, we can eliminate answer choice (A).
Let’s move on to answer choice
(B). The first element in this pair is
nitrogen. Nitrogen has seven electrons: two
in the K-shell and five in the L-shell. To fill the L-shell and attain a
stable electronic configuration, atoms of nitrogen tend to gain three electrons. So the valence of nitrogen is
three, not one. So we can eliminate answer choice
(B).
Next up is calcium with an atomic
number of 20. Calcium has a lot of electrons, so
we won’t worry about the electronic configuration here. We just need to know that calcium
has two electrons in the outer shell. To attain a stable electronic
configuration, atoms of calcium tend to lose two electrons. This will leave a calcium ion with
a full outer shell. So calcium has a valence of two,
meaning we can eliminate this answer choice as well.
Next is lithium, which has an
atomic number of three. This corresponds to an electronic
configuration of 2,1. Atoms of lithium tend to lose one
electron to attain a stable electronic configuration. This leaves two electrons in the
K-shell. Since lithium tends to lose one
electron, it has a valence of one. Since lithium has a valence of one,
answer statement (D) could be the right answer. So let’s check fluorine to
confirm.
Fluorine has the atomic number
nine, corresponding to the electronic configuration 2,7. To attain a stable electronic
configuration, atoms of fluorine tend to gain one electron to fill the L-shell. So the valence of fluorine is also
one.
Of the pairs of elements we were
given, the pair that had a valence of one was answer choice (D), lithium and
fluorine.
