There are over 100 elements in nature. Each element is composed of an innumerable group of atoms which are identical with one another and different from the atoms that make up the y other elements. Normally they are found in pairs or in combinations with other kinds of atoms. We call these arrangements of atoms ‘molecules’.
Why do some atoms combine while others do not? What determines the manner in which atoms combine? The answers have to do with the electrons that circle the nucleus of the atom. As we know, an atom is composed of three kinds of particles: protons and neutrons, which are found in a very small region at the center of the atom, and electrons, which orbit the nucleus. The number of electrons in an atom is the same as the number of protons, and this number determines the chemical properties of the element.
The number of neutrons in the atoms of a given element is not constant, though it is usually slightly greater than the number of protons. The orbits of the electrons about the nucleus are something like the orbits of the planets in our solar system about the sun, except that each atomic orbital can contain only a certain maximum number of electrons. For example, the first atomic orbital, corresponding to the planet Mercury, can contain as many as two electrons, no more; the second atomic orbital, corresponding to the planet Venus, can contain as many as eight electrons, no more; and so on.
The inner orbitals of atoms are the first to take electrons, and because of certain factors that depend upon energy, atoms like to have their last, outer orbital full. The inert gases – Helium, Neon, Argon, Krypton, Xenon, and Radon – are elements whose atoms have full electron orbitals. Consequently, these elements do not combine with other elements; they are chemically inactive, inert.
The atoms of all other elements tend to combine with other atoms so as to fill up their electron orbitals. Hydrogen atoms always have a single electron and a single proton, so their electron shell (orbital) is one electron short of being full. In the gaseous state, two hydrogen atoms are combined to form a single molecule (H2).
Each electron circling about both nuclei makes it appear as if there were only one electronic orbital. Oxygen atoms have eight electrons, two of which fill the first orbital; the remaining six are contained in the second orbital, leaving the second orbital two short of the preferred number eight. Often in nature we find a molecule where two hydrogen atoms have given their electrons to a single oxygen atom, which fills the second orbital of the oxygen atom.
This arrangement of oxygen and hydrogen is very stable. This molecule is called ‘water’. The carbon atom has four of its six electrons in its outer orbital. Depending upon how you look at it, it has either four too few or four too many electrons in its outer orbital. It is willing either to borrow or to lend four electrons. When carbon combines with oxygen, the carbon atom gives two electrons to each of two oxygen atoms; the result is the gas carbon dioxide (CO2), which is quite common in nature.
Chemical reactions are simply the arrangements and rearrangements atoms and molecules go through to have full 5 electron orbitals. Any destruction or creation of molecules is a chemical reaction.
1. Use your own words for expressing the general idea of the first paragraph.
2. Use your own words for expressing the general idea of the second paragraph.