Bronies Coklat

An atom is a basic unit that makes up all matter. There are many different types of atom each with its own name, mass and size. These are called chemical elements, such as hydrogen or gold. Atoms are very small, the exact size changes depending on the element - they range from 0.1 to 0.5 nanometers.^[1] One nanometer is around a hundred times smaller than the width of a human hair. This makes atoms almost impossible to see. The manner in which they work, and how they interact with each other has to be investigated using equations. The field of physics that is concerned with 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, a form of a chemical reaction. Atoms themselves are made up of three kinds of smaller particles, called protons, neutrons and electrons, respectively. The protons and neutrons are in the middle of the atom. They are called the nucleus. The nucleus is surrounded by a cloud of electrons with a negative charge which are bound to nucleus by an electromagnetic force. 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 one electron; the element Sulfur has 16 protons, 16 neutrons and 16 electrons.

Atoms move faster when in gas form (as they are free to move) than liquid and solid matter. In solid materials the atoms are tightly next to each other so they vibrate, not are not able to move (there is no room) unlike liquid does.

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 development 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 tiny, solid spheres made of atoms. Dalton believed that all atoms of the same element have the same mass. He said that compounds are formed when atoms of more than one element combine. According to Dalton, in a compound, atoms of different elements always combine the same way.

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 realized they had a negative charge, unlike protons (positive) and neutrons (no charge). Thomson created the plum pudding model, which stated that an atom was like plum pudding: the dried fruit (electrons) were stuck in a mass of pudding (protons). In 1909 a scientist named Ernest Rutherford used the Rutherford Experiment to prove that electrons are very small compared to protons and neutrons. Rutherford took a photo plate and surrounded it with with gold foil, and then shot alpha particles at it. Many of the particles went through the gold foil, which proved that atoms are mostly empty space. Electrons are so small they make up only 1% of an atoms mass.^[5]

In 1913, Neils Bohr introduced the Bohr model. This model showed that electrons orbit the nucleus in fixed circular orbits. This was more accurate than the Rutherford model. However, it was still not completely right. Improvements to the Bohr model have been made since it was first introduced.

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 isotope 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. 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.

The most accurate model so far comes from the Schrödinger equation. Schrödinger realized that the electrons exist in a cloud around the nucleus, called the electron cloud. In the electron cloud, it is impossible to know exactly where electrons are. The Schrödinger equation is used to find out where an electron is likely to be found. This area is called the electron's orbital.

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.

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 join 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 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]