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Linné on line Physics and the Cosmos Physics and the Cosmos Atoms – the smallest parts of the elements

## Atoms – the smallest parts of the elements

#### How much can you divide an element?

The modern concept of the atom was formulated in the beginning of the 19th century by John Dalton. According to Dalton's theory of atoms all matter consists of atoms that are indivisible and indestructible. Further, the atoms of an element are identical so that they have the same weight and chemical properties. Compound matter is obtained by combining different atoms in given ratios. Dalton arrived at his theory after thorough investigations of many different experiments that studied the weight-ratios in different chemical reactions. For example, to form water one needs 8 grams of oxygen for each gram of hydrogen to come out even. Dalton's theory of atoms meant a final disclaimer of the alchemist teachings about the four elements that had been around since the old Greeks.

 Today we know that Dalton was wrong in one respect: the atoms are not indivisible. An atom consists of an atomic nucleus with positive electric charge surrounded by an electron cloud with negative electric charge. In fact, the atom mostly consists of empty space, the nucleus is about ten thousand times smaller than the atom. The fact that the atom has a nucleus where almost all the mass is concentrated was found out by Ernest Rutherford in 1911. His conclusion was based on an experiment performed by Hans Geiger and Ernest Marsden. They irradiated a thin foil of gold with alpha-particles and studied how the alpha-particles were scattered as they passed through the foil. To their surprise they found that most of the alpha-particles went straight through the foil without being deflected while some of them were scattered at very large angles.

One measure of the smallness of the atoms is given by Avogadro's number, NA=6.02*1023, which gives the number of atoms in one mole of an element. One mole of atomic hydrogen weighs about 1 gram, so each hydrogen atom weighs no more than 0.000 000 000 000 000 000 000 0017 grams. Another way of understanding how large Avogadro's number is, is the following thought experiment: Imagine that you have 1 gram of hydrogen atoms that you divide in two equal parts, keeping one of them but throwing away the other. Then you divide the part you kept in two equal parts and keeps one of them etc. In order to be left with only one atom you have to repeat this procedure 79 times. Compare this with how many times you can fold a paper in the middle.

#### The periodic system

In 1869 Dimitri Mendeleev ordered the different elements in the periodic system according to their atomic weight. Elements with similar chemical properties were ordered in columns. This also led Mendeleev to be able to predict the atomic weight and chemical properties of elements that so far had not been discovered, based on the empty places in the periodic system. In the modern periodic system the elements are ordered after atomic number, i.e. the number of protons in the atomic nucleus. Hydrogen has the atomic number 1, helium 2, lithium 3 etc.

The electrons in the electron cloud that surrounds the atomic nucleus are bound to the nucleus by the electric force. The atomic nucleus, that consists of protons and neutrons, has a positive electric charge whereas the electrons are negatively charged. The number of electrons surrounding the nucleus depends on the electric charge of the nucleus in such a way that the atom as a whole is electrically neutral. Since the protons are positively charged and neutrons are neutral the charge of the atomic nucleus is given by the number of protons, i.e. the atomic number.

 The electrons that surround the atomic nucleus can only exist in certain energy levels, or orbitals as they are also called. This follows from quantum mechanics, which says that the energy cannot have an arbitrary value but is instead quantized. In addition to the energy being quantized the so called Pauli principle also applies for the electrons. According to this principle two electrons cannot be in the same quantum mechanical state. For each energy level there are two possible quantum states for the electrons. This means that the lowest level is filled by two electrons and that further electrons have to be in a higher level.

Usually one divides the different possible energy levels for the electrons into shells. The innermost shell, called K, can contain two electrons. The next shell, called L, can contain 8 electrons etc. The electrons in the outermost shell are called the valence electrons and they decide the chemical properties of an element.

#### How come neon tubes shine?

The electrons in the electron cloud are not always in the lowest energy level but they can also be excited to higher levels if energy is added, for instance in the form of heat. When this happens a so called hole is formed in one of the shells. Since the electron always tries to be in a state with minimum energy, this hole will be filled by one of the electrons from a higher energy level. When this happens the energy of the atom is lowered by emitting a photon, in other words by electromagnetic radiation. Depending on the energy of the photon, it can give visible light (as happens in a neon tube) or light with shorter or longer wavelength that is not visible (for instance UV-light in a solarium or infra-red light from an infra-red heater). The wavelength of a photon depends on its energy: the shorter the wavelength the higher the energy, or with a mathematical formula,

E=hf

where E is the energy, h is Planck's constant and f is the frequency which is inversely proportional to the wavelength.