Oleh: adiriyadhi | 18 Februari 2008



In 1804, John Dalton proposed the existence of atoms. He not only postulated that atoms exist, as had ancient Greek philosophers, but he also attributed to the atom certain properties. His postulates were as follows:

  1. Elements are composed of indivisible particles, called atoms.
  2. All atoms of a given element have the same mass, and the mass of an atom of a given element is different from the mass of an atom of any other element.
  3. When elements combine to form a given compound, the atoms of one element combine with those of the other element(s) in a definite ratio to form molecules. Atoms are not destroyed in this process.
  4. Atoms of two or more elements may combine in different ratios to form different compounds.
  5. The most common ratio of atoms is 1:1, and where more than one compound of two or more elements exists, the most stable is the one with 1:1 ratio of atoms. (This postulate is incorrect.)

From 50 to 100 years after Dalton proposed his theory, various discoveries were made that show that the atom is not indivisible, but really is composed of parts. Natural radioactivity and the interaction of electricity with matter are two different types of evidence for this subatomic structure. The most important subatomic particles are Proton (+1) , Neutron (0), and Electron (-1), along with their most important properties. The protons and neutrons are found in a very tiny nucleus (plural, nuclei). The electrons are found outside the nucleus.

Atoms having the same number of protons but different numbers of neutrons are called isotopes of one another. The number of neutrons does not affect the chemical properties of the atoms appreciably, so all isotopes of a given element have essentially the same chemical properties. Different isotopes have different masses (contrary to Dalton’s second postulate) and different nuclear properties, however. The sum of the number of protons and the number of neutrons in the isotope is called the mass number of the isotope. Mass number is symbolized A. Isotopes are usually distinguished from one another by their mass numbers, given as a superscript before the chemical symbol for the element. Carbon-12 is an isotope of carbon with a symbol 12C.

The periodic table is an extremely useful tabulation of the elements. It is constructed so that each vertical column contains elements that are chemically similar. The elements in the columns are called groups, or families. (Elements in some groups can be very similar to one another. Elements in other groups are less similar. For example, the elements of the first group resemble one another more than the elements of the fourth group from the end, headed by N.) Each row in the table is called a period. There are three distinct areas of the periodic table—the main group elements, the transition group elements, and the inner transition group elements

The arrangement of electrons in successive energy levels in the atom provides an explanation of the periodicity of the elements, as found in the periodic table. The charges on the nuclei of the atoms increase in a regular manner as the atomic number increases. Therefore, the number of electrons surrounding the nucleus increases also. The number and arrangement of the electrons in the outermost shell of an atom vary in a periodic manner. For example, all the elements in Group IA (H, Li, Na, K, Rb, Cs, Fr) corresponding to the elements that begin a new row or period, have electronic configurations with a single electron in the outermost shell, specifically an s subshell.

H = 1s1
Li = 1s2 2s1
Na = 1s2 2s2 2p6 3s1
K = 1s2 2s2 2p6 3s2 3p6 4s1
Rb = 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s1
Cs = 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s1
Fr = 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f 14 5d10 6p6 7s1

The noble gases, located at the end of each period, have electronic configurations of the type ns2 np6, where n represents the number of the outermost shell. Also, n is the number of the period in the periodic table in which the element is found.


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