Atomic radius, half the distance between the nuclei of identical neighbouring atoms in the solid form of an element. An atom has no rigid spherical boundary, but it may be thought of as a tiny, dense positive nucleus surrounded by a diffuse negative cloud of electrons. The value of atomic radii. An atomic radius can be described in two ways. Nonbonding atomic radius, or van der Waals radius of an atom, is one-half of the distance between adjacent nuclei in the atomic solid. Conversely, a bonding atomic radius, or covalent radius, distinguishes between metals and nonmetals. The atomic radius is the distance from the nucleus of an atom to the outermost electrons. Since the orbitals around an atom are defined in terms of a probability distribution in quantum mechanics, and do not have fixed boundaries, determining where an atom 'stops' is not very straightforward. Trends in atomic radius in Periods 2 and 3. Trends in atomic radius down a group. It is fairly obvious that the atoms get bigger as you go down groups. The reason is equally obvious - you are adding extra layers of electrons. Trends in atomic radius across periods. You have to ignore the noble gas at the end of each period. Atomic Radius of Chromium The atomic radius of Chromium atom is 139pm (covalent radius).
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Ionic radii are difficult to measure with any degree of certainty, and vary according to the environment of the ion. For example, it matters what the co-ordination of the ion is (how many oppositely charged ions are touching it), and what those ions are.
There are several different measures of ionic radii in use, and these all differ from each other by varying amounts. It means that if you are going to make reliable comparisons using ionic radii, they have to come from the same source.
What you have to remember is that there are quite big uncertainties in the use of ionic radii, and that trying to explain things in fine detail is made difficult by those uncertainties. What follows will be adequate for UK A level (and its various equivalents), but detailed explanations are too complicated for this level.
Trends in ionic radius in the Periodic Table
Trends in ionic radius down a group
This is the easy bit! As you add extra layers of electrons as you go down a group, the ions are bound to get bigger. The two tables below show this effect in Groups 1 and 7.