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In this article we will discuss about Atoms. After reading this article you will learn about: 1. Atomic Structure of Atom 2. Atomic Number and Atomic Weight.
Atomic Structure of Atom:
Dalton’s concept of indivisibility of the atom was discarded after the discovery of various fundamental particles like electron, proton, neutron, positron, etc., which constitute atoms of all elements. The idea about these particles resulted from Faraday’s famous work on electrolysis (1834).
Rutherford proposed a nuclear theory of atomic structure which is as follows:
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(i) The atom consists of a very minute nucleus at its centre in which is concentrated the entire mass of the atom. The dimension of the nucleus is negligible compared to the radius of the atom.
(ii) The entire positive charge also resides in the nucleus. The positive charge is Ze, where e is the unit charge equivalent in magnitude to that of an electron and Z is the number of such units of charge existing in the nucleus. These units of positive charge are called protons.
(iii) Outside the nucleus, there are Z electrons, called extra-nuclear electrons, so that the atom is ultimately neutral. The electrons move constantly around the nucleus in different orbits. The arrangement is thus quite similar to that of the solar system.
(iv) The number of positive charges (Z) in the nucleus is the atomic number of the element. Z is thus the atomic number of electrons in the atom. Structurally the atoms of all elements are similar but they differ only in their atomic number, which is the fundamental characteristic of each element. The chemical properties of an element are, however, based on its extra-nuclear electrons and their arrangement.
Hydrogen and helium are simple elements. The hydrogen atom consists of only one proton and one electron, and the helium atom consists of two protons and two electrons. The nucleus of the hydrogen atom has no neutron and the single electron moves constantly around the nucleus forming an envelope.
Atoms of other elements are more complex. Carbon (C) has six protons, 6-8 neutrons and six electrons. Of all the elements on earth the most complex one is uranium (U), having 92 protons, 140-147 neutrons and 92 electrons.
Atomic Number and Atomic Weight:
The atomic number (Z) of an element is the number of positively charged protons in its nucleus. The fundamental characteristic of an element depends on its atomic number (Z). The atomic nucleus contains two kinds of particles — neutrons and protons, the exception being that of hydrogen, which lacks any neutron.
The total number of protons and neutrons together represent the atomic weight. Thus, the carbon atom (at. no. 6, and at. wt. 12) has six protons and six neutrons in its nucleus. In chlorine (at. no. 17, at. wt. 35.5), 17 protons and 18 neutrons constitute the nucleus.
An element is conveniently expressed by its symbol with a subscript and superscript. The subscript represents the atomic number whereas the superscript represents the atomic weight. Thus hydrogen, helium, lithium can be written as 1H1, 4He2,7Li3. respectively.
Scientists use powerful machines called particle accelerators to smash atoms or subatomic particles together at high speed to make other subatomic particles like kaon, upsilon, charmed lambda, etc. We know that the nucleus of every atom contains protons and neutrons. These in turn are made up of smaller particles, called quarks. The quarks are held together by other particles, called gluons.
