Introduction:The atom is the fundamental building block of all stuff, or what scientists like to call "matter". An individual atom is very small. In fact, the smallest type of atom, hydrogen, has a diameter of 10-8 cm. This means that if the hydrogen atom was the size of a soccer ball, then a soccer ball would be 6450 kilometers (4008 miles) high. Every single object is composed of atoms. Your body is made up of many, many individual atoms. There are also many different types of atoms. In fact, there are over a 100. These different types are called elements. Examples of some elements are hydrogen, oxygen, iron, copper, and helium. Under normal conditions many atoms can stick together to form larger, different stuff. Scientists call material that results from the joining of different types of atoms "compounds". An example of a compound is water, which is a group of two hydrogen atoms and one oxygen atom. Notice that we said that these types of compounds can only form under what we called "normal conditions". In the type of environment in which nuclear fusion occurs, the joining of atoms, also known as bonding, can't happen. We will explain why later.
Smaller Than the Atom:So, are atoms made of even smaller stuff? The answer to this question is yes. Atoms are mostly empty space, but in the center of the atom is a structure called a nucleus. The nucleus is a congregation of particles. These particles are called protons and neutrons. Neutrons are neutral, or have no electrical charge. Protons, however, carry a positive electrical charge of 1. So, in a carbon atom, which has 6 protons in its nucleus, the overall electric charge of the nucleus would be 6. However, a regular atom is electrically neutral. This is because swirling around the nucleus in what is called the "electron cloud". The electrons in the electron cloud counteract the positive charges of the protons in the atomic nucleus with their negative electrical charges. This generates the neutral charge of the atom. The number of electrons and number of protons correlate in a one to one ratio. This means that there are the same number of protons and electrons in one atom. So, if an atom has 6 protons, like carbon, it will also have 6 electrons. The 6 electrons each have a charge of -1. This means that the total charge of all the electrons is -6, or -1x6. The charge of carbon's nucleus is 6 (from the protons), so when you add the two: 6 + -6, you get 0, which means that the atom, overall, has no charge.
Note: Picture NOT to scale: An atom is more than 99% empty space, and the protons and neutrons make up a very small amount of the volume of an atom. Additionally, the electrons are much smaller in proportion to the nucleons (protons and neutrons) than we have depicted. The nucleons are actually about 1800 times the size of an electron.
Ionization:Before, we mentioned that in an environment in which nuclear reactions occur, atoms don't form groups. Scientists would say "They don't bond." So, why don't they "bond"? When atoms bond together, they do so by sharing electrons. Additionally, when electrons gain energy, they move out farther from the nucleus of the atom. In some environments, there is extremely intense heat. Heat is a form of energy, and electrons can absorb this energy. So, electrons in the atoms become VERY excited and energetic. As a result, they move out very far away from the nucleus. So far, in fact, that they are really not part of the atom anymore. When this happens, we call the atom "ionized". (This is sort of like when a young child eats a lot of sugar. He or she can get really excited and tends to wander away from his parents!) If an atom is ionized, or has no electrons, it can't form chemical bonds with other atoms. This is because electrons are required for those bonds, and they aren't present because the atom has become "ionized". One of the places this happens to an atom is in a star.
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