ISOTOPE EXPLAINED: Same Element, Different Identity

 

ISOTOPE (ISOTOPY)

Did you know that atoms of the same element can have different masses and still remain the same element? Welcome to the fascinating world of isotopes.  

An isotope refers to atoms of the same elements that have the same atomic number (same number of protons) but different mass numbers because they contain different numbers of neutrons.

            Since isotopes belong to the same element, they have nearly identical chemical properties but may differ in some physical properties.

“Isotope can also be defined as “atoms of the same element that possess the same number of protons but different numbers of neutrons, resulting in different mass numbers”.

ATOMIC NUMBER (Z)

The number of protons in the nucleus of an atom

EXAMPLE

H = 1               Be = 4

He = 2            B = 5

Li = 3              C = 6 …

 

MASS NUMBER (A)

Total number of: Protons + Neutrons

[Mass Number (A) = atomic number (Z) + number of neutrons (N)]

 

                                                A = Z + N

Where

A = Mass number

Z = Atomic number

N = Number of neutrons

 

NEUTRON NUMBER

Number of neutrons in the nucleus

Number of Neutrons = Mass Number – Atomic Number

                                                N = A – Z

 

REPRESENTATION OF ISOTOPES

Isotopes are written as:                           

                           

                                                                       

Where

A = Mass number

Z = Atomic number

X = Element symbol

 

EXAMPLE:  

                        ¹²C          ¹³C          ¹⁴C

These are isotopes of carbon

 

A well-known example of isotope occurs in chlorine.

Analysis of the chlorine isotopes  

                                                                   

Isotope  35Cl	Isotope  37Cl
Mass number, A                           35 Atomic number, Z                        17 Number of protons                      17 Number of electrons                   17 Number of neutrons (A-Z)          35 – 17                                                          = 18 Abundance in nature (%)            75	37 17 17 17 37 – 17 =20 25

 

RELATIVE ATOMIC MASS AND ISOTOPES

The atomic mass shown on the periodic table is usually a weighted average of naturally occurring isotopes.

Formula: Relative atomic mass = (M₁ X a₁) + (M₂ + a₂)

                                                                      100

Where:

M = Isotope mass

A = Percentage abundance

 

For example

The relative atomic mass of chlorine is 35.5 because a given quantity of chlorine contains:

75% of + ³⁵CL and 25% of ³⁷Cl

Relative Atomic mass = (M1 X a1) + (M2 X a2)                                      

                                                       100

= (35 X 75) + (37 X 25) = 35.5

                  100

Relative atomic mass = 35.5

The relative atomic mass can also be calculated using the ratio instead of the percentage abundance.

For example:

            Two isotopes of chlorine with mass number 35 and 37 exist in the ratio 3:1 respectively. What is the relative atomic mass of chlorine?.

 

Solution

Add ratio (3:1) of the occurrence together

3 + 1 = 4

(3 X 35) + (1 X 37)

105         +      37

=142

>> Relative atomic mass = 142/4

                                             = 35.5

 

Alternatively

 

Ratios of abundance (i.e. 75:25) %

= 3:1 = 4

 

Element ³⁵Cl = ¾ = 0.75

Element ³⁷Cl = ¼ = 0.25

 

Relative atomic mass:

= (35 X 0.75) + (37 X 0.25)

            26.25 + 9.25

            = 35.5.

 

WHY ISOTOPE HAVE SIMILAR CHEMICAL PROPERTIES

 

            Chemical properties depend mainly on the electronic configuration. Since isotopes have the same atomic number, they have the same number of electrons.

  Example

Carbon 12 – 2:4

Carbon 14 – 2:4

Therefore, they react similarly.

 

CHARACTERISTICS OF ISOTOPES

1.     Same atomic number

2.     Same number of electrons (neutral atoms)

3.     Same chemical properties

4.     Different mass numbers

5.     Different numbers of neutrons

6.     May have different physical properties.

 

 

DIFFERENTS BETWEEN ISOTOPES

property	Isotopes
Atomic number	Same
Mass number	Different
Number of protons	Same
Number of neutrons	Different
Chemical properties	Similar
Physical properties	May differ