In case of d block elements as we move from left to right across the period, atomic number increases. This is due to the presence of then d – block elements. Radii of aluminium and gallium are equal in spite of the fact they belong to the same group. The inert gases have the largest atomic radii in the period because for them van der Wall’s radii are considered. Thus in a period alkali metals have the largest atomic radius and it gradually decreases across the period and it is minimum for the halogen elements. Hence the attractive force on the electrons in the outermost shell increases. The effective nuclear charge increases across the period. The valence electrons are added to the same orbit of all the elements in the same period, hence screening effect and number of shells are the same. ![]() ElementsĪs we move from left to right in a period atomic number increases, hence the nuclear charges increases. The atomic radii of the elements of the second period and the graphical representation of variation for the second period are given below. Periodic Trend in Atomic Radius Along the Period: The atomic radius is inversely proportional to the effective nuclear charge. For a given quantum shell, the shielding ability of inner electrons decreases in the order of s > p > d > f.Įffective nuclear charge: The effective nuclear charge is the difference between the actual nuclear charge and the screening effect constant.charge. Thus atomic radius is directly proportional to the screening effect. As the screening effect increases, the atomic radius increases. This effect is known as the screening effect. Thus they act as a screen or shield between the nucleus and electrons of outer orbit. In an atom having more electrons and particularly more electron shells, it is observed that the inner orbits decrease the attraction between the electrons in the outer orbit and nucleus. Thus atomic radius is inversely proportional to the nuclear charge. Nuclear Charge:Īs the nuclear charge increases the atomic radius decreases due to an increase in the attractive force on the outermost electrons. ![]() Thus atomic radius is directly proportional to the number of electronic shells. The covalent radius is the smallest of all the radii because covalent bonds are formed due to overlapping of orbitals and there is penetration of one atom in another.įactors Affecting Atomic Size: Number of Shells:Ītomic size increases with the increase in the number of electronic shells. van der Wall’s forces are weaker hence the distance between the atoms is larger. Generally, atomic radii of inert gases are expressed in terms of van der Walls’ radius. the closest distance between two hydrogen atoms without forming the bond is 240 pm. Hence van der Walls’ radius of sodium = 240 / 2 = 120 pm. Van der Walls’ radius is defined as one half of the distance between the nuclei of the two atoms of the same substance at their closest approach. This property can be used to find the internuclear distance between two molecules forming a single covalent bond among themselves. ![]() Hence crystal radius of oxygen = 132 / 2 = 66 pm. the distance between two oxygen atoms in molecular oxygen is 132 pm. Hence crystal radius of sodium = 372 / 2 = 186 pm.Ĭovalent radius is defined as one half of the distance between the centres of the two similar nuclei of two similar atoms bonded together by a single covalent bond. For e.g. the distance between two sodium atoms in a sodium crystal is 372 pm. It is defined as one half of the distance between the centres of nuclei of two adjacent atoms in a metallic crystal. Other Terms Related to Atomic Radius: Crystal Radius: Hence the definition given in above point is arbitrary. The electron density in an atom is greatly influenced by the presence of other atoms around the bonding atom and the nature (type) of bonding with neighbouring atoms, Depending upon this the terms like crystal radius, covalent radius, van der Walls’ radius, tetrahedral radius, etc. The atomic radius (atomic size) may be regarded as the distance from the centre of the atom to the outermost (valence) shell of electrons. One practical approach of finding the size of an atom of a non-metallic element is to measure the distance between two atoms when they are bound together by a single bond in a covalent molecule and from this value, the “Covalent Radius” of the element can be calculated. Secondly, since the electron cloud surrounding the atom does not have a sharp boundary, the determination of the atomic size cannot be precise. The size of an atom is very small (120 pm).
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