An element is a pure substance: It cannot be broken down into simpler substances. Because an element is pure, every atom of that element is like every other atom of that element with the exact same physical and chemical properties. Atoms, the smallest units of matter, are made of subatomic particles: protons, neutrons and electrons. Electrons surround the nucleus and are negatively charged particles with almost no mass. Protons and neutrons, at the center of the atom, do have mass. Protons are positively charged particles, while neutrons are neutral.

In most atoms, the number of electrons and protons are the same, balancing the atom's electrical charge. Sometimes, however, a few of the electrons can become shared with another atom. This creates a new molecule called a compound. Compounds have the atoms of two or more different elements.

Hydrogen bromide is a chemical compound. It’s formed from atoms of hydrogen and bromide, two
A
electrons.
B
elements.
C
neutrons.
D
nuclei.