why carbon can form chain 1,2,3?

Carbon (C) appears in the 2nd row of the periodic table and has atomic number of 6. Given our discussion of electron shells it is easy to see that carbon has 4 electrons in its valence shell. Since carbon needs 8 electrons to fill its valence shell, it forms 4 bonds with other atoms (each bond consisting of one of carbon's electrons and one of the bonding atom's). Every valence electron participates in bonding, thus a carbon atom's bonds will be distributed evenly over the atom's surface. An organic molecule (hydrocarbon) is formed when carbon bonds to hydrogen. The simplest hydrocarbon consists of 4 hydrogen atoms bonded to a carbon atom (called methane). in fact, there appears to be almost no limit to the number of different structures that carbon can form. To add to the complexity of organic chemistry, neighboring carbon atoms can form double and triple bonds in addition to single carbon-carbon bonds. each carbon atom has 4 bonds. As you add carbon to a molecule, the empty carbon bonds are filled with hydrogen atoms (or other elements, as we will soon see). You can calculate the number of H atoms in the simple alkanes. The number of H atoms in a simple alkane equals two times the number of carbon atoms plus 2, or (2n + 2), where n is the number of carbon atoms in the molecule. The simple alkenes have 1 double bond and 2 fewer H atoms in the molecule; the number of H atoms in the simple alkenes = (2n). Simple alkynes contain 1 triple bond 2 fewer H atoms than the alkene, or (2n - 2) H atoms. The simple hydrocarbons are fairly common. Methane, for example, is released by decaying organic matter and is the main compound in natural gas. These chemicals are generally gases or liquids in nature and are very flammable. Butane is used in cigarette lighters. Ethyne, also known as acetylene, is used in welding. In addition to carbon and hydrogen, hydrocarbons can also contain other elements. The alcohols, for example, are a group of hydrocarbons in which a hydroxol (-OH) group is bound to a carbon skeleton. These compounds are named like the simple hydrocarbons, a prefix attached to a root ending (-anol for the alcohols).