Atomic Structure
In 1773 Joseph Priestly isolated oxygen gas. A few years later, Antoine-Laurent Lavoisier recognized oxygen as an element. He was the first person to give a practical definition of an element. An element is a substance which cannot be broken down into simpler substances by chemical processes. Even if elements are physically separated, there will come a point at which there is only one unit representing that element. This unit is known as an atom. Atoms are the smallest unit of an element that can exist as a stable entity. Even though atoms can be broken down further, any separation past this level causes the constituent parts to lose any recognition as different elements. This idea of a basic unit of matter was important to future understanding and use of the lements. Even though many elements were identified, exactly what they were and how they interacted was not understood until the atomic model became known.
An atom is comprised of two major parts, the nucleus at the center, and the electron cloud orbiting about the nucleus. Two types of particles contribute to the nucleus, protons and neutrons. Though most of an atom's mass is contained in the nucleus, the nucleus does not contribute very much to the overall size of the atom. The radius of a typical atom is 1 to 2.5 Angstroms. The radius of a typical nucleus is only about 0.00001 Angstroms. 1 Angstroms is equal to .0000001 millimeters. To help visualize this, imagine that you are standing in the middle of a golf course holding a golf ball. If that golf ball was the nucleas of an atom, the atom would extend for approximately three miles in every direction. As can be seen by this example, an atom is comprised of mostly empty space through which the electrons travel.
Protons and neutrons are almost exactly the same size and mass but protons carry a positive charge with them. The mass of atomic particles is measured in Atomic Mass Units or amu's. One amu is defined as exactly one twelfth the mass of a carbon atom which has six protons and six neutrons in its nucleus. By this definition, both protons and neutrons have a mass that is nearly but not exactly one amu. By definition, an Avogadro's number of protons or neutrons is equal to 1 gram.
The mass of a proton is 1.00728 amu and the mass of a neutron is 1.00867amu. By caparision, the mass of an electron is only 0.000549 amu, about 1/1836 that of either a proton or a neutron.
Compared to these particles, the electrons that travel around the positively charged nucleus have almost no mass at all. Despite this relative lack of mass, about 0.00054 times that of a proton, electrons do have a negative charge that is equal to the positive charge of the proton.
In an atom that does not have a charge, the number of protons equals the number of electrons. In this way, the charges balance. If the number of electrons does not equal the number of protons, the atom is said to be an ion. Ions can be positively or negatively charged depending on whether there are more electrons or more protons.
This number of protons or electrons in a neutral atom is called the atomic number. Atomic numbers are important because each element has a unique number. Though the number of neutrons in a nucleus can vary, each additional proton signifies a new element. Different from the atomic number is the atomic mass. The mass of an atom is calculated by adding the number of protons and neutrons together to come to a total. This mass can be different for different atoms of the same element depending on the number of neutrons in the nucleus. This is why the atomic mass of an atom can vary, while the atomic number remains constant. Atoms of the same element which have different atomic masses are called isotopes. These concepts of atomic number and mass will become very important later on in this discussion.
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