Question

An atom of an element is represented as ${}_{11}{}^{23}\mathrm{X}$

1. What does the numeral 23 indicate?
2. What does the numeral 11 indicate?
3. What is the number of protons in X?
4. What is the number of electrons in X?
5. How are the electrons distributed around the nucleus of X?
6. What is the number of neutrons in X?

Answer 1

Mass number and Atomic number of an element is denoted as ${}_{\mathrm{Atomic}\mathrm{number}}{}^{\mathrm{Mass}\mathrm{number}}\mathrm{Symbol}$

(i) Numeral 23 indicates the mass number of the element X as the superscript denotes the mass number.

(ii) Numeral 11 indicates the atomic number of the element X as the subscript denotes the atomic number.

(iii) Number of protons in X

• As Atomic number= Number of protons
• Therefore, Number of protons in X= 11

(iv) Number of electrons in X

• In an electrically neutral species like an atom, Number of electrons= Number of protons
• Therefore, Number of electrons in X= 11

(v) Electrons distribution: Electron distribution or electronic configuration is the arrangement of electrons in the shells around the nucleus.

The 11 electrons are distributed as 2, 8, 1 in K, L and M shell respectively.

• The maximum number of electrons that can be accommodated in a shell is given by 2n².
• Therefore for first or K shell n=1 and maximum number of electrons that can be filled in it is 2. Hence 2 electrons out of 11 go in K shell.
• For second or L shell, n= 2 and maximum number of electrons that can be filled in it is 8. Hence 8 electrons out of 11 go in L shell.
• For third or M shell, n= 3 and maximum number of electrons that can be filled in it is 18 but as only 1 electron is left, So 1 electron is filled in M shell.

(vi) Number of neutrons in X:

• Mass number= Number of protons+ Number of neutrons
• Therefore, Number of neutrons= Mass number- Number of protons
• Number of neutrons= 23- 11= 12.