Structure of atom

Structure of atom

Bohr-Bury scheme for distribution of electrons in different orbits

Maximum number of electrons that can be accommodated in a shell is given by 2n², where n is the shell number i.e. first shell can accommodate 2 electrons, second shell can accommodate 8 electrons, third shell can accommodate 18 electrons and so on.

Outermost orbit of an atom  can accommodate a maximum number of 8 electrons 3. Electrons are not accommodated in a given shell, unless the inner shells are filled.

i.e. the shells are filled in a step-wise manner

Combining capacity of an atom is called its valency

 Outermost shell of an atom is called valence shell

Electrons present in valence shell is called valence electrons.

 If the outermost shell of an atom is completely, its valency is 0

 Valency of elements having 1-4 electrons in outermost shell are generally determined by the rule: Valency = Number of electrons in outermost shell

Valency of elements having number of electrons in outermost shell close to 8 is determined by the

formula:

Valency=8- Number of electrons in outermost shell

Significance of valence electrons:

 Valence electrons are responsible for chemical changes

 Elements having same number of valence electrons in their atoms possess similar chemical properties

because chemical properties of an element are determined by the number of valence electrons in an atom

Elements having different number of valence electrons in their atoms possess different chemical properties

Atomic number(Z) is defined as the total number of  protons present in nucleus of an atom

Protons and neutrons together are called nucleons

Mass number (A) is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom.

Isotopes:

•       Isotopes are the atoms of same element having same atomic number but different mass number

•       Isotopes have similar chemical properties because they have same number of valence electrons

•       Isotopes have different physical properties like boiling point, melting point etc because they have different mass number

Fractional atomic mass of elements are due fact that all elements have isotope Applications of isotopes: Uranium isotope (₉₂U²³⁵)  is used in nuclear reaction

 Cobalt isotope (⁶⁰U)  is used to remove brain tumours and in their treatment

 Isotope of sodium (₁₁²⁴Na)has been used to diagnose restricted circulation of blood

Isobar

 Isobars are the atoms of different element with different atomic number but same mass number 

First Part