The chemistry of every element is best understood by knowing the structure of the atoms in that element. The number of protons in the nucleus, for example, defines what the element is: a nucleus containing one proton is always Hydrogen, and a nucleus containing two protons is always Helium. This article explains simply what each component is, and what it does. Some relevant definitions are also included.
Basic Structure of the Atom
An atom is the most basic unit of a chemical element. It comprises two main parts. The first part is a very dense and very small core called a nucleus. Most of the mass of the atom is concentrated in the nucleus, and it is made up from protons and neutrons. The second part is a "cloud" of electrons, that move around the nucleus in various positions called "energy levels".
The Nucleus of the Atom
The nucleus is made up of protons and neutrons. Protons and neutrons have roughly the same mass, although the neutron is slightly heavier, and they have a mass of about 1,840 times that of an electron. Protons and neutrons are collectively known as nucleons. Nuclei that contain the same number of nucleons are called isobars.
Protons
The number of protons in the nucleus of the atom is what makes the chemical element unique. All atoms of Carbon, for example, contain six protons. Not all carbon atoms contain the same number of neutrons: the most common carbon atom ("Carbon-12") contains six protons and six neutrons, whereas another type ("Carbon-14") contains six protons and eight neutrons. Carbon-12 and Carbon-14 are known as isotopes of carbon.
Neutrons
Neutrons, as the name suggests, are electrically neutral particles. They are needed in the nucleus in the right number, to make it stable. When the number of neutrons is fixed and the number of protons is allowed to change, the result is known as isotones of the nucleus. For example, a nucleus containing one proton and two neutrons ("Hydrogen-3", or Tritium) and a nucleus containing two protons and two neutrons ("Helium-4"), are known as isotones.
The neutron was discovered in 1932 by James Chadwick. It is stable when in the nucleus of an atom, but alone it decays into a proton, and electron and another particle called an antineutrino.
Electrons
Each proton has a positive electrical charge of one unit, so for an atom to be electrically neutral it must have the same number of electrons. These electrons do not "fall" into the nucleus, attracted by the protons. The reason is explained by Quantum Theory, which states that they may only exist at certain, specific energy "levels", or "shells".
The first shell can hold two electrons, the second can hold eight electrons, and the nth can hold 2 x n-squared electrons. As a general rule, each shell is more stable when it is full. Shells that are more than half full tend to want extra electrons, and shells that are less than half-full tend to want to lose electrons. The number of electrons that an atom wants to gain or lose is called the "valency".
Summary of the Structure of the Atom
Protons are what make a chemical element what it is. They reside in the nucleus along with neutrons, which are slightly heavier but which have no electrical charge. Electrons are much lighter, and revolve around the nucleus in "shells" of given energy levels. Each shell is more stable when it is "complete", and the degree of completeness gives every element its chemical bonding properties.
Martin Bell - Martin holds a B.Sc. degree in chemical engineering, and an M.Sc. degree in electronics and computing. He has spent more than 25 years ...
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