Question

Which element has the lowest atomic number?

$$A.\;\;\;\;{\;_{19}}K \\\ B.\;\;\;\;{\;_{24}}Cr \\\ C.\;\;\;\;{\;_{11}}Na \\\ D.\;\;\;\;{\;_{29}}Cu \\\$$

Answer 1

Atomic number represents the number of protons or electrons present in any given atom. This number is unique to every element and is the distinguishing property of all elements.

Complete Step-by-Step Solution:
Atomic number of any element can be determined by the electronic configuration of the elements. By adding the number of electrons present in each orbital of a particular element, we would get the total number of electrons present in the atom.
Now, in an element, the number of protons present is equivalent to the number of electrons present. Hence, the number of electrons present also represent the atomic number of the element. Hence, the electronic configurations and the total number of electrons present in the given elements are:
1. Potassium or K:
The electronic configuration of potassium can be given as: $1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^1}$
Hence the total number of electrons in K = 2 + 2 + 6 + 2 + 6 + 1 = 19
Hence the atomic number of K is 19
2. Potassium or Cr:
The electronic configuration of potassium can be given as: $1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^2}3{d^4}$
Hence the total number of electrons in Cr = 2 + 2 + 6 + 2 + 6 + 2 + 4 = 24
Hence the atomic number of Cr is 24
3. Potassium or Na:
The electronic configuration of potassium can be given as: $1{s^2}2{s^2}2{p^6}3{s^1}$
Hence the total number of electrons in Na = 2 + 2 + 6 + 1 = 11
Hence the atomic number of Na is 11
4. Potassium or Cu:
The electronic configuration of potassium can be given as: $1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^1}3{d^9}$
Hence the total number of electrons in Cu = 2 + 2 + 6 + 2 + 6 + 1 = 19
Hence the atomic number of Cu is 19
Hence, the element has the lowest atomic number is Na or Sodium

Hence, Option C is the correct option.

Note: The three main atomic particles are protons, neutrons and electrons. The atomic number of an atom identifies the number of protons in the atom. While the number of neutrons can be determined by subtracting the atomic number from the atomic mass number.