Atoms having 8 electrons in their outermost shell are very stable and they did not form compounds. Other atoms such as hydrogen, sodium, chlorine etc. which do not have 8 electrons in their outermost shell undergo chemical reactions. They can stabilize by combining with each other and attain the above configurations of noble gases i.e. 8 electrons (or 2 electrons in case of helium) in their outermost shells.
Before the beginning of the eighteenth century, only a few elements were known, so it was quite easy to study and remember the properties of those elements and their compounds individually. However, by the middle of the nineteenth century, more the than sixty elements had been discovered.
The periodic table based on the modern periodic law is called the Modern Periodic Table. Presently, the accepted modern periodic table is the Long Form of Periodic Table. It may be regarded as an extended form of Mendeleev’s table in which the subgroups A and B have been separated.
Though Mendeleev’s periodic table included all the elements, yet at many places a heavier element had to be placed before a lighter one. Such pairs of elements (called anomalous pairs) violated the periodic law. Also, there was no place for different isotopes of an element in the periodic table.
D'mitri Mendeleev, a Russian chemist, studied the properties of all the 63 elements known at that time and their compounds. On arranging the elements in the increasing order of atomic masses, he observed that the elements with similar properties occur periodically.
In 1864, an English chemist John Alexander Newlands arranged the elements in the increasing order of their atomic masses (then called atomic weight). He observed that every eighth element had properties similar to the first element. Newlands called it the Law of Octaves.
Before the beginning of the eighteenth century, only a few elements were known, so it was quite easy to study and remember the properties of those elements and their compounds individually. However, by the middle of the nineteenth century, more than sixty elements had been discovered.
The electrons move in definite paths called orbits or shells around a central nucleus. These orbits or shells have different energies and can accommodate different number of electrons in them. The question arises that how are the electrons distributed among these shells? The answer to this question was provided by Bohr and Bury.
The nucleus of atom contains positively charged particles called protons and neutral particles called neutrons. The number of protons in an atom is called the atomic number and is denoted by the symbol Z. All atoms of an element have the same atomic number.
Rutherford's model was unable to explain the relationship between the atomic mass and the atomic number (the number of protons). According to the Rutherford’s model, the mass of helium atom (containing 2 protons) should be double that of a hydrogen atom (with only one proton).
All matter is made of atoms and all the atoms are electrically neutral. Having discovered electron as a constituent of atom, Thomson concluded that there must be an equal amount of positive charge present in an atom. On this basis he proposed a model for the structure of atom.
Much before the discovery of electron, Eugen Goldstein (in 1886) performed an experiment using a perforated cathode (a cathode having holes in it) in the discharge tube filled with air at a very low pressure. When a high voltage was applied across the electrodes in the discharge tube, a faint red glow was observed behind the perforated cathode.
Dalton's atomic theory, proposed in the year 1803, considered the atom to be the smallest indivisible constituent of all matter. The Dalton's theory could explain the law of conservation of mass, law of constant composition and law of multiple proportions known at that time.
An ancient Greek philosopher Democritus (460 - 370 BC) and Leucippus suggested that if we go on dividing matter, a stage will come when further division of particles will not be possible. Democritus called these individual particles 'atoms' (which means indivisible).
When a substance gains oxygen during a reaction, it is said to be oxidized and when a substance loses oxygen during a reaction, it is said to be reduced. During the reaction process, one reactant gets oxidized while the other gets reduced. Such reactions are called oxidation reduction reaction or Redox Reactions.
Water pollution is the contamination of water bodies like lakes, rivers, ground water and oceans. It occurs due to the discharge of untreated pollutants into water bodies. It not only affects plants and organisms living near the location of discharge but also travels to other locations through transportation of polluted water.
When a capillary tube with a fine bore is dipped in water, water rises in the capillary. The extent to which water rises depends on the diameter of the capillary. The smaller the diameter of the capillary, the higher will be the rise of water in the capillary tube.
Genetic Engineering is also called recombinant DNA technique. In this technique, gene from an organism of a species can be transferred to become part of the or genome of an organism belonging to another species which is then termed GMO genetically modified organism.
Genes control all features of an organism. Some times a gene may change or mutate either in the gamete or zygote. Mutated gene may not remain normal. Also, sometimes a defective gene present in the parent may not be expressed in the parent as the dominant normal member of its pair may mask the effect of the defective gene.
The combination of sex chromosomes with autosomes determines whether the foetus will be a boy or a girl. The foetus develops from the zygote which is formed by the fusion of the two gametes, the male gamete or sperm and the female gamete or egg. Gametes are haploid while the zygote is diploid.
Genes Genes are present on chromosomes. The Genes are the "Mendelian factors" present in pairs (one received from the father, other from the mother), on the chromosomes. Thus, one member of a pair of genes present on the chromosomes has its pair on the homologous chromosome at the same location.
The question about heredity intrigued many scientists. Gregor Johann Mendel (1822 -1884), an Austrian monk undertook the laborious task of finding the answers. He selected some pea plants, grew them year after year, compiled a lot of data, analysed and postulated certain laws of inheritance for the first time.
All organisms resemble, in structure, to their parents. The passing down of similar characters generation after generation is termed heredity. Heredity is controlled by genes. Differences in gene combinations lead to variations or differences even among members of the same family. The science of heredity and variation is termed Genetics.