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A LITTLE ABOUT ATOMS - 6

1. Introduction.
2. Types of Atoms.
3. Charged Atoms.
4. Plasma.
5. Nuclear Reactions - Fission.
6. Nuclear Reactions - Fusion.

NUCLEAR REACTIONS - FUSION

If Hydrogen atoms are placed under conditions of extreme pressure and very high temperature they can be forced together to produce Helium atoms.

In a practical reaction an atom of the Hydrogen isotope Deuterium and one of the Hydrogen isotope Tritium fuse to produce one atom of Helium and a neutron. Because the total mass of the Helium atom and the neutron is less than the total mass of the two Hydrogen atoms the missing mass is converted into energy.

Nuclear fusion

To date because of the high temperature and pressure involved it has not been possible to harness this reaction for the production of energy. A lot of research is being conducted and it is hoped that one day our energy needs might be met using nuclear fusion. The fuels required can be obtained from sea water and there is no radioactive waste.

Nuclear fusion also takes place in a hydrogen bomb. The energy is released in a short time resulting in a massive explosion.

Stars (including the Sun) are powered by fusion although the reactions are different to the one discussed.

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A LITTLE ABOUT ATOMS - 5

1. Introduction.
2. Types of Atoms.
3. Charged Atoms.
4. Plasma.
5. Nuclear Reactions - Fission.
6. Nuclear Reactions - Fusion.

NUCLEAR REACTIONS - FISSION

Atoms such as Uranium and Plutonium have large numbers of protons and neutrons in their nuclei. This makes the nucleus unstable. A small disturbance to the nucleus such as the addition of an extra neutron causes the nucleus to vibrate and break apart. This is called nuclear fission.

If you were to note the mass of the original nucleus before fission and then add up the masses of all the pieces after fission you would find that the total mass after fission is less than the total mass before fission. This missing mass is turned into large amounts of energy. This is the energy that powers nuclear reactors and atom bombs.

Splitting a Uranium atom

Assume we have an atom of Uranium 235, that is it has 92 protons and 143 neutrons in the nucleus. If we add an extra neutron we create Uranium 236 which is unstable. The nucleus vibrates and splits in two creating two new nuclei of approximately equal mass. In addition three neutrons are also produced. As mentioned before the total mass of the final products is less than the mass of the U235 and the initial neutron resulting in release of energy.

You will notice that the reaction was started with one neutron and three neutrons were produced in the reaction. Each of these resulting neutrons can split another nucleus. One nucleus can split three nuclei which can split nine nuclei which can split 27 and so on. In a very short time large numbers of nuclei become involved releasing huge amounts of energy. This is called a chain reaction.

Depending on how the reaction is set up the reaction can be made to proceed quickly with a rapid, explosive release of energy as in an atom bomb or the reaction can proceed much more slowly as in a nuclear reactor resulting in the energy being released over a longer period of time in a controlled fashion.

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A LITTLE ABOUT ATOMS - 4

1. Introdution.
2. Types of Atoms.
3. Charged Atoms.
4. Plasma.
5. Nuclear Reactions - Fission.
6. Nuclear Reactions - Fusion.

PLASMA

Neutral Hydrogen atoms.

Imagine a cloud of Hydrogen gas. Further imagine that the Hydrogen atoms have been ionised, that is the electrons have been removed from their orbits around the nucleus ( a proton) and are moving through the cloud seperately from the protons. We have a mixture of independently moving protons and electrons. This is called a plasma.

Hydrogen plasma.

Because electrons are 2000 times less massive than protons it is easier to cause them to move through the plasma. Large clouds of electrons can be made to move in unison by electric or magnetic fields in the plasma. Such a coordinated movement of electrons constitutes an electric current. Unlike a gas, a plasma will readily conduct an electric current.

Electrons flowing in unison through the plasma creating an electric current.

Plasmas are also affected by magnetic fields. The plasma will flow along magnetic field lines as illustrated by this image of a magnetic loop on the Sun.

IMAGE CREDIT: TRACE TEAM, NASA.

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A LITTLE ABOUT ATOMS - 3

1. Introduction
2. Types of Atoms.
3. Charged Atoms.
4. Plasma.
5. Nuclear Reactions - Fission.
6. Nuclear Reactions - Fusion.

CHARGED ATOMS

Under conditions like we have on Earth it is usual for atoms not to have an electric charge. The number of positively charged protons equals the number of negatively charged electrons so the amount of positive charge is exactly balanced by the amount of negative charge. The positive and negative charges cancel out leaving the atom without a charge.

Uncharged helium atom

Consider the Helium atom on the left. Two protons balance out the charge of the two electrons and the atom overall has no charge.
Assume that for some reason that one of the electrons is removed as shown on the next diagram down.
The atom now has two positive charges due to the two protons but only one negative charge because it now has only one electron. One positive charge is cancelled out by one negative charge but the other positive charge has no negative charge to cancel it so the atom has one overall positive charge. A charged atom is called an ion, in this case the atom is a positive ion as it has a positive charge.

