The Composition of Matter

All matter is made up of chemical elements (the elements of the periodic table), which are in turn made up of atoms. A chemical elements cannot chemically be broken down into simpler substances. today there are 109 known elements, 93 are ound in nature and the remaining 16 are artifically created. Elements are ranked according to mass. Hydrogen(H) is the lightest natural element, and Uranium(U) the heaviest.

Atoms and nuclei. An atom consists of a positively carged nucleus and one or more negatively carged electrons. The nucleus makes up nearly all of the atom's mass. The electrons, which are almost weightless, revolve around the nucleus. They determine the various chemical combinations that an atom enters into with other kinds of atoms. Electrons do not play an active part in nuclear reactions.
The nuclei of of every chemical element except Hydrogen(H) consists of particles called protons and neutrons. An ordinary nucleus of Hydrogen, the lightest element, has one proton and no neutrons. The heaviest elements, such as Uranium and Thorium, have the largest number of protons and neutrons in thei nuclei.
Protons carry a positive charge. Neutrons carry no electrical charge. Extremely strong forces, called Nuclear Forces, hold the protons and neutrons together in the nucleus. The nuclear forces of each type of nucleus determine the amount of energy that would be required to release its neutrons and protons. This energy is known as binding energy.

Isotopes. Most chemical elements have more than one form. These different forms are called the isotopes of the element. The atoms that make up each of the different forms have different weights and are also called isotopes.
Isotopes are identified by its mass number- that is, the total number of protons and neutrons in each of its nuclei. All the isotopes of a given element has the same number of protons in each nucleus. Hydrogen for example has one proton in its nucleus and every Uranium nucleus has 92 protons. However, each isotope of an element has a different number of neutrons in its nuclei and thus has a different mass number. For example, the most plentiful isotope of Uranium has 146 neutrons. Its mass number therefor is 238 (the sum of 92 and 146). Scientifically this isotope is called Uranium 238 or U-238. The Uranium isotope that almost all nuclear reactors use as feul has 143 neutrons, and so its mass number is 235. This isotope is called Uranium 235 or U-235. The isotopes of all chemicals are identified in the same way.
No two elements has the same number of protons in their atoms. If an atom gains or loses one or more of its protons or neutrons, it changes into an atom of a different element. If an atom gains or loses one or more of its neutrons, it changes into another isotope of the same element.

BACK

NEXT