Chemistry quick n dirty

Earth is made up of elements. Many of these, such as hydrogen, oxygen, nitrogen, carbon, and iron, are familiar, while others, such as berkelium, iridium, and thorium, are probably not. All elements are made up of atoms, which can be considered to be the smallest particle of any element that still retains the properties of that element. Atoms, in turn, are made up of protons, neutrons, and yet smaller electrons, which are collectively termed subatomic ("smaller-than-atomic") particles. Protons and neutrons reside in the central core, or nucleus of the atom. The electrons are located in a cloud surrounding the nucleus. The electrons are bound within the cloud in a series of energy levels; that is, some electrons are more tightly bound around the nucleus and others are less tightly bound. Those that are less tightly bound are, as one might expect, more easily removed than those that are more tightly bound (Figure A2.1).

Neutrons Total number Atomic _ Atomic

0 charge of neutrons number weight

Protons Total number + I charge of protons

Atomic number

Electrons

Total number of protons

Atomic number

Figure A2.1. Diagram of a carbon atom. In the nucleus are the protons and neutrons. In a cloud around the nucleus are the electrons, whose position relative to the nucleus is governed by their energy state.

Keeping that in mind, let us further consider the subatomic particles. Protons and electrons are electrically charged; electrons have a charge of -1 and protons have a charge of +1. Neutrons, as their name implies, are electrically neutral and have no charge. To keep a charge balance in the atom, the number of protons (positively charged) must equal the number of electrons (negatively charged). This number - which is the same for protons and electrons - is called the atomic number of the element, and is conventionally displayed to the lower left of the elemental symbol. For example, the element carbon is identified by the letter C, and it has 6 protons and 6 electrons. Its atomic number is thus 6, and it is written 6C.

Along with having an electrical charge, some subatomic particles also have mass. Rather than work with the extremely small mass of a proton (one of them weighs about 6.02 X 10 23 grams!), it is assigned a mass of 1. Neutrons have a mass of 1 as well. Because relative to protons and neutrons, the masses of electrons are negligible, the mass number of an element is composed of the total number of neutrons plus the total number of protons. In the case of the element carbon, for example, the mass number equals the total number of neutrons (6) plus the total number of protons (6); that is, 12. This is usually written 12C and is called carbon-12. Note that 12C has 6 protons and therefore must also have 6 neutrons, so its atomic number remains 6, and is written 12C. Because the atomic number is always the same for a particular element, it is commonly not included when the element is discussed. Thus "C is usually abbreviated 12C.

Variations in elements exist in nature and those variations that have the same atomic number but different mass numbers are called isotopes. For example, a well-known isotope of carbon-12 (12C) is carbon-14 (14C). Since 14C is an isotope of carbon, it has the same atomic number as 12C (based upon 6 electrons and 6 protons). The change in mass number results from additional neutrons. Carbon-14 has 8 neutrons, which, with the 6 protons, increase its atomic mass to 14. Because it is carbon, of course, its atomic number remains 6.

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