The modern atomic model has not been greatly modified since the development of quantum chemistry. It is now understood that atoms have a dense, massive nucleus at their center. This is the location of positively charged particles called protons and particles that have no charge called neutrons. The nucleus occupies a very minute space in the overall volume of an atom. Surrounding the nucleus is the electron cloud. There is no rigid, tangible boundary to the electron cloud. It is here that the electrons freely move within certain specified regions based on their energy. These regions of space are called orbitals. Electrons have very little mass compared to the nucleons (protons and neutrons), but they have a negative charge.
The proton is the positively charge particle in the nucleus (nucleon). It is massive (for a subatomic particle), and each has a mass of 1.673E-27 kg. Since it is composed of three quarks (one down/two up), protons are a type of baryon. The identity of an atom is defined by the number of protons in its nuclues. That is, a chemist's definition of gold says nothing about its value, color, or use. Simply, a chemist defines gold as any atom that has 79 protons. Note that the periodic table is arranged by atomic number, or the number of protons in an atom's nucleus.
The neutron is a particle that lacks charge, and like the proton, resides in the nucleus. It is slightly more massive than the proton, with a mass of 1.675E-27 kg per particle. Like the proton, it is a baryon. Unlike the proton, its composition has two down quarks and one up quark. Due to its lack of charge, it was the last of the three main subatomic particles to be discovered. Atoms of the same element do not necessarily have the same number of neutrons. Particles of the same element that have different numbers of neutrons (and therefore mass) are called isotopes. Sometimes the ratio of neutrons to protons in an atoms causes the particle to be unstable. Isotopes like this are radioactive, meaning they will spontaneously release energy, often accompanied by smaller particles.
Electrons are small, negatively charged particles that orbit the nucleus in special regions of space called orbitals. The mass of a single electron is 9.109E-31 kg. This is more than three orders of magnitude smaller than the proton and neutron. The electron cloud is composed of all the individual orbitals needed to hold the atom's electrons. The vast majority of the volume of an atom is the electron cloud. Atoms of the same element do not necessarily have the same number of electrons. Since this causes an inbalance of protons (positively charged) and electrons (negatively charged) particles, the term ion is used to describe an atom with a charge. Furthermore, ions that are negatively charged (caused by the addition of electrons) are referred to as anions. Ions that are positively charged (caused by the loss of electrons) are referred to as cations.
(1) Asimov, Isaac. Atom: Journey Across the Subatomic Cosmos; Truman Talley Books: New York, 1992.
(2) Asimov, Isaac. Understanding Physics, Volume III; Dorset Press: New York, 1988.
(3) Cohen, Bernard L. The Heart of the Atom; Anchor Books: Garden City, NY, 1967.
(4) Grimes, Robin W. and Nuttall, William J. Science. 2010, 329, 799-803.
(5) Kaplan, Irving. Nuclear Physics, 2nd ed.; Addison-Wesley: Reading, MA, 1964.
(6) Romer, Alfred. The Restless Atom; Anchor Books: Garden City, NY, 1960.
(7) Smith, Timothy Paul. The Anatomy of a Neutron. American Scientist. 2010, 98, 478-485.