Learning Objectives Describe the three main subatomic particles. State how the subatomic particles are arranged in atoms. Prior to the 1900's atoms were considered to be indivisible. We know now that atoms not only can be divided but also are composed of three different kinds of particle, subatomic particles , with their own properties that are different from the chemical properties of atoms. Subatomic Particles The first subatomic particle was identified in 1897 and called the electron. It is an extremely tiny particle, with a mass of about 9.109 × 10 −31 kg. Experiments with magnetic fields showed that the electron has a negative electrical charge. By 1920, experimental evidence indicated the existence of a second particle. A proton has the same amount of charge as an electron, but its charge is positive, not negative. Another major difference between a proton and an electron is mass. Although still incredibly small, the mass of a proton is 1.673 × 10 −27 kg, which is almost 2,000 times greater than the mass of an electron. Because opposite charges attract each other (while like charges repel each other), protons attract electrons (and vice versa). Finally, additional experiments pointed to the existence of a third particle. Evidence produced in 1932 established the existence of the neutron, a particle with about the same mass as a proton but with no electrical charge, it is neutral . We understand now that all atoms can be broken down into subatomic particles: protons, neutrons, and electrons. Table \(\PageIndex{1}\) lists some of their important characteristics and the symbols used to represent each particle. Particle Symbol Mass (kg) Relative Mass (proton = 1) Relative Charge proton p+ 1.673 × 10−27 1.00000 +1 neutron n0 1.675 × 10−27 1.00000 0 electron e− 9.109 × 10−31 0.00055 −1 The Nucleus How are these subatomic particles arranged? in the early 1900's much experimental work happened. A model was proposed to explain the structure of the atom. Protons and neutrons are concentrated in a central region called the nucleus of the atom. Electrons are outside the nucleus and orbit about it because they are attracted to the positive charge in the nucleus. Most of the mass of an atom is in the nucleus, while the orbiting electrons account for an atom’s size. As a result, an atom consists largely of empty space. This description is the “planetary model” of the atom. It is essentially the same model that we use today to describe atoms but with one important modification. The planetary model suggests that electrons occupy certain specific, circular orbits about the nucleus. We know now that this model is overly simplistic. A better description is that electrons form fuzzy clouds around nuclei. Figure \(\PageIndex{1}\) shows a more modern version of our understanding of atomic structure. Concept Review Exercises What are the charges and the relative masses of the three subatomic particles? Describe the structure of an atom in terms of its protons, neutrons, and electrons. Answers proton: +1, large; neutron: 0, large; electron: −1, small Protons and neutrons are located in a central nucleus, while electrons orbit about the nucleus. Key Takeaways Atoms are composed of three main subatomic particles: protons, neutrons, and electrons. Protons and neutrons are grouped together in the nucleus of an atom, while electrons orbit about the nucleus.