2.2—The Structure of an Atom - AP Chemistry - Chemistry - 2011-09-09

2.2—The Structure of an Atom

An atom is the basic unit of an element that can enter into chemical combination.

Scientists later found out that atoms possessed an internal structure made up of even smaller particles, which are known as subatomic particles. This led to the discovery of protons, neutrons, and electrons.

The Electron

In the 1890’s, many scientists became caught up in the study of radiation, which is the emission and transmission of energy through space in the form of waves.

The cathode ray tube was used to investigate further into the structure of the atom.

English Physicist J.J. Thompson, used a cathode ray tube to determine th ration of electric charge to the mass of an individual electron. The number he came up with was -1.776 x 108 C/g (where C stands for coulonb).

From 1908-1917 R.A. Millikan carried out a series of experiments and succeeded in measuring the charge of an electron to be -1.6602 x 10-19 C.

From that, Millikan determined the mass of an electron to me 9.10 x 10-28 g.


In 1895, the German physicist Wilhelm Röntgen noticed that cathode rays caused glass and metals to emit very unusual rays. He called them X rays because of their unknown nature.

Marie Curie, one of Antoine Becquerel’s students (he was a physics professor in Paris at the time) suggested the name radioactivity to describe the spontaneous emission of particles and/or radiation.

3 types of rays are produced by the decay, or breakdown, or radioactive substances such as uranium. 2 of the 3 are deflected by oppositely charged metal plates.

            Alpha (a) rays consist of positively charged particles, called a particles, and therefore car deflected by the positively charged plate.

            Beta (b) rays are electrons and are deflected by the negatively charged plate.

The third type of radioactive radiation consists of high-energy rays called gamma (g) rays. Like X rays, gamma rays have no charge and are not affected by and external field.

The Proton and the Nucleus

Thomson proposed that an atom could be thought of as a uniform, positive sphere in which the electrons are embedded into.

In 1910 the New Zealand physicist Ernest Rutherford conducted his gold foil experiment where he used very thin gold foils and other metals as targets for a particles. It surprised him when in some instances the a particles bounced back in the direction from which it had come.

Rutherford later explained his findings in terms of a new model for the atom. He said that most of the atom was empty space. He proposed that the atom’s positive charge was all concentrated in the nucleus, which is a dense central core within the atom.

The positively charged particles in the nucleus are called protons.

In separate experiments, it was found that the proton carries the same quantity of charge as an electron and has a mass of 1.67262 x 10-24 g, which is about 1840 times the mass fo the electron.

Atomic and molecular dimensions are expressed in SI units called picometers (pm) where

            1 pm = 1 x 10-12 m

A typical atomic radius is about 100 pm, whereas the radius of an atomic nucleus is only about 5 x 10-3 pm.

The Neutron

An English physicist named James Chadwick provided the proof that there was a third type of subatomic particle in an atom.

When he bombarded a thin sheet of beryllium with a particles, a very high-energy radiation similar to gamma rays was emitted by the metal. Later experiments show that the rays actually consisted of a third type of subatomic particles, which Cahdwick named neutrons, because they proved to be electrically neutral particles hacing a mass slightly greater and that of protons.

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