Science 9 : Models of the Atom Page

John Dalton’s Model of the Atom (1827) Dalton like the Greeks thought that atoms were hard indivisible, uncuttable particles, solid like billiard balls.

Electricity and Vacuum Tubes Shed Light on Atoms Michael Faraday (1838) found that if a tube with positive and negative electrodes had the air pumped out, it began to glow at the negative electrode end. With a better vacuum pump, Heinrich Geissler (1857) found that a glowing beam formed between the two electrodes.

The Laws of Electric Charges Like charges repel each other away. Unlike charges attract each other.

Where is the glowing beam coming from? Experiments by William Crookes (1870s) and J.J. Thompson (1897) showed that the glowing beam was coming from the negative electrode (cathode) and that the glowing beam was negatively charged. The glowing beam was called cathode rays because it came from the cathode.

Cathode rays found to be in all elements (all atoms) The exact same glowing beam formed no matter what element was used for the cathode. This indicated that matter (atoms) was made up of negative things and because matter overall is neutral, it must also have something positive to neutralize the negative. A new model of the atom, the raisin bun or plum pudding model was proposed by J.J. Thompson (1904).

The Gold Foil Experiment and The Nuclear Atom Ernest Rutherford designed an experiment in which a beam of heavy positive alpha particles was shot at a very thin gold foil sheet that was only a few atoms thick. Rutherford expected that the alpha particles would plow directly through the gold atoms (supposedly soft plum pudding atoms) like an artillery shell would plow through a piece of tissue paper.

Detecting Alpha Particle Motion A circular screen coated with zinc sulphide was placed around the gold foil sheet. The zinc sulphide glowed whenever an alpha particle crashed into it.

Gold Foil Experiment : Unexpected Results Rutherford found that 99 % of the alpha particles went straight through the gold foil but a small number of particles were scattered way off track and some even bounced back!

Rutherford was Astonished The results of the Gold Foil Experiment astonished Rutherford. He wrote, “… It was almost as incredible as if you fired a 15 inch shell at a piece of tissue paper and it cam back and hit you.”

A New Model of the Atom: The Nuclear Atom Rutherford’s experiment showed him that there was a small positive centre in the atom that he called the nucleus. Around this centre, the negative electrons were thought to rotate much like the planets rotate around the sun. Rutherford’s atom was mostly empty space with a small positive centre and moving negative electrons around the centre.

What is in the Nucleus? More experiments convinced Rutherford that the nucleus must have both positive and neutral particles in it. The positive particles were called protons. In 1932, an associate of Rutherford, James Chadwick, discovered the neutron, a particle that is neutral and weighs the same as a proton.

Atom Composition: Three Particles Atoms are composed of protons, neutrons and electrons. The nucleus contains protons (+) and neutrons (0). Electrons (-) are moving rapidly around the nucleus. The number of electrons equals the number of protons in a neutral atom. The charge on one electron exactly balances the charge on one proton. 1+ added to 1- gives a charge of 0. A proton and neutron weigh 1832 X as much as an electron.

What About the Electrons of an Atom? Rutherford’s planetary model of the electrons was shown to be incorrect since the laws of physics predicted that a negative charge orbiting a positive nucleus would lose energy, fall inward and crash into the nucleus, producing a flash of violet light called the violet catastrophe. This does not happen in nature so Rutherford’s model was wrong.

The Bohr Model of the Atom : Electron Energy Levels In 1913, Niels Bohr proposed that an atom’s electrons are arranged in energy levels (shells) around the nucleus. The electrons moved not in circling orbits but within the shells. The inner shell holds 2 electrons maximum, the second shell holds 8 electrons maximum and the third shell 18.

The Bohr Model of the Atom : Electron Energy Levels This model helped to explain the spectra of elements.

The Bohr Atom and Spectra Electrons can move up to higher energy levels if energy is added to them. These electrons are said to be excited. When electrons drop to lower energy levels (due to their attraction to the nucleus {- and + charges}), they release or give off energy.

Falling Electrons Give off Specific Energies of Light Bright line spectra are produced when energized electrons fall back to lower levels giving off specific energies, the specific colours of light in the bright line spectrum.

Element Spectra : Unique Like Fingerprints Each element has its own unique bright line and absorption spectrum which identify it like different fingerprints identify different people.

The Bohr Model of the Atom : Main Ideas The nucleus is just 1/10,000 of the atom’s volume. Most of the atom is empty space. The nucleus contains 99.9% of an atom’s mass/weight. The nucleus contains positive protons and neutral neutrons which are 1832 times heavier than electrons. The negative electrons are found in energy shells around the nucleus. The number of electrons is always equal to the number of protons

The Sodium Atom has 11 electrons (and protons) The first shell (n=1) can hold only 2 electrons (2*(1)2. The second shell (n=2) can hold 8 electrons (2*(2)2. The third shell (n=3) can hold 18 electrons (2*(3)2 .

Visualizing an Atom If an atom were blown up in size to the size of a football field, the nucleus (protons and neutrons) would be the size of a single pea on the 50 m (yd) line and the electrons would be like salt grains buzzing around the field in their shells.

How Small is an Atom? In an “average” grain of salt from a salt shaker, there are 1.2 X 1018 atoms or 1,200,000,000,000,000,000 atoms.

How do the Atoms of Different Elements Differ? The number of positive protons in the nucleus (the atomic number) of an atom determines what kind of atom it will be (what kind of element it will be).

The Quantum Mechanical Model of the Atom A number of persons working from 1925 - 1928 eventually produced the modern highly mathematical and complex model of the atom called the Quantum Mechanical Model of the Atom. This is the model still accepted today. Electrons move rapidly within three-dimensional shapes called orbitals. Many features of the Bohr atom are kept in the Quantum Mechanical Model of the atom.

Review of the Models of the Atom from 1800 - 1940

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