Page 143 - Chemistry ICSE Class IX
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The Periodic Table 131
Periodicity in Properties
The properties of elements depend on their electronic con gurations.
Electronic con gurations change regularly along the period and down the
group of the periodic table. Therefore, the properties of elements change
across the period and down the group of the periodic table. There are quite
a few atomic/physical properties of elements. At this stage, we describe
only the following properties of elements:
Atomic number Atomic size (or atomic radius) Signboards like these use one of
Valence electrons Valency the noble gases
Tendency to lose electrons Tendency to gain electrons
Metallic and nonmetallic character Chemical reactivity
Nature of oxides
Atomic Number
The number of protons inside the nucleus of an atom of any element is
called its atomic number.
Element Li Be B C N O F Ne
Atomic number 3 4 5 6 7 8 9 10
Gold and platinum are called
No. of protons inside the nucleus 3 4 5 6 7 8 9 10 noble metals because of their
Thus, limited reactivity
the atomic number increases as we go down the group.
the atomic number increases by one at each element as we go from left
to right in a period.
Atomic Size
The atomic size of an element can be expressed as the radius of its atom
(assumed to be spherical). The atomic radius may be de ned as:
“The distance from the centre of the nucleus to the outermost shell
of electrons in an isolated atom is called its atomic radius.”
For nonmetallic elements, atomic radius is de ned as follows:
Fig. 5.1 Atomic (or covalent) radius of
“Half the distance between the nuclei of the two similar atoms H atom
covalently bonded to each other by a single bond is called its atomic (or
covalent) radius.”
For example, the H – H covalent bond has a length of 74 pm (pm is
called picometre; 1 pm = 1 × 10 –12 m). So,
74 pm
Atomic radius of hydrogen atom = = 37 pm
2 dŚĞ ĂƚŽŵŝĐ ƌĂĚŝƵƐ ŽĨ ŵĞƚĂůƐ ŝƐ ĂůƐŽ
For metals, atomic radius is de ned as follows: ĐĂůůĞĚ ŵĞƚĂůůŝĐ ƌĂĚŝƵƐ͘
“Half of the distance between the nuclei of the two adjacent metal
atoms in the metallic crystal is called its atomic radius.”
How does atomic size vary in a group of the periodic table
In any group of elements, the atomic size increases while going from top
to bottom. This can be explained as follows.
When we go down the group, a new shell of electrons is added to the
atom. This increases the size of the atom at each new element down the
group. For example, the atomic size of group IA and group VIIA elements
follows the order:
