User:Lights and freedom/damagetest

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A drawing of a Lithium atom. In the middle is the nucleus, which in this case has four neutrons (blue) and three protons (red). Orbiting it are its three electrons.
Helium atom model
Showing nucleus with two protons (blue)
and two neutrons (red),
orbited by two electrons (waves).
Smallest recognised division of a chemical element
Mass: 1.66 x 10(−27) to 4.52 x 10(−25) kg
Electric charge: zero

An atom is a basic unit that makes up all matter. There are many types of atom each with its own name, mass and size. These are called chemical elements. For example hydrogen or gold. Atoms are very small, the exact size changes depending on the element but they range from 0.1 to 0.5 nanometers.[1] To put that into perspective, one nanometer is around a hundred times smaller than the width a human hair. This makes atoms almost impossible to see, and so how they work and interact with each other has to be worked out through complex mathematics and equations. This is called quantum physics.

Atoms combine to make molecules or particles, for example two hydrogen atoms and one oxygen atom combine to make a water molecule. Atoms themselves are made up of three smaller particles called protons, neutrons and electrons. The protons and neutrons are in the middle of the atom. They are called the nucleus. The electrons orbit or go around them in a similar way to the planets going around the Sun. Protons and neutrons are made up of even smaller particles called quarks. Electrons are elementary or fundamental particles, they cannot be split into smaller parts. The number of protons, neutrons and electrons an atom has determines what element it is. Hydrogen, for example, has one proton, no neutrons and no electrons whereas the element Sulfur has 16 protons, 16 neutrons and 16 electrons.


  • combine to make everything En Da De
  • size En De
  • contain proton neutron and electron Ep En De
  • electromagnetic force En De
  • nuclear force En De
  • types of atoms are chemical elements with similar properties Ep En De
  • atomic number En De
  • isotopes have same number of protons but different numbers of neutrons Ep En
  • radioactivity Ep En
  • molecules Ep En
  • ions En De
  • field of chemistry En
  • microscope observing atoms De

History[change | change source]

The word "atom" comes from the Greek ἀτόμος, indivisible, from ἀ-, not, and τόμος, a cut. The first historical mention of the word atom came from works by the greek philosopher Democritus, around 400BC. [2] Atomic theory stayed as a mostly philosophical subject, with not much actual scientific investigation or study, until the devlopment of chemistry in the 1600's. In 1777 French chemist Antoine Lavoisier defined the term element for the first time. He said that an element was any basic substance that could not be broken down into other substances by the methods of chemistry. Any substance that could be broken down was a compound.[3] In 1803 English philosopher John Dalton suggested that elements were made of atoms and compounds happen when atoms come together or combine.

In 1827 British scientist Robert Brown looked at pollen grains in water and used Dalton's atomic theory to describe patterns in the way they moved. This was called Brownian Motion. In 1905 Albert Einstein used mathematics to prove that the seemingly random movements were down to the reactions of atoms, and by doing so he conclusively proved the existence of the atom.[4] In 1869 scientist Dmitri Mendeleev published the first version of the periodic table. The periodic table groups atoms by their atomic number (how many protons they have. This is always the same as the number of electrons). Elements in the same column, or period, usually have similar properties. For example Helium, Neon, Argon, Krypton and Xenon are all in the same column and have very similar properties. All these elements are gases that have no colour and no smell. Together they are known as the noble gases.[3]

The physicist J.J. Thomson was the first man to discover electrons, while he was working with cathode rays in 1897. He realised they had a negative charge, unlike protons (positive) and neutrons (no charge). In 1909 a scientist named Ernest Rutherford proved that electrons are also very small compared to protons and neutrons. They are so small they make up only 1% of an atoms mass.[5]

File:Ernest Rutherford.jpg
Ernest Rutherford in 1910, shortly before he won the Nobel Prize for physics.

In 1925 chemist Frederick Soddy found that some elements in the periodic table had more than one kind of atom.[6] For example an atom with 2 protons should be a Helium atom. However some Helium atoms have three neutrons as well. This means they are still Helium, as the element is defined by the number of Protons, but they are not normal Helium either. Soddy called an atom like this, with a different number of neutrons, an isotope. To get the name of the isotope we look at how many neutrons it has and add this to the name of the element. So a Helium atom with three neutrons is called Helium-3, and a Carbon atom with twelve neutrons instead of six is called Carbon-12. However when he developed his theory Soddy could not be certain neutrons actually existed. To prove they were real physicist James Chadwick and a team of others created the mass spectrometer.[7] The mass spectrometer actually measures the mass and weight of individual atoms. By doing this Chadwick proved that to account for all the weight of the Atom, neutrons must exist.

In 1937 German chemist Otto Hahn became the first person to create nuclear fission in a laboratory. He discovered this by chance when he was shooting neutrons at a Uranium atom, hoping to create a new isotope.[8] However he noticed that instead of a new istope the Uranium simply changed into a Barium atom. This was the worlds first recorded nuclear fission reaction. This discovery eventually led to the creation of the atomic bomb. The very first of these was dropped on Hiroshima in 1945, ending World War II. Further into the 20th century physicists went deeper into the mysteries of the atom. Using particle accelerators they discovered that protons and neutrons were actually made of other particles, called quarks.

