The nucleus of an atom contains neutrons and protons. Protons are positively charged, whereas neutrons are neutral. Electrons, while part of the atom, are not part of the nucleus.
Atoms are electrically neutral, which means that they have an equal amount of negative and positive charge. The net charge of the nucleus of an atom is positive because of the positive protons. To counteract this, negatively charged electrons orbit the nucleus.
A neutral atom has the same number of protons as electrons. Unlike protons, the number of neutrons in the nucleus can vary. An atom that contains the correct amount of protons and electrons, but with a different number of neutrons in the nucleus, is called an isotope.
Most of the mass of an atom is located in the nucleus. Electrons are much lighter than protons and neutrons.
Even though a nucleus contains most of an atom's mass, it is very small compared with the atom's total size. This is because the protons and neutrons are held closely together by nuclear force. Without this nuclear force, the positive protons would repel each other and the nucleus wouldn't exist. If the nuclear force is too weak, the nucleus breaks up, resulting in radioactive emissions.