Video Transcript
Which of the following best describes a
compound? (A) Atoms chemically bonded together in a
discrete unit. (B) A substance composed of two or more
elements chemically bonded together in a fixed ratio. (C) A single atom of an element. (D) A mixture of different elements.
If you don’t remember what a compound is,
it might help to think about the role of the word in other contexts, for instance, like
compound interest, which is interest on interest. Or you could think about a compound
fracture, which is a fracture of the bone, which also breaks the skin. These have associations of togetherness
or combination or somehow layering together. Water is a good example of a
compound. A molecule of water, H2O, is composed of
one oxygen atom and two hydrogen atoms. So water is composed of two different
elements. Sodium chloride is another example of a
compound, but the chemical structure of it is completely different to water. Sodium chloride is made up of positive
sodium ions and negative chloride ions, bound together in a lattice that, theoretically,
could extend in all directions for infinite distance.
What these two compounds have in common
is that they consist of two or more elements chemically bonded together. In the case of water, there are covalent
bonds, and in sodium chloride, there are ionic bonds. The last thing we need to figure out is
what fixed ratio means. For a compound containing two elements,
it means that there’s a fixed ratio between atoms or ions of that element and the other
element in the compound. So we have a one-to-one ratio in sodium
chloride between sodium ions and chloride ions. The formula for sodium chloride is
NaCl. Since it’s a one-to-one ratio, we don’t
normally write the subscripts. But before we select this as our final
answer, let’s have a look at the others.
The first statement suggests that
compounds are atoms chemically bonded together in a discrete unit. An example of this would be two oxygen
atoms bonded together in an oxygen molecule, which is not a compound because it contains
only one element. So statement one actually refers to a
definition of molecule. A single atom of an element is simply an
atom. A single atom, for instance, an atom of
argon, is definitely not a compound because it doesn’t contain two or more elements. A good example of a mixture of different
elements is a combination of nitrogen and oxygen gases, which makes up the majority of the
air we breathe. These molecules would not be chemically
bound to each other, so we would not consider this mixture a compound. Therefore, of the four descriptions we’ve
been given, the one that best describes a compound is a substance composed of two or more
elements chemically bonded together in a fixed ratio.
