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Understanding Atomic Radius Trends: The 2 Key Principles
Home / Blog / Understanding Atomic Radius Trends: The 2 Key Principles
Admin August 19, 2019 Homework 0
An atom is defined as one of the basic unit of any chemical element. Any chemical element from small to large weight has large atomic number has some definite atomic radius. The radius is defined as the distance between the centre of a circular substance to its outer perimeter. Hence, the atomic radius is the distance between the centre of nuclei and the electron in the outermost shell. The atomic radius are typically measured in very small SI unit of pico-meters, when 1 pm = meters. The smallest average atomic radius of hydrogen (H) is about 25 pico-meters and caesium (Cs) has the largest average radius of about 260 pico-meters.
The Basics Of Atomic Radius Trend
There is an interesting trend about the atomic radius of elements in periodic table. The two main trends of atomic radius in the periodic table can be seen when the movement is from left to right i.e. moving within a period and other trend is when moving from top to bottom. The periodic table shown below gives the trend of increase of atomic radii in the direction of arrow. The very first atomic radius trend is that the size of atom decreases as we move from left to right through a period. Now, for each period of elements one new electron is being added to the very same shell. Now, at the time of addition of electron to an atom a proton is also added to the nucleus of the atom. The electrons are basically attracted to the nucleus and thus makes the radius of the atom small. For an example the atom of a carbon C has 6 atomic number of 6 electrons and 6 protons. Whereas, the fluorine atom contain 9 electrons and 9 protons. Hence, from the trend of atomic radius the atom of Carbon C has a larger radius than the radius of an atom of fluorine F as with the three extra protons of fluorine will attract the electrons with higher force than the Carbon atom and thus making the radius of Fluorine smaller than that of Carbon. This is indeed true as the atomic radius of the Carbon atom is about 70 pico-meters and the atomic radius of Fluorine is 50 pico-meters.
The second trend of atomic radius suggests that atomic radius increases as we move to the bottom of the periodic table from the top. As we move through each group the atom gets an additional orbit of electron and thus each orbit becomes further away from the nucleus of the atom and hence the atomic radius is increased. The valence band electrons that is the electrons in the outermost shell are not much attracted by the nucleus to make the atomic radius smaller due to the electron shielding effect. Electron shielding is basically an effect by which the inner shell electrons shields the electrons in the outer shell from electro-magnetic attraction of the positive charged nucleus to electrons. This attraction force is also reduced also when there are more electrons in one shell. Hence, due to electron shielding the electrons in valence band can't get particularly close to the nucleus of the atom and as they can't get any close thus the atom has a large radius. For example, the potassium (K) has a large radius than the sodium (Na) atom as the potassium atom has one extra orbit of electrons than the Na atom or the valence electrons are further away from the nucleus and hence the potassium has a large radius of an atom. The atomic radius of potassium atom is about 220 pm which is larger than the atomic radius of sodium which is 180 pm.
Empirical atomic radii:
The empirical atomic number table shows the element name, symbol and their corresponding atomic number and empirical atomic radius in pico-meters from which the two trends can be identified.
Atomic Number
Atomic Symbol
Name of Element
Empirical Atomic Radius (in pm)
Caesium
Lanthanum
Praseodymium
Astatine
Actinium
Curium
Mendelevium
Nobelium
Lawrencium
Meitnerium
Nihonium
Moscovium
Livermorium
Oganesson
Atomic Radius Trend Exceptions
By observation of the above table it can be seen that in some cases, specifically in three cases. The first case is the exception for noble gases which are helium, neon, argon, krypton, xenon and radon. This exception occurs because the bonding of the noble gases are different from the bonding of other atoms. As the atomic radius is the half of the distance between the two atoms and hence the difference in distance between the atoms affects the length of atomic radius. In case of noble gases the outermost orbit is completely filled and hence there exist no covalent force between the atoms of noble gases as electrons are not shared by the atoms. The noble gas atoms are kept together by Van-der Waals force which is weaker than the covalent force. Thus the gapping between the two nuclei is larger for noble gases compared to the gapping between other atoms. Hence, if the empirical radius is calculated for noble gases then it will be overestimated. Hence, any of the noble gases has the empirical radius and thus don't follow the trend of atomic radius. It can be also be seen that the lanthanide and actinide series does not follows the two trends like they don't follow many other periodic table trends.
Significance Of The Trend Information
This atomic radius trends can be used to estimate the reaction properties between elements while forming a compound as the atoms with smaller atomic radii are more reactive than the larger ones. This is why the halogens in group 17 are more reactive than other group elements. Also, among the halogens fluorine has the lowest atomic radii and thus it is the most reactive element.
Top Periodic Trends In Atomic Radius
Thus the two atomic radius trends, in brief, are i) reduction of atomic radii as we move downwards in a group ii) The second trend of atomic radii is that the atomic radii decrease by moving left to right through a particular period. The few exceptions that exist in the periodic table for the radius trends are for halogens, lanthanides and actinides are explained for unusual atomic characteristics.
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