Chemistry – Dr. May Notes
Composition of the Atom
The Discovery of Subatomic Particles
In 1807, John Dalton visualized the atom as an indivisible, uniformly dense, solid sphere that was involved in all reactions but was not changed in the process. He used this idea to explain why 100 grams of CO always contained 27.3 grams of carbon and 72.7 2
grams of oxygen. This theory helped chemists determine and explain the percent composition of many compounds.
The Kinetic Molecular Theory of gases was developed in the 1800’s. The concept
of the atom is consistent with the explanation that gases consist of atoms or molecules.
Crookes’ experiment projected electrons from a cathode (–) to an anode (+). By
applying a magnet he showed the particles to be charged when they were deflected.
In 1897 J.J. Thompson modified Crookes’ cathode ray tube by placing a fluorescent screen at one end and was able to measure the charge on the particles. The deflection caused by the magnet depends on:
1. The mass of the particle,
2. The velocity of the particle,
3. The electric charge on the particle,
4. The strength of the magnet, and
5. The amount of charge on the plates
From his work, Thompson concluded that all the particles are identical and that they are subatomic particles found in atoms. He discovered the electron.
An Electron is a negatively charged particle found in the atom
Once electrons were discovered, all scientists assumed there must be a positive particle since the atom is neutral (no overall charge).
Thompson discovered that when a high voltage was applied to hydrogen that electrons were formed as before but he was now able to detect a positive stream going in the other direction toward the cathode (–). He had discovered the proton or the positive
hydrogen ion. Thompson visualized the atom as a “pudding” with an equal thick mixture of electrons and protons.
A Proton is a positively charged particle found in the atom
If an atom loses one or more electrons it becomes positive and if it gains one or more electrons it becomes negative. This is the formation of ions. A positive ion is a cation and a negative ion is an anion.
The formation of ions or the sharing of electrons
is the key to chemical reactions.
Charge and Mass Measurements
By using the relationship between charge (the same for the electron and the proton) and mass (the proton is heavier than the electron), scientists were able to determine the following:
–191. The charge on an electron = 1.602 x 10 coulombs (SI unit) –282. The mass of an electron = 9.10953 x 10 gram –243. The mass of a proton = 1.67265 x 10 gram (1 atomic mass unit)
The proton has 1836 times more mass than the electron.
Rutherford’s Model of the Atom
The Discovery of Radioactivity
By 1900 three different types of radiation had been discovered by Becquerel, and Marie and Pierre Curie. In the process of discovering the alpha, beta, and gamma radiation the Curies discovered two new elements, radium and polonium.
Alpha radiation consists of rapidly moving helium ions that have no electrons. 8They are emitted at the speed of light. (The speed of light is 3 x 10 meters per second.)
Beta radiation consists of electrons emitted at very high speeds.
Gamma radiation is electromagnetic and is similar to but more powerful than X-rays.
Rutherford's Gold Foil Experiment
Ernest Rutherford shot alpha particles at gold foil in 1909 and found that some passed through (most), some bounced back (occasional), and some were deflected (few).
The Rutherford Atom
He concluded that the atom is mostly space, most of the mass of the atom (99.9%) is located in the nucleus, and the electrons “buzz” around the nucleus like bees around a hive.
The diameter of a nucleus is about 1/100,000 that of the atom. If the hydrogen nucleus were the size of a Ping-Pong ball, the atom (with a single electron) would have a diameter of 2 kilometers (1.2 mile).
Atomic Number and Isotopes
In 1932 Irene and Frederic Joliot-Curie discovered a type of radiation that had no charge but mass. This was identified as the neutron.
The Neutron has about the same mass as a proton but no charge
The nucleus was found to contain about the same number of protons and neutrons and have an overall charge of zero.
Atomic Number and Mass Number
The atomic number of an element is defined as the number of protons in the nucleus. Since the overall charge of an atom is zero, the number of protons always equal
the number of electrons.
The atomic mass of an element (mass number) is the total number of protons and neutrons. To determine the number of neutrons, subtract the atomic number (number of protons) from the mass number (protons plus neutrons).
Isotopes are atoms of the same element that contain a different number of neutrons. Since the number of protons is the same, it is the same element but has a different atomic mass.
The atomic mass of an element as given on the periodic table is an average of all the naturally occurring isotopes.