I. The Atomic Theory
In the fifth century B.C the Greek philosopher Democritus expressed the belief that all matter consists of very small, indivisible particles, which he named ‘atomos’ (meaning uncuttable or indivisible).
In 1808 an English scientist and teacher, John Dalton † formulated a precise definition of the indivisible building blocks of matter that we call atoms.
Dalton’s work marked the beginning of the modern era of chemistry.
The hypotheses about the nature of matter on which Dalton’s atomic theory is based can be summarized as follows:
- Elements are composed of extremely small particles called atoms.
- All atoms of a given element are identical, having the same size, mass, and chemical properties. The atoms of one element are different from the atoms of all other elements.
- Compounds are composed of atoms of more than one element. In any compound, the ratio of the numbers of atoms of any two of the elements present is either an integer or a simple fraction.
- A chemical reaction involves only the separation, combination, or rearrangement of atoms; it does not result in their creation or destruction.
II. Structure of Atoms
A series of investigations that began in the 1850s and extended into the twentieth century clearly demonstrated that atoms actually possess internal structure, they are made up of even smaller particles, which are called subatomic particles. This research led to the discovery of three such particles electrons, protons, and neutrons.
J. Thomson † , an English physicist, used a cathode ray tube and his knowledge of electromagnetic theory to determine the ratio of electric charge to the mass of an individual electron. The number he came up with was :
where C stands for coulomb, which is the unit of electric charges phenomenon was a cathode ray tube, the forerunner of the television tube.
2.The Proton and the Nucleus
By the early 1900s, two features of atoms had become clear: they contain electrons, and they are electrically neutral. To maintain electric neutrality, an atom must contain an equal number of positive and negative charges. Therefore, Thomson proposed that an atom could be thought of as a uniform, positive sphere of matter in which electrons are embedded like raisins in a cake. This so-called “plum-pudding” model was the accepted theory for a number of years.
In 1895, the German physicist, Wilhelm Röntgen † noticed that cathode rays caused glass and metals to emit very unusual rays. This highly energetic radiation penetrated matter, darkened covered photographic plates, and caused a variety of substances to fluoresce. These rays could not be deflected by a magnet, so they could not contain charged particles as cathode rays do. Röntgen called them X rays because their nature was not known.
Not long after Röntgen’s discovery, Antoine Becquerel, † a professor of physics in Paris, began to study the fluorescent properties of substances. Purely by accident, he found that exposing thickly wrapped photographic plates to a certain uranium compound caused them to darken, even without the stimulation of cathode rays. Like X rays, the rays from the uranium compound were highly energetic and could not be deflected by a magnet, but they differed from X rays because they arose spontaneously.
One of Becquerel’s students, Marie Curie, suggested the name radioactivity to describe this spontaneous emission of particles and/or radiation. Since then, any element that spontaneously emits radiation is said to be radioactive.
Rutherford’s model of atomic structure left one major problem unsolved. It was known that hydrogen, the simplest atom, contains only one proton and that the helium atom contains two protons. Therefore, the ratio of the mass of a helium atom to that of a hydrogen atom should be 2:1. (Because electrons are much lighter than protons, their contribution to atomic mass can be ignored.) In reality, however, the ratio is 4:1.
Rutherford and others postulated that there must be another type of subatomic particle in the atomic nucleus; the proof was provided by another English physicist, James Chadwick, † in 1932. When Chadwick bombarded a thin sheet of beryllium with a particles, a very high-energy radiation similar to g rays was emitted by the metal. Later experiments showed that the rays actually consisted of a third type of subatomic particles, which Chadwick named neutrons, because they proved to be electrically neutral particles having a mass slightly greater than that of protons.
III. Atomic Number, Mass Number, and Isotopes
The atomic number (Z) is the number of protons in the nucleus of each atom of an element.
The mass number (A) is the total number of neutrons and protons present in the nucleus of an atom of an element
Most elements have two or more isotopes, these atoms that have the same atomic number but different mass numbers. For example, there are three isotopes of hydrogen:
- One, simply known as hydrogen, has one proton and no neutrons.
- The deuterium isotope contains one proton and one neutron, and
- Tritium has one proton and two neutrons.
Thus, for the isotopes of hydrogen, we write:
IV. The Periodic Table
More than half of the elements known today were discovered between 1800 and 1900. During this period, chemists noted that many elements show strong similarities to one another. Recognition of periodic regularities in physical and chemical behaviour and the need to organise the large volume of available information about the structure and properties of elemental substances led to the development of the periodic table, a chart in which elements having similar chemical and physical properties are grouped together.
The periodic table shows the modern periodic table in which the elements are arranged by:
- Atomic number (shown above the element symbol) in horizontal rows called periods
- and in vertical columns known as groups or families, according to similarities in their chemical properties.
The elements can be divided into three categories—metals, nonmetals, and metalloids.
- A metal is a good conductor of heat and electricity
- A nonmetal is usually a poor conductor of heat and electricity.
- A metalloid has properties that are intermediate between those of metals and nonmetals.
Elements are often classified by their periodic table group number (Group 1A, Group 2A, and so on). However, for convenience, some element groups have been given special names.
- The Group 1A elements (Li, Na, K, Rb, Cs, and Fr) are called alkali metals
- The Group 2A elements (Be, Mg, Ca, Sr, Ba, and Ra) are called alkaline earth metals
- Elements in Group 7A (F, Cl, Br, I, and At) are known as halogens
- Elements in Group 8A (He, Ne, Ar, Kr, Xe, and Rn) are called noble gases or rare gases
The periodic table is a handy tool that correlates the properties of the elements in a systematic way and helps us to make predictions about chemical behaviour.