Chemistry

Structure of the Atom

Relationship Between Mass Number and Atomic Number

We will now derive a relationship between mass number and atomic number of an element. We have just seen that:
Mass number = No. of protons + No. of neutrons
Since the number of protons in an atom is equal to the atomic number of the element,we can rewrite the above relation by putting "Atomic number" in place of "No. ot protons.Thus.
Mass number = Atomic number + No.of neutrons
The atomic number and mass number can be indicated on the symbol of an element. The is written on the lower left side of the symbol whereas mass number is written on the upper left side symbol of the element. This is shown below :
For example, an atom of carbon whose atomic number is 6 and mass number 12 is represented as :

A = \begin{matrix} 12 \\ 6 \end{matrix}

C
The lower figure (6) indicates the atomic number and the upper figure (12) indicates the mass number of carbon. Let us solve some problems now.
Sample Problem 1. Calculate the atomic number of an element whose atomic nucleus has mass number 23 and neutron number 12. What is the symbol of the element ?
Solution. We know that :
Mass number = Atomic number + No. of neutrons
So 23=atomic number+12
And, Atomic number = 23-12
= 11
The element having atomic number 11 is sodium and its symbol is Na. If, however, we indicate the atomic number and nis number also, then the symbol becomes

A = \begin{matrix} 23 \\ 11 \end{matrix}

Na, where 11 is the atomic number and
Sample Problem 2. The number of electrons in an atom is 8 and the number of protons is also 8.
(a) What is the atomic number of the atom ?
(b) What is the charge on the atom ?
Solution, (a) Atomic number is equal to the number of protons in one atom. Since this atom contains 8 protons, so the atomic number is 8.
(b) This atom contains an equal number of positively charged protons and negatively charged el-, i rons(8 each), so it has no overall charge. That is, the charge on this atom is 0 (zero).
Sample Problem 3. Helium atom has an atomic mass of 4 u and two protons in its nucleus Ho. mam neutrons does it have ?
Solution.We know that atomic mass is numerically equal to mass number of an atom.Since the helium atom has an atomic mass of 4 u, therefore, the mass number of helium atom will be 4. And the number of protons in the helium nucleus has been given to be 2. Now,
Mass number = No. of protons + No.of neutrons
So, 4 = 2+ No. of neutrons
And, No. of neutrons =4-2
= 2
Thus, the helium atom has 2 neutrons.

Notes

Structure of the Atom - Notes

1. Drawback of Rutherford’s Model of the Atom