Valence Electrons and Valency

Predict the valency of phosphorous (atomic number $15$).

Write the electronic configuration of an element whose atomic number is 12.

Electrons hold the key to the chemical world. Our present-day understanding of $117$ elements and their compounds rests primarily on the number of electrons present in the atoms of these elements and the electronic structures of their atoms.
How is valence electron related to the stability and reactivity of an element?
 

Electrons hold the key to the chemical world. Our present-day understanding of $117$ elements and their compounds rests primarily on the number of electrons present in the atoms of these elements and the electronic structures of their atoms.
How is the valence electron related to the valency of an element?

Electrons hold the key to the chemical world. Our present-day understanding of $117$ elements and their compounds rests primarily on the number of electrons present in the atoms of these elements and the electronic structures of their atoms.
What is the valence electron?

If $\mathrm{Z}=3$, what would be the valency of the element? Also name the element.  

Sodium has completely filled $\mathrm{K}$ and $\mathrm{L}$ shells. Explain. 

What is the maximum number of electrons which can be accommodated in the $5^{\mathrm{th}}$ orbit? (Write in number(s))

What is the maximum number of electrons which can be accommodated in the $6$^th orbit? (Write in numerals)

What is the maximum number of electrons which can be accommodated in the N shell?

Predict the valency of argon (atomic number $18$). (Zero / One / Two)

Predict the valency of calcium (atomic number $20$)

Predict the valency of aluminium (atomic number. $13$) (Give the answer in numerals)

Which of the following is the electronic configuration of sulphur?

The electronic configuration of sulphur is $\left[\mathrm{Ne}\right] 3{\mathrm{s}}^2 3{\mathrm{p}}^{\mathrm{k}}$(In terms of quantum mechanical model of atom). The numerical value of k is _____.

Out of the elements ${}_{17}{}^{35}\mathrm{Cl}$ and, ${}_{18}{}^{40}\mathrm{Ar}$, which is chemically more reactive and why?

Write the electronic configuration of an element whose atomic number is$17$.

Define valency. 

What are valence electrons?

What are valence shell?