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How many valence electrons are in an atom of phosphorus?

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Hint:. The answer lies in the fact that the valence electron of an atom is nothing but the total number of electrons present in the outermost orbit of the shell of that particular atom and this fact gives the correct answer.

Complete step by step answer:
- We are familiar with the basic concepts of chemistry that deal with the topic about the general facts including the calculation of the number of protons, neutrons and electrons and also the valence electrons in an atom.
- We shall now focus on the calculation of valence electrons in general which helps us to deduce the answer for the given question that is for phosphorus atoms.
- Valence electrons are those electrons which are situated in the outermost orbit of the atom.
- These valence electrons are capable of forming a chemical bond only if the outer shell remains unclosed.
Now, based on these above facts, for phosphorus atom, to know the outermost electrons, let us write the electronic configuration of this atom first that is,
Atomic number of phosphorus is 15 with the symbol $P$ and the electronic configuration is $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{3}}$
- Here, in this the outer orbit is having the number 3 and therefore, the total valence electrons in the shell are two 3s electrons and three 3p electrons which gives a total of five valence electrons.
Therefore, the correct answer is phosphorus has 5 valence electrons in its outermost orbit of its shell.

Note: Note that for neutral atoms, the number of valence electrons equal to the number of the main group of that particular atom and to know this, you must be thorough with the periodic table.