2.3 The Structure Of Atoms

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Subatomic Particles

The first subatomic particle was identified in 1897 and called the electronA subatomic particle with a negative electric charge.. 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 protonA subatomic particle with a positive charge. 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 neutronA subatomic particle with no electric charge., a particle with about the same mass as a proton but with no electrical charge.

We understand now that all atoms can be broken down into subatomic particles: protons, neutrons, and electrons. Table 2.4 "Properties of the Subatomic Particles" lists some of their important characteristics and the symbols used to represent each particle.

Table 2.4 Properties of the Subatomic Particles

Particle Symbol Mass (kg) Relative Mass (proton = 1) Relative Charge
proton p+ 1.673 × 10−27 1 +1
neutron n0 1.675 × 10−27 1 0
electron e− 9.109 × 10−31 0.00055 −1

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