Structure and Parts[change | change source]

Parts[change | change source]

The atom is made up of thee main particles; the proton, the neutron and the electron. The isotope of Hydrogen Hydrogen-1 has no neutrons, and a positive hydrogen ion has no electrons. These are the only known exceptions, all other atoms have at least one proton, neutron and electron each.

Electrons are by far the smallest of the three, their mass and size is too small to be measured using current technology.[9] They have a negative charge. Protons and neutrons are similar sizes[9] Protons are postively charged and neutrons have no charge. Most atoms have a neutral charge; because the number of protons (postive) and electrons (negative) are the same, the charges balance out to zero. However in ions (different number of electrons) this is not always the case and they can have a positive or a negative charge. Protons and Neutrons are made out of quarks, of two types; up quarks and down quarks. A proton is made of two up quarks and one down quark and a neutron is made of two down quarks and one up quark.

Nucleus[change | change source]

The nucleus is in the middle of an atom. It is made up of protons and neutrons. Usually in nature two things with the same charge repel or shoot away from each other. So for a long time it was a mystery to scientists how the postively charged protons in the nucleus stayed together. They solved this by finding a particle called a Gluon. It's name comes from the word glue as Gluons act like atomic glue, sticking the protons together using the strong nuclear force. It is this force which also holds the quarks together that make up the protons and neutrons.

A diagram showing the main difficulty in nuclear fusion, the fact that protons, which have positive charges, repel each other when forced together.

The number of neutrons in relation to protons defines whether the nucleus is stable or goes through radioactive decay. When there are too many neutrons or protons, the atom tries to make the numbers the same by getting rid of the excess particles. It does by emitting radiation in the form of aplha, beta or gamma decay.[10] Nuclei can change through other means too. Nuclear fission is when the Nucleus splits into two smaller nuclei, releasing a lot of stored energy. This release energy is what makes nuclear fission useful for making bombs and electricity, in the form of nuclear power. The other way nuclei can change is through nuclear fusion, when two nuclei joing together, or fuse, to make a heavier nucleus. This process requires extreme amounts of energy in order to overcome the electrostatic repulsion between the protons, as they have the same charge. Such high energies are most commonly found in stars like our Sun, which fuses Hydrodgen for fuel.

Electrons[change | change source]

Electrons orbit or go around the nucleus. They are called the atom's electron cloud. They are attracted towards the nucleus because of the electromagnetic force. Electrons have a negative charge and the nucleus always has a positive charge, so they attract each other. Around the nucleus some electrons are further out than others. These are called electron shells. In most atoms the first shell has two electrons, and all after that have eight. Exceptions are rare, but they do happen and are difficult to predict.[11] The further away the electron is from the nucleus, the weaker the pull of the nucleus on it. This is why bigger atoms, with more electrons, react more easily with other atoms. The electromagnetism of the nucleus is not enough to hold onto their electrons and they lose them to the strong attraction of smaller atoms [12]

Radioactive Decay[change | change source]

Some elements, and many isotopes, have what is called an unstable nucleus. This means the nucleus is either too big to hold itself together[13] or has too many protons, electrons or neutrons. When this happens the nucleus has to get rid of the excess mass or particles. It does this through radiation. An atom that does this can be called radioactive. Unstable atoms continue to be radioactive until they lose enough mass/parfticles that they become stable. All atoms above atomic number 82 (82 protons) are radioactive.[13]

There are three main types of radioactive decay; alpha, beta and gamma.[14]

  • Alpha decay is when the atom shoots out two protons and two neutrons. This is essentially a Helium nucleus. The result is an element with atomic number two less than before. So for example if a Beryllium atom (atomic number 4) went through alpha decay it would become Helium (atomic number 2). Alpha decay happens when an atom is too big and needs to get rid of some mass.
  • Beta decay is when a neutron turns into a proton or a proton turns into a neutron. In the first case the atom shoots out an electron, in the second case it is a positron (like an electron but with a positive charge). The end result is an element with one higher or one lower atomic number than before. Beta decay happens when an atom has either too many protons, or too many neutrons.
  • Gamma decay is when an atom shoots out a gamma ray, or wave. It happens when there is a change in the energy of the nucleus. This is usually after a nucleus has already gone through alpha or beta decay. There is no change in the mass, or atomic number or the atom, only in the stored energy inside the nucleus.

Every radioactive element or isotope has something called a half life. This is how long it takes half of any sample of atoms of that type to decay until they become a different stable isotope or element.[15] Large atoms, or isotopes with a big difference between the number of protons and neutrons will therefore have a long half life.

References[change | change source]

  1. "Size of an Atom".
  2. "History of Atomic Theory".
  3. 3.0 3.1 "A Brief History of the Atom".
  4. "Brownian motion - a history".
  5. "Ernest Rutherford on Nuclear spin and Alpha Particle interaction" (PDF).
  6. "Frederick Soddy, the Nobel Prize in chemistry: 1921".
  7. "James Chadwick: The Nobel Prize in Physics 1935, a lecture on the Neutron and its properties".
  8. "Otto Hahn, Liese Meitner and Fritz Strassman".
  9. 9.0 9.1 "Particle Physics - Structure of a Matter".
  10. "How does radioactive decay work?".
  11. "Chemtutor on atomic structure".
  12. "Chemical reactivity".
  13. 13.0 13.1 {{citeweb|url=
  14. "S-Cool: Types of radiation".
  15. "What is half-life?".

Other websites[change | change source]