Atom

The existence of the smallest particles that make up matter, first proposed by two Greek philosophers, Leucippus and Democritus ie about 450 years before Christ. Both men state that all matter is composed by particles are very small and can not be divided again called atoms. Atom comes from the Greek, which atomos (a meaning not and Tomos mean divided).
Mentioned earlier that until now there has been no man who is able to see the smallest particle of a substance directly or with the aid of the most sophisticated microscopes. Thus, the atoms form nobody ever knew. Based on a variety of phenomena, John Dalton (1766-1844) who was a chemistry teacher from England, in 1808 filed the idea of ​​the smallest particles that make up the material. The essence of John Dalton thought about the atom, namely:

1. Each element consists of the smallest particles that can not be divided again, called atoms.
2. All atoms of the same element have the same size and mass. Atoms of different elements have different masses. Thus, many kinds of atoms with the same number of elements.
3. Atoms can not be undermined. Atoms can not be destroyed or created through a chemical reaction.
4. Through a chemical reaction, the atoms of the reactants will have a new structure and will be mutually bonded to each other with a certain ratio or number.

Thought of the atom above Dalton is known as Dalton's atomic model. With Dalton thought about the atom, it can be said that the various (thousands even millions) of substances that exist in nature is actually derived from the smallest particles of elements (atoms) which are relatively very small (see the number of elements contained in the periodic system element).
In the images contained in these books are often chemically different atoms of the elements are given different colors. For example, the color of the carbon atoms are colored black while the oxygen atoms are colored red, as shown in Figure 7.2. This coloring is not the color of the atom itself. Different coloring only to show that the atoms of different elements derived from. It must be remembered that the atoms in a single state alone or do not have certain properties, such as color, shape, density, electrical conductivity, boiling point, melting point, and so on. New properties that arise when atoms combine to form a large collection of atoms in certain ways, for example
are graphite and diamond. Both substances have different properties, diamond is very hard and transparent, while graphite is soft, black, and not transparent. Both substances have different properties, but both are formed by atoms of the same element, but in ways different binds. Atom in question here is a carbon atom. If they burned the two substances would produce the same substance, namely carbon dioxide (CO2).
In general, it can be said that the way the atoms are bonded in determining the nature of the substances formed. In graphite, shown in Figure 7.3, that the carbon atoms forming separate layers. In setiaplapisan, one carbon atom has three atoms adjacent (neighboring). While the diamond, the atoms are arranged more meetings, one carbon atom surrounded by four carbon atoms.

Figure 7.3 Graphite and diamonds along with models of the arrangement of the atoms.

Metal elements such as sodium, calcium, copper, gold, and iron in the free state or not combine with other elements, composed of the smallest particles of matter that belongs to the class of atoms. So, a piece of iron metal prepared by iron atoms that have a particular structure or pattern (see Figure 7.4).

Model Atomic Structure of Iron

In addition to metals, there are also non-metallic elements which in its free state (in a state not combine with other elements) is composed of atoms. For example, the elements of the group of noble gases helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). Look for information about the elements that you know that in the free state is composed of atoms.
To facilitate the study of the properties of elements and compounds, chemical experts construct elements of the periodic system of elements. Periodic system of elements is a table that lists all the elements that are known or unknown. On the periodic table of elements, listed 118 kinds of elements that have been identified by the expert presence. These elements are in a free state or its compounds in nature even elements that only exist in the laboratory alone.
Each column in the periodic system of elements termed class. In each group there is only one class of agents. In one class, the elements will be prepared in accordance with the increase in mass numbers. Main group elements given additional symbol A faction behind the numbers, eg IA, IIA, IIIA, and so on (see Figure 7.5).

Figure 7.5 The periodic system of elements, there are 8 class A and class B 8 and 7 periods.

Some of the main group gets special mention. The elements contained in the first main group (IA), such as lithium (Li) and sodium (Na) are called elements of the alkali metal group (hydrogen is not a metal that does not include alkali metals). The second major group (IIA), such as beryllium (Be) and magnesium (Mg) are called elements of the alkaline earth metals. The elements contained in the seven main groups (VIIa), such as fluorine (F) and chlorine (Cl) group elements called halogens. The elements contained in the eight main groups (VIIIA), such as helium (He) and neon (Ne) group elements called noble gases.
Each line begins with the elements of the periodic table alkali metal and ends with the noble gas elements. The elements which constitute one group will be rediscovered nature periodically atoms in each row. Therefore, the line in the periodic system of elements are called periods. The first period consists of only two kinds of elements, hydrogen and helium. Different ways of writing classes that use Roman letters, numbers written period with the numbers 1, 2, 3, 4, 5, 6, and 7. In the periodic system of elements, each element is written in the form of the element symbol along with the identity of the element, in the form of atomic number and mass number.