The atomic number of an element is equal to the number of _____ in the nucleus.

The atomic number of an element is equal to the number of protons in the nucleus.

An element is the purest form of a substance. When divided, it results in a large number of identical atoms—assuming no isotopes are there. Atoms contain three main types of particles: protons, neutrons and electrons. The protons are positively charged particles, while the electrons are negatively charged particles. In comparison, neutrons do not have any charge. The nucleus consists of neutrons and protons, while the electrons are outside the nucleus. The number of protons and neutrons combined together are known as the mass number of the element. 

In the case of isotopes, atoms may contain different numbers of neutrons, but they still contain the same number of protons and electrons.

Hope this helps.