Explanation of The General Trends
The way the atomic radius varies with increasing atomic number can be explained by the arrangement of electrons in shells of fixed capacity. The shells are generally filled in order of increasing radius, since the negatively charged electrons are attracted by the positively charged protons in the nucleus. As the atomic number increases along each row of the periodic table, the additional electrons go into the same outermost shell; whose radius gradually contracts, due to the increasing nuclear charge. In a noble gas, the outermost shell is completely filled; therefore, the additional electron of next alkali metal will go into the next outer shell, accounting for the sudden increase in the atomic radius.
The increasing nuclear charge is partly counterbalanced by the increasing number of electrons, a phenomenon that is known as shielding; which explains why the size of atoms usually increases down each column. However, there is one notable exception, known as the lanthanide contraction: the 5d block of elements are much smaller than one would expect, due to the shielding caused by the 4f electrons.
The following table summarizes the main phenomena that influence the atomic radius of an element:
| factor | principle | increase with... | tend to | effect on radius |
|---|---|---|---|---|
| electron shells | quantum mechanics | principal and azimuthal quantum numbers | increase atomic radius | increases down each column |
| nuclear charge | attractive force acting on electrons by protons in nucleus | atomic number | decrease atomic radius | decreases along each period |
| shielding | repulsive force acting on outermost shell electrons by inner electrons | number of electron shells | increase atomic radius | reduces the effect of the 2nd factor |
Read more about this topic: Atomic Radius
Famous quotes containing the words explanation of the, explanation of, explanation, general and/or trends:
“To develop an empiricist account of science is to depict it as involving a search for truth only about the empirical world, about what is actual and observable.... It must involve throughout a resolute rejection of the demand for an explanation of the regularities in the observable course of nature, by means of truths concerning a reality beyond what is actual and observable, as a demand which plays no role in the scientific enterprise.”
—Bas Van Fraassen (b. 1941)
“We live between two worlds; we soar in the atmosphere; we creep upon the soil; we have the aspirations of creators and the propensities of quadrupeds. There can be but one explanation of this fact. We are passing from the animal into a higher form, and the drama of this planet is in its second act.”
—W. Winwood Reade (1838–1875)
“How strange a scene is this in which we are such shifting figures, pictures, shadows. The mystery of our existence—I have no faith in any attempted explanation of it. It is all a dark, unfathomed profound.”
—Rutherford Birchard Hayes (1822–1893)
“Suppose we think while we talk or write—I mean, as we normally do—we shall not in general say that we think quicker than we talk, but the thought seems not to be separate from the expression.”
—Ludwig Wittgenstein (1889–1951)
“Thanks to recent trends in the theory of knowledge, history is now better aware of its own worth and unassailability than it formerly was. It is precisely in its inexact character, in the fact that it can never be normative and does not have to be, that its security lies.”
—Johan Huizinga (1872–1945)