A(n) is a substance that is made up of a single kind of atom.

An element is a substance that is made up of a single kind of atom, the building blocks of matter.   It is a substance that consists of atoms with the same number of protons or atomic number. It is a material which cannot be changed into another form nor broken down.  Examples of elements are Oxygen, carbon, Mercury, and many others.  There are about 117-118 known elements.
You can see the atomic number, atomic weight,  and other information about certain elements on the periodic table.

The answer would be dogs.