How did J.J. Thomson discover electrons?

Thomson observed a beam of charged particles traveling toward the positive electrode in a cathode ray tube. He deflected these beams using electric and magnetic fields, concluding the particles were negatively charged, low-mass constituents of atoms. These particles were later named electrons, fundamentally changing our understanding of atomic structure.

What did Rutherford's gold foil experiment reveal about atomic structure?

Rutherford directed positively charged alpha particles at thin gold foil. Most passed through undeflected, indicating atoms contain mostly empty space. A few scattered at large angles or bounced back, revealing a tiny, dense, positively charged nucleus at the atom's center containing most of the atom's mass.

What are the three fundamental subatomic particles and their charges?

Atoms contain protons, neutrons, and electrons. Protons carry a charge of +1, neutrons have no charge, and electrons carry a charge of −1. The charge of a proton and electron are equal in magnitude but opposite in sign, allowing atoms to maintain electrical neutrality when electron and proton numbers are equal.

How do the masses of subatomic particles compare?

Protons and neutrons have nearly identical masses of approximately 1 amu each, while electrons have a negligible mass of about 0.00055 amu. It would take roughly 1800 electrons to equal the mass of one proton. The atomic mass unit (amu) is defined as one-twelfth the mass of a carbon-12 atom.

What is the atomic mass unit and why is it used?

The atomic mass unit (amu) is a standard unit for expressing the extremely small masses of subatomic particles. One amu equals 1.66 × 10−27 kg, or one-twelfth the mass of a carbon-12 atom. This unit is more practical than kilograms for describing atomic-scale masses and charges.

Where are protons, neutrons, and electrons located in an atom?

Protons and neutrons are located in the nucleus at the atom's center, which contains most of the atom's mass. Electrons orbit around the nucleus in the surrounding empty space. The nucleus is roughly 100,000 times smaller in diameter than the entire atom, yet contains nearly all of its mass.

How did James Chadwick's discovery of neutrons explain isotopes?

Chadwick discovered neutrons as uncharged particles in the nucleus with mass similar to protons. This explained why isotopes of the same element have different masses: they contain different numbers of neutrons. However, isotopes remain chemically identical because they have the same number of protons, which determines the element's identity.

2.2

Subatomic Particles

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Dalton’s atomic theory naturally led to the question of whether atoms themselves are divisible. A series of subsequent experiments demonstrated that an atom is in fact made of three fundamental particles called the subatomic particles – electrons, protons, and neutrons.

Electrons were discovered by J.J. Thomson, when he observed a beam of charged particles traveling in a straight line towards the positive electrode in a cathode ray tube and was able to deflect these beams using electric and magnetic fields. Thomson concluded that these particles were negatively charged, low mass constituents of an atom. These particles were later given the name electrons.

Robert Millikan determined that an electron has a charge of –1.6 × 10^–19 coulombs, where the coulomb is the SI unit of charge.

With the discovery of electrons, atoms were no longer viewed as indestructible, and models were proposed to describe the atomic structure.

The most successful was the nuclear model by Ernest Rutherford — based on his famous gold foil experiment, in which he directed positively charged alpha-particles at a thin sheet of gold foil. As most of the alpha particles passed through the gold foil undeflected, he concluded that most of the space inside an atom is empty. A few, however, were scattered at large angles and a small number bounced back, which confirmed that all of the positive charge and most of the mass of an atom reside in a tiny central core called the nucleus.

Rutherford also postulated that an atom has as many electrons, located in the empty space around the nucleus, as needed to counterbalance the positive charge of the nucleus and maintain the electrical neutrality of the atom.

Subsequently, Rutherford himself discovered protons as the positively charged particles in the nucleus, and James Chadwick discovered neutrons as the electrically neutral particles in the nucleus.

Charge and mass are two fundamental properties of subatomic particles.

The charge of the proton and the electron are equal in magnitude but opposite in sign. In relative units, the electron is assigned a charge of −1 and the proton is assigned a charge of +1. The neutron has no charge.

The mass of a proton is 1.673 × 10^–27 kg, while the mass of an electron is 9.1 × 10^–31 kg, and the mass of the neutron is 1.675 × 10^–27 kg.

A more common unit to express these extremely small masses is the atomic mass unit, or amu. 1 amu is equal to 1.66 × 10^–27 kg, or one-twelfth the mass of a carbon atom containing six protons and six neutrons. Thus, the mass of a proton or neutron is approximately 1 amu. Electrons have an almost negligible mass of 5.5 × 10^–4amu.

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Dalton was only partially correct about the particles that make up matter. All matter is composed of atoms, and atoms are composed of three smaller su…