Charged helium atom

We can use the same reasoning in the case of a Hydrogen atom which gains an electron. In this case the atom has an overall negative charge because it has more electrons than protons, and is called a negative ion.
Material made up of charged atoms or ions is said to be ionised.

Uncharged hydrogen atom

Charged hydrogen atom

It is not only Hydrogen and Helium that can become ionised, any atom under the right conditions can gain or loose electrons and become charged.
On Earth charged atoms are not common but in most of the universe the majority of matter is ionised. Hydrogen between the stars, much of the material of stars and the upper atmosphere of planets (including the Earth) is ionised. The vast clouds of glowing gas seen in the photo of the Orion Nebula are clouds of Hydrogen ionised by ultra-violet radiation from new stars imbedded in the nebula.

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A LITTLE ABOUT ATOMS - 2

1. Introduction.
2. Types of Atoms.
3. Charged Atoms.
4. Plasma.
5. Nuclear Reactions - Fission.
6. Nuclear Reactions - Fusion.

TYPES OF ATOMS

Hydrogen atom

The simplest atom is Hydrogen consisting of a proton in the nucleus and an electron orbiting it. It is also the most abundant atom in the universe making up some 75% of ordinary matter.
Next is another form of Hydrogen called Deuterium. It has one proton and one neutron. It is still Hydrogen because it has one proton, the number of protons determine which element it is, but the extra neutron makes it heavier than normal Hydrogen. For this reason it is sometimes called Heavy Hydrogen. Note that the charges of the electron and the proton balance each other out, the neutrons have no effect so the whole atom has no charge.

Deuterium atom

Atoms with the same number of protons but a different number of neutrons are called isotopes. Deuterium then is an isotope of Hydrogen.

A third rare form of Hydrogen is Tritium one proton and two neutrons.

The next heaviest element Helium occurs naturally in two forms – Helium 3 with two protons and one neutron and Helium 4 with two protons and two neutrons. The two protons make it Helium and the different number of neutrons makes the different isotopes. Because Helium has two protons, that is two positive charges in the nucleus, it requires two electrons to balance out the positive charge so there are two electrons in orbit around the nucleus.

Helium 4 atom

Other elements are made by adding more protons. Lithium has 3 protons, Berilium 4 protons, Boron 5 protons, Carbon 6 protons and so on. In each case for an uncharged atom, the number of electrons will equal the number of protons. The number of neutrons can vary giving different isotopes.

Under conditions that we have on Earth most atoms do not have a charge as the number of electrons equals the number of protons and the positive charges on the protons are balanced out by the negative charges on the electrons.

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A LITTLE ABOUT ATOMS - 1

1. Introduction.
2. Types of Atoms.
3. Charged Atoms.
4. Plasma.
5. Nuclear Reactions - Fission.
6. Nuclear Reactions - Fusion.

INTRODUCTION

An element - one kind of atom

All matter is made of atoms. Atoms can be thought of a very small spheres measuring about one hundred millionth of a centimetre across. There are some 92 naturally occurring atoms though the exact number depends on who you talk to.

Some substances consist of only one kind of atom, a lump of iron consists of iron atoms. These materials are called elements.
Other substances consist of more than one kind of atom, water consists of one oxygen atom combined with two hydrogen atoms. These materials are called compounds. A number of atoms joined together to form one unit is called a molecule.

A compound - atoms join

Yet other substances are mixtures of elements and/or compounds, air is an example. These are called mixtures and they differ from compounds in that the atoms mix without combining as they do in a compound.

Although atoms are extremely small they are made up of smaller structures. In the centre of the atom is the nucleus only about a trillionth of a centimetre across. Electrons orbit around the nucleus.

A mixture - atoms don’t join

Electrons are unimaginably small specks of matter which carry a negative electric charge. All electrons have an identical electric charge and have the same mass.

A nucleus

The nucleus consists of protons and neutrons. Protons are about 2000 times heavier than an electron and carry a positive charge, opposite to that of an electron but of the same magnitude. Under normal conditions an atom will have the same number of electrons and protons so the number of positive charges will equal the number of negative charges and their effects will cancel out leaving the atom with no overall charge.

In all atoms except Hydrogen, the nucleus also contains neutrons. These are particles with slightly more mass than a proton but with no electric charge. The number of neutrons does not necessarily equal the number of protons. Because protons and neutrons have more mass than electrons most of the mass of the atom is concentrated in the nucleus.

Here are a few important constants.

Mass of electron 9.1093 X 10^-28gram

Mass of proton 1.6726 X 10^-24gram

Mass of neutron 1.6749 X 10^-24gram

Charge of electron 1.6022 X 10^-19coulomb

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