An isotope of which element could have 19 protons and 21 neutrons? calcium zirconium scandium potassium

You will need Periodic Table for this problem.

As we know, the number of protons specifies Atomic number. So, the atomic number for this element is 19. 
Now, element having 19 protons must have 19 electrons, So, writing electronic configuration for 19 electrons,

                                 1s², 2s², 2p⁶, 3s², 3p⁶, 4s¹

So, valence shell is 4, means element is present in 4rt period. And number of electrons in 4rth shell are 1. Means element belong to group 1.

Take periodic table go to period 4 and group 1. Its Potassium (K).

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