Atomic number is the basis for modern periodic table. Atomic number is the number of protons in the nucleus, it is also equal to the number of electrons in the atom.
Electronic configuration is the arrangement of electrons in an atom around its nucleus. The important features of Bohr’s atomic theory are:
- Electrons occur in definite electronic shells.
- The shells in an atom can be named alphabetically as K,L,M,N and so on.
The main features of modern periodic table are:
- Elements are arranged in the increasing order of atomic numbers.
- There are 7 horizontal rows called periods and 18 vertical columns called groups.
Elements are placed in periods based on the number of shells in their atoms.
The shortest period is the first period which contains only two elements – hydrogen and helium.
The second period contains elements from lithium to neon.
The third period contains elements from sodium to argon.
The fourth period contains elements from potassium to krypton.
The fifth period contains elements from rubidium to xenon.
The sixth period contains elements from cesium to radon and seventh period is incomplete.
Classification of elements into groups is based on the number of valence electrons in an atom.
The first group has hydrogen, lithium, sodium, potassium, rubidium, cesium and francium. Their general electronic configuration is ns1 and are known as alkali metals.
Beryllium, magnesium, calcium, strontium, barium and radium are second group elements whose general electronic configuration is ns2 and are known as alkaline earth metals.
Boron, aluminium, gallium, indium and thallium are the elements of third group whose electronic configuration is ns2np1. They form covalent compounds.
Carbon, silicon, germanium and tin are the fourth group elements whose general electronic configuration is ns2np2. Nitrogen, phosphorous, arsenic, antimony and bismuth are the elements of fifth group whose general electronic configuration is ns2np3. Oxygen, sulphur, selenium, tellurium and polonium are the sixth group elements whose general electronic configuration is ns2np4. Fluorine, chlorine, bromine, iodine and astatine are the seventh group elements whose electronic configuration is ns2np5. Helium, neon, argon, krypton, radon and xenon are elements of eighth group or zero group. Their electronic configuration is ns2np6 . they are also known as inert gas elements