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Atoms and Elements

Ernst Rutherford and Niels Bohr developed the current atomic model.

The idea that matter is made up of tiny particles is very old. Democritus was a Greek who lived in the 5th century BCE, and is known as the 'Father of the Atom' for his idea that every substance had a limit to how much it could be divided and remain that substance. The Greeks also thought that the atoms of different types of materials had different geometric shapes, which is not really true.

However, following the ancient Greeks, the idea of a fundamental basic building block of all matter was lost for two thousand years. People in this time thought matter was composed of four elements: air, water, earth, and fire. These express characteristics of matter much like the modern concepts of gas, liquid, solid and energy. However, this idea did not explain the great variety of differences between substances.

Microscopic world revealed by glass

In the 17th century the microscope was invented, and the modern sciences of chemistry and biology began in earnest. In the 18th century, scientists began to ask what the air was made of. Experiments showed that it was not a single substance, but consisted of different gases. With this, and many similar discoveries, people began to wonder if the traditional 4-element model was correct.

Brownian Motion gets everyone thinking

In 1827, a biologist called Robert Brown observed tiny particles of pollen moving around on their own in still water. He observed it, but could not explain it.

It was not until the late 19th century that a German physicist, Ludwig Boltzman, reproposed the atomic structure model. It seems surprising today to hear that his fellow scientists thought this idea to be preposterous!

In 1905, Albert Einstein demonstrated how this atomic model explained Brownian motion, and soon after Niels Bohr and Ernest Rutherford put forward what we still believe to be the structure of the atom.

Gold Foil (or Gold Leaf) Experiment
Gold foil Experiment
Gold Foil Experiment: demonstrating that the nucleus of an atom exists

In 1911, Ernest Rutherford and two students, Geiger and Marsden, conducted an experiment which established the model of the atom we still use today. In this experiment, the scientists fired alpha particles at a very thin sheet of gold (perhaps only a few atoms thick), to measure the diffraction pattern. To their surprise, they saw that a very small percentage (one in ten thousand) reflected (or bounced back), while a few others deflected at unexplainable angles. This was unexpected and not the objective of the experiment.

Rutherford's brilliant insight was to see that this demonstrated that atoms were mainly empty space, with a tiny nucleus at the centre, and a 'cloud' of electrons at the periphery.

Before this accidental discovery, the prevailing model of the atom was a solid 'plum pudding', with electrons evenly distributed through a 'sea of positive charge'.

The discovery led to a complete rethink of the atomic model, quantum mechanics, and the discovery of the proton and neutron.

The Bohr-Rutherford Atom

All the matter that makes up the physical world around us and the universe consists of atoms. These come in 92 varieties, called elements. The Periodic Table is a representation of these elements, ordered by the number of particles they consist of.

These particles are the neutron, proton and electron.

Particle Relative Mass Charge
neutron heavy no charge
proton heavy positive
electron very light negative

The protons and neutrons are in a very tiny nucleus. The electrons are in 'shells' around the nucleus. In a neutral (uncharged) atom, there are exactly the same number of electrons as protons. The number of neutrons can vary, but, except for hydrogen, there are always at least as many, or usually more, neutrons than protons.

Hydrogen is the only element with more protons than neutrons - only one!

Hydrogen is the first and smallest element. In fact, most hydrogen atoms are just a single proton and a single electron. There are some rarer forms (isotopes) of hydrogen with one and two neutrons. These forms, called deuterium and triterium, are very important for nuclear energy.

Helium has two protons, two electrons, and two neutrons

The next largest atom is helium. This atom therefore has two protons, and two electrons. An atom with more than one proton must have neutrons. This is because protons are positive, and if they are next to each other in the nucleus, they would repel each other and the nucleus would fall apart. Neutrons, which have no charge, act like a glue, holding the nucleus together, preventing it from being blown apart by the positive charges of the protons.

Carbon has an atomic number of 6, and a mass number of 12 (6 protons + 6 neutrons = 12 nucleons)

Carbon is the element of 'life'. It is the backbone of all organic compounds. Most carbon is carbon-12 (6 protons and 6 neutrons), but there is an isotope, carbon-14 (6 protons and 8 neutrons), which is radioactive and is used to date organic residue in archeology.

Dmitri Mendeleev

Dmitri Mendeleev, 1834 - 1907, was a Russian chemist who developed the modern Periodic Table of Elements.

Mendeleev was able to use his Periodic System to predict successfully the existence of 'missing' elements, and their expected properties.

Dmitri Mendeleev, 1834 - 1907, Russian chemist
Dmitri Mendeleev, 1834 - 1907, Russian chemist

Mendeleev's Periodic Table

In his paper submitted to the Russian Chemical Society on 6 March 1869, The Dependence between the Properties of the Atomic Weights of the Elements, Mendeleev describes the elements in the original classification by atomic weight and valence. He proposed 8 characteristics for the Periodic System:

  1. The elements exhibit a periodicity of properties when ordered by atomic weight.
  2. Similarity of properties occur between elements if they have either similar atomic weights, or their atomic weights increase by regular increments (horizontal rows).
  3. There is a correspondence in valency and certain chemical properties between groups arranged by atomic weight (vertical columns).
  4. The lower atomic weight elements are the most widely diffused.
  5. The atomic weight determines the character of the element, in the same way as the size of a molecule determines the characteristics of a compound.
  6. It is possible to predict the existence of unknown elements from this pattern.
  7. Atomic weight will increase in correspondence to position in the table (actually not necessarily the case: atomic number increases regularly, but atomic weight can vary due to the large number of isotopes, particularly with very large elements).
  8. Certain characteristic properties of elements can be predicted from their atomic weights.

Content © Andrew Bone. All rights reserved. Created : August 21, 2015 Last updated :March 26, 2016

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