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Atomic History
Democritus: 2000+ years ago in ancient Greece, Democritus and Leucippus
suggested all matter was made up of tiny, invisible, indestructible, particles.
These particles were called atomos.
There was NO experimental data to support this idea, just careful
thinking and reasoning.
Antoine Lavoiser: A French chemist who in the 1770’s gave the 1st
experimental evidence for the law of conservation of matter
law of conservation of matter-matter can neither be created or
destroyed.
Joseph Proust: A French chemist who in 1799 worked on the problem of
determining the composition of compounds. Credited with the law of
definite proportions.
law of definite proportions-the proportion by mass of the element in
a given compound is always the same.
Dalton’s Atomic Theory: Early 1800’s John Dalton, 1st stated law of
multiple proportions.
law of multiple proportions-In all cases, the masses of 1 element that
combine with a fixed mass of another element will form simple, wholenumber rations.
From the evidence of the above 3 laws, in 1803 Dalton proposed an
atomic theory of matter.
Dalton’s Atomic Theory
1. All elements are composed of atoms which can not be
created or destroyed.
2. Atoms of the same element are identical
3. Atoms of different elements are different…different
masses
4. compounds are formed by the joining of atoms of 2 or
more elements. In any compound the atoms of the
elements in the compound are joined in a definite
whole number ratio.
Dalton’s Atomic model…solid sphere
Discovery of subatomic particles:
J.J. Thompson: 1st discovered electrons in the 1870’s using a Cathode Ray
Tube, deflecting and attracting the ray with an electric field as well as a
magnetic field.
Cathode Ray Tube-or a discharge tube is a glass tube in which 2
metallic electrodes are sealed. Most of the air is removed. A high voltage
current flows through the tube and a glow appears at the opposite end of the
cathode (positive end).
Thompson proposed the “plum pudding model” of an atom
Electrons are 1/1840 lighter than protons
Protons: single positively charged particle with a mass of 1 amu
Named by Rutherford
James Chadwick: 1932 discovered the neutron. Has approximately the
same mass as a proton but 0 charge.
Ernest Rutherford: 1909 showed that atoms are not solid spheres, by
emitting alpha particles (positively charged Helium atoms, He+)
Rutherford’s said atoms consist of a small, massive, positively
charged nuclei, surrounded mainly by empty space. Electrons move about
the empty space, and the negative charge of the electrons offsets the positive
charge of the nucleus accounting for the electrically neutral atom.
Rutherford’s atomic model
Niels Bohr: Stated there are certain definite orbits in which the electrons
can travel.
1. Each orbit is a circular orbit at a fixed distance from the nucleus.
2. An electron in a given orbit has a certain definite amount of energy.
3. The greater the radius of the shell, or distance of the electron from
the nucleus, the greater the energy of an electron in that shell.
4. Orbits or shells are also know as energy levels
Under normal conditions electrons in an atom occupy the lowest energy
levels available to them.
Ground State: When all the lowest energy levels are occupied in an atom.
Excited State: When an atom absorbs energy from an outside source, the
energy may cause an electron or electrons to move to a higher energy level.
Electrons do NOT remain at these higher levels for very long. The
electrons return (or fall back) to the original ground state. When they do, the
electrons emit (give off) a quantum of energy (the same amount of energy
needed to raise the electron to the higher state)…in the form of light
Summary: