An atom is the most simple type of particle that makes up matter. Matter is anything that has mass and uses space. It is the smallest part of an element that still has the properties of that certain element. When you see atoms as ball-shaped, they have diameters of about 10-10 meters (that's 0.0000000001 m). They have masses around 10-23g (that's 0.00000000000000000000001 g.)
The word "atom" comes from the Greek ἀτόμος, "indivisible", from ἀ-, "not," and τόμος, "a cut." Until the end of the 19th century, many people believed atoms were very tiny, solid balls which could never be divided. This idea comes from Democritus and Leucippus, Greek philosophers in the 5th century BC. People believed this until the early 20th century when tests by physicists trying to provide nuclear power disproved it.
Types of atom.
Atoms are made of 3 kinds of sub-atomic particles - protons, neutrons and electrons. A proton has a positive charge. A neutron has no charge. An electron has a negative charge. Electrons have much less weight than the protons and neutrons. Electrons move very fast around the nucleus. In a neutral atom, the number of protons is the same as the number of electrons. In other atoms, such as [b]isotopes[/b] the number of neutrons is not the same as the number of protons.
Charged atoms are called ions. Atoms with fewer electrons than protons have a positive charge. These atoms are called cations. Atoms with more electrons than protons have a negative charge. These atoms are called anions.
The nucleus is at the center of an atom. The "nucleus" contains "protons" and "neutrons", which are tightly packed together. Because of this, the nucleus is positively charged. Protons and neutrons have similar mass.
A nucleus is much smaller than its atom. The nucleus is more than 100,000 times smaller than the atom - the size of the atom is determined by the size of the outer electron shell. For an example of size difference, if the atom were the size of a football stadium, the atom's nucleus would be the size of a pea in the middle. Most of an atom is empty space which is occupied by electrons spinning around.
Electrons orbit at different distances from the nucleus. We say those closest to the nucleus are in one "shell," and those a little farther away are in another "shell." Each shell is given a number. The one closest to the nucleus is shell 1, the next one out is shell 2, and so on.
Atoms are usually put into groups based on their atomic number. The atomic number of an atom is the number of protons in the atom. The atomic number decides what family or "element" the atom belongs to. For example, carbon atoms are atoms with 6 protons in their nucleus. All atoms with the same atomic number share many of physical properties and have the same chemical behavior. The different kinds of atoms are listed in the Periodic table. The mass number or "nucleon number" is the total of protons and neutrons.
The number of neutrons does not change the element of the atom - within an element family are several members, each with the same atomic number but different mass numbers. These are called isotopes. To write the name of an isotope, we write the name of an element, then its mass number. For example, Carbon-14 (which has 6 protons and 8 neutrons in each atom). Some isotopes are radioactive, meaning they give off radiation. Others are called stable, meaning they do not give off any radiation. Any atom that has an atomic number of 83 or higher is also radioactive, whatever isotope it is.
Radioactive atoms and isotopes are dangerous to any living thing except in very small amounts. They should only be handled by experts because they can make people very sick if not handled correctly. Doctors sometimes use isotopes. For example, the isotopes of Barium and Iodine are used to help find illnesses, but they should still be handled very carefully, even though they use very small amounts which are not enough to make a person sick.
Only 94 different types of atoms are found in nature, A few more have been made in laboratories). The next element we make could be an ultra-light solid. It could be a liquid with a boiling point of over 2,000,000 degrees Celsius.
The Periodic Table.
Every different kind of atom has a name and every name has an abbreviation. The 94 kinds of atoms are usually listed in a table. The table is called the periodic table. A scientist called Dmitri Mendeleev was the first to think of the table. Because of this and other things he did, element 101 was called Mendelevium when it was discovered. Lots of atoms that have higher numbers are radioactive, like radium and uranium, and should never be handled without a lot of care and protection because they give off so much radiation.
The simplest atom is the hydrogen atom, which has atomic number 1. The most common type of hydrogen atom has 1 proton and 1 electron (with no neutron). The hydrogen isotope containing 1 additional neutron is called "deuterium"; the hydrogen isotope with 2 additional neutrons is called "tritium". It has been the subject of much interest in science, particularly in the early development of quantum theory.
The first 11 kinds of atoms (also called elements) are called hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, neon and sodium. Their abbreviations are H, He, Li, Be, B, C, N, O, F, Ne, Na.
You may notice that Sodium does not fit with Na. This is because sodium was most commonly called "natrium" in the past. This has carried over into other parts of science as well. "Hyponatremia" is an illness where the body lacks enough salt to function properly.
The purpose of an atom.
All matter and all substances are made up of these 95 kinds of atoms. Most things contain far fewer types of atoms. Water, for example, contains only atoms of hydrogen and oxygen. In water these atoms form combinations called molecules.

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