Video Transcript
Which of the following pairs of
elements is likely to bond covalently? (A) P and F, (B) Mg and Cl, (C) Fe
and H, (D) S and Ne, or (E) Ba and K.
Covalent bonds are chemical bonds
that are formed when two atoms share one or more pairs of electrons. Two atoms are more likely to form a
covalent bond if the atoms have similar electronegativity and atomic size.
Shown here are some examples of
molecules that contain covalent bonds. In diatomic hydrogen, the hydrogen
atoms share a pair of electrons to form a single covalent bond. One of the shared electrons was
originally from the hydrogen atom on the left, while the other shared electron
originally belongs to the atom on the right. In a molecule of carbon dioxide,
each oxygen atom shares two of its electrons. The central carbon atom also shares
two of its electrons with each oxygen atom. This forms a double covalent bond
between the carbon atom and each oxygen atom. In diatomic nitrogen, each nitrogen
atom shares three electrons for a total of six shared electrons between them,
forming a triple covalent bond.
All of the atoms that form a
covalent bond in these examples share electrons in such a way that each atom now has
a full valence shell. We can say that covalent bonds are
typically formed so that each atom has a full valence or outer electron shell. In addition, all of the atoms in
the examples are nonmetals. So, we can say that typically
covalent bonds are formed between nonmetal elements.
To answer this question, we need to
determine which of the pairs of elements are likely to bond covalently. So, we are looking for a pair of
elements that are nonmetals where neither nonbonded atom has a full valence
shell.
We can determine which elements are
nonmetals by looking at the periodic table. Elements of the periodic table are
categorized as metals, nonmetals, or metalloids. We can see that of the elements
given, barium, potassium, magnesium, and iron are metals, while the rest are
nonmetals. Answer choices (B), (C), and (E)
either consist of two metals or a metal and a nonmetal. So, none of these answer choices
can be the answer to this question. Answer choices (A) and (D) do
consist of two nonmetals. But only one of these pairs of
elements is likely to bond covalently.
Let’s take a closer look at the
atoms of these elements. We can see that phosphorus,
fluorine, and sulfur do not have full valence shells, while neon does. We have seen that covalent bonds
are typically formed so that each atom gains a full valence shell. This means that the nonbonded atoms
do not initially have a full valence shell. As neon already has a full valence
shell, it is extremely unlikely to react and is considered inert. So, answer choice (D) cannot be the
answer to this question.
Therefore, the pair of elements
that is likely to bond covalently is answer choice (A), P and F.
