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A single atom is composed of electrons (small, negatively charged particles of mass and charge ), protons (larger, positively charged particles of mass and charge ), and neutrons (particles almost equal in size to protons without charge).
The nucleus of an atom is the highly dense core of an atom composed of neutrons and protons. The number of protons in the nucleus (the atomic number) is a defining characteristic of elements, that is, each element found on the periodic table of elements has a unique integer number of protons in the nucleus. The number of neutrons often varies, and these variations define several isotopes of an element. Adding or losing neutrons in the nucleus only affects the overall mass of the atom and does not affect the identity of the element since the number of protons remains constant.
The electrons exist in a large electron cloud surrounding the nucleus. Due to the indeterminacy of the position and momentum of electrons at any given moment and the wave-particle duality of electrons, we can't say with certainty where electrons exist in the electron cloud at any given moment. Instead, we describe probability distributions of where an electron may exist in space around the nucleus at any given point in time. These probability distributions are called orbitals, and these orbitals are defined by specific potential energy levels of the electrons existing in those orbitals.
These electron probability distribution shells surrounding the nucleus have a large volume, and since only a few electrons exist in each orbital, and the nucleus is so dense, the atom itself is mostly empty space, 99.9999% empty space. The attraction between the negative electrons and the positive nucleus keeps the electrons moving around the nucleus.
Typically the number of electrons equals the the number of protons, so that the overall charge of the atom (the sum of the charges of all the particles) is 0.