The Atom

The Atom

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  • About 91 elements found in nature.
  • Over 20 made in laboratories.
  • Each is unique (has different properties)

Antoine Lavoisier

https://en.wikipedia.org/wiki/Antoine_Lavoisier

Law of conservation of mass (1785)

“The mass of the products of a chemical reaction is exactly equal to that of reactants”

Joseph Proust

https://en.wikipedia.org/wiki/Joseph_Proust

Law of constant proportions (1799)

“A given mass of single compound is made of a great number of identical molecules; each one of them is made of the same small number of atoms, present in some ratios”

John Dalton

Dalton’s Postulates (of his atomic theory - basically treating atoms like billiard balls)

  1. The matter is made of ATOMS, very small particles of a given element cannot be divided, created or destroyed.
  2. The atoms of an element cannot turn into atoms of another element: in a chemical reaction the original substances get separated into atoms that do recombine to form different substances. (a la mierda la alquimia).
  3. The atoms of an element show identical mass and properties and are different from any atom of other elements.
  4. The compounds are formed by the chemical combination of a specific number of different atoms.

Couldn’t explain:

  • existence of sub-atomic particles that were electrically charged

Law of simple multiple proportions

“If A and B react to form two or more compounds (e.g. AB and AB_2), the two different masses of B that get combined with the same mass of A are multiples of a small prime number”

Johnstone Stoney

Discovered that compounds could be decomposed with eletricity to their repective constituents (Electrolysis)(1891)

  • therefore discovered that electricity exists in discrete units and proposed the name electron for such unit.

Joseph John Thomson

Discovered that glass with electrodes with a potential difference inside fluoresces (faint green light)(1897)

Jean Perrin

Discovered that the cathode consist of negatively charged particles, using a magnet (1895)

J. J. Thomson again

The cathodic tube (

and therefore his atomic model like a cake with raisin inside, IMPORTANT to note: Equally distributed

  • cloud of positive particles
  • equally distributed negative particles

Robert Millikan

The charge and mass of an electron

Rutherford

Experiment deducing that most of the mass of the atom is in the center (later called nucleus)

Gaps in the deductions from the gold leaf experiments

  • If nucleus consists of positively charged particles tightly packed together how come they don’t repel each other
  • Also where is the extra mass of the nucleus coming from (e.g. Beryllium has 4 protons but weighs 9 amu not 4 amu)(we later find out that neutrons exist) Rutherford therefore proposed that there exists another particle in the nucleus of an atom - it is called a neutron It has the same mass as a proton (1 amu) and has no charge

Rutherford’s model of the atom

Limitations of Rutherford model

  • Couldn’t explain emission/absorption spectra
  • Photoelectric effect
  • And why don’t electrons just collapse inwards and spiral into the nucleus like plenatry orbits would Anyways the model of the atom was an improvement over Dalton’s model

Inside an Atom