History of the atom
since the discovery of the atom, it was thought that matter is made up of simple, indivisible particles. Greek philosophers thought that matter could be divided into smaller and small particles to reach a basic unit, which could not be further subdivided. Democritus (460-370 B.C) called these particles atoms, derived from the word ‘’atomos’’ means indivisible. However, the ideas of Greek philosophers were not based on experimental evidence.
In the late 17th century, the quantitative study of the composition of pure substances disclosed that a few elements were the components of many different substances. It also investigated how elements combined to form compounds and how compounds could be broken down into their constituent elements.
In 1808, an English school teacher, John Dalton, recognise that the law of conservation of matter and the law of definite proportions could be explained by the existence of atoms. He developed an atomic theory; the main postulate of which is that all matter is composed of atoms of different elements, which differ in their properties.
What is the definition of the atom?
an atom is the smallest possible particle of an element that can take part in a chemical change.
or , it can be defined as:
Atom is the smallest particle of an element, which can take part in a chemical reaction.
Examples of Atoms
For example, He and Ne, etc. Have atoms, which have independent existence while atoms of hydrogen, nitrogen, and oxygen do not exist independently.
Modern researchers have clearly shown that an atom is further composed of subatomic particles like electrons, protons, hyperons, neutrino, anti-neutrino, etc. More than 100 such particles are thought to exist in an atom. However, electrons, protons, and neutrons are regarded as the fundamental particles of an atom.
A Swedish chemist J. Berzelius (1779-1848) determined the atomic masses of elements. A number of his values are close to the modern values of atomic masses. Berzelius also developed the system of giving elements a symbol.
Evidence of the Atom
It is not possible actually to see the atoms but the nearest possibility to its direct evidence is by using an electron microscope. A clear and accurate image of an object that is smaller than the wavelength of visible light, cannot be obtained.
Thus an ordinary optical microscope can measure the size of an object up to or above 500 mm (1nm=10-9m). However, objects of the size of an atom can be observed in an electron microscope. It uses a beam of electrons instead of visible light because the wavelength of electrons is much shorter than that of visible light.
Bellow electron microscopic shows a photograph of a piece of graphite that has been magnified about 15 million times. The brighter band in the figure are layers of carbon atoms.
In the 20th century, X-ray work has shown that the diameter of atoms is of the order 2×10-10 m which is 0.2 nm. masses of atoms range from 10-27 to 10-25kg.
They are often expressed in atomic mass units (amu) when 1 amu is = 1.661x 10-27kg. The students can have an idea about the amazingly small size of an atom from the fact that a full stop may have two million atoms present in it.
Atomic Particle: protons, neutrons, and electrons
All elements are made up of atoms. The atoms of one individual element are identical in every respect and differ from the atoms of all other elements. Atoms are made up of sub-atomic particles called protons, neutrons, and electrons.
The table below shows the relative atomic masses and charges of these particles. The mass of a proton is equal to the mass of a neutron. However, an electron`s mass is only 1/1840 that of either a proton or neutron. As the largest atoms still have less than a hundred electrons, the electron`s mass is regarded as so small it can be ignored.
|Particle||Symbol||Relative mass||Relative charge|
An element`s identity: proton number and nucleon number
Every element has its own proton number (Z) and nucleon number (A).
The proton number of an element is the number of protons in its atom (which is equal to the number of electrons).
The nucleon number of an element is the total number of protons and neutrons in its atom.
Consider a sodium atom, whose nucleon number is 23. This means its atom contains in total of 23 protons and neutrons. However, the proton number of sodium is 11, which means it contains 11 protons and 11 electrons. By subtracting the proton number from the nucleon number, we obtain the number of neutrons present in the atom.
To represent these numbers, we write:
Nucleon number (A)
Proton number (Z)
From this, we can deduce that a sodium atom has 11p, 11e, and 12 n. In the next table, the number of sub-atomic particles is worked out in a similar way, for the first twenty elements listed in the periodic table.
Proton numbers and nucleon numbers of elements
|Element||Proton number (Z)||Nucleon number (A)||Number of protons||Number of electrons||Number of neutrons (A-Z)|