History of Atomic Theory and Discoveries

Study Notes

Democritus believed matter was made of tiny indivisible particles called atoms.
Democritus did not have any experimental evidence for his theory.
Aristotle disagreed with Democritus, believing that matter was made of four elements: fire, dust, water, and air.
According to Aristotle, any matter can change into another by changing the proportions of these four elements.

Dalton proposed four key points to his theory:
- Elements are composed of tiny indivisible particles called atoms.
- Atoms are solid and very small.
- Atoms of the same element have similar masses, but they differ from atoms of other elements.
- Compounds are formed by the combination of atoms of different elements in simple whole-number ratios.

J.J. Thomson discovered the electron in 1897 while studying the properties of cathode rays.

Cathode rays are invisible rays emitted from the negative pole (cathode).
These rays cause a green fluorescent glow when they hit the wall of the tube.
Cathode rays behave like negatively charged particles in the presence of an electric field.

J.J. Thomson measured the charge-by-mass ratio (e/m) of cathode ray particles using the deflection in electric and magnetic fields.
The value of e/m is -1.76 x 10⁸ coulombs per gram (C/g).

Robert Millikan determined the charge of the electron using the oil drop experiment.
The experiment involved suspending tiny oil droplets between electrically charged plates.
By studying the motion of the droplets under the influence of gravity and electric fields Millikan determined the fundamental unit of charge, which is the charge of an electron.

Millikan found that all charges measured were multiples of -1.6 x 10⁻¹⁹ Coulombs.
The charge of the electron is -1.6 x 10⁻¹⁹ Coulombs.

Using the charge-by-mass ratio (e/m) and the charge of the electron, the mass of the electron can be calculated.
The mass of the electron is 9.11 x 10⁻³¹ kg.

Rutherford's experiment bombarded a thin gold foil with alpha particles.
Most of the alpha particles passed straight through the gold foil , suggesting that the atom is mostly empty space.
Some alpha particles were deflected at small angles, indicating the presence of a small, positively charged center called the nucleus.
A few alpha particles were deflected at large angles, implying that the nucleus is small, dense, and positively charged.

The atom has a dense, positively charged center called the nucleus.
The nucleus occupies a very small fraction of the total volume of the atom.
The atom is not a solid sphere.
Electrons revolve around the nucleus in closed circular paths at high speeds.
The number of negatively charged electrons outside the nucleus is equal to the number of positively charged protons in the nucleus.
The overall electrical neutrality of an atom is explained by the balance between protons and electrons.

Rutherford's model proposes that the atom resembles the solar system, with a central nucleus (like the sun) and electrons orbiting it (like planets).

Rutherford's model could not explain the stability of the atom. According to classical physics, electrons orbiting the nucleus would lose energy and spiral into the nucleus, making the atom unstable.
Rutherford's model could not explain the radiation emitted by some atoms.

When alpha particles bombarded Beryllium nuclei, radiations were observed.
These radiations were found to be neutral, meaning they weren't deflected by electric fields.
Chadwick's observations led to the discovery of the neutron.

Isotopes are atoms of the same element that have the same number of protons and electrons but different numbers of neutrons.
They have the same atomic number but different mass numbers.

The atomic mass (weight) of an element is the average mass of an atom of that element, expressed in atomic mass units (amu).

The average atomic mass (weight) of an element is calculated based on the abundance of its isotopes in nature.
It is determined by weighting the atomic mass of each isotope by its relative abundance.
Average Atomic Mass (Weight) = (Atomic mass of isotope 1 x its abundance) + (Atomic mass of isotope 2 x its abundance) + ... etc.