Question

What is the oxidation number of $As$ in the chemical formula for arsenic acid, ${H_3}As{O_4}$?
A.$ + 1$
B.$ + 2$
C.$ + 3$
D.$ + 4$
E.$ + 5$

Answer 1

Hint: Valence shell: The shell which is at the last of the electronic configuration, is known as valence shell and the number of electrons present in the valence shell is known as valence electrons and they decide the valency of an atom. And the number of electrons shared by the atom in the molecule is known as its oxidation number.

Complete step by step solution:
First of all let us talk about orbits, orbitals.
Orbits: These are the spaces in the atom where electrons revolve. The orbits are as $1,2,3,...$
Orbitals: The space where electrons are likely to be found. The orbitals are as $s,p,d$ and $f$ orbitals.
For $s - $orbitals maximum number of electrons it can have is two.
For $p - $orbitals maximum number of electrons it can have is six
For $d - $ orbitals maximum number of electrons it can have is ten.
For $f - $ orbitals maximum number of electrons it can have is fourteen.
Valence shell: The shell which is at the last of the electronic configuration, is known as valence shell and the number of electrons present in the valence shell is known as valence electrons and they decide the valency of an atom.
Oxidation number: It is also known as oxidation state. It is defined as the number of electrons which are gained or loss of an atom or ion to form a molecule, is known as oxidation number.
Electro-positive elements: Those metals which can donate electrons to the other element during the formation of molecules i.e. the positive oxidation number, are known as electropositive elements.
Electro-negative elements : Those metals which can accept electrons from the other element during the formation of molecules i.e. the negative oxidation number, are known as electronegative elements.
Oxidation number of any element is calculated as the sum of the oxidation number of all atoms present in the molecule is equal to zero in case of neutral molecule or the given charge in case of ion. For example: the oxidation number of $As$ in the chemical formula for arsenic acid, ${H_3}As{O_4}$ is calculated as: we know that the oxidation number of hydrogen is one and that of oxygen is $ - 2$. And let the oxidation number of Arsenic as $x$ and the molecule i.e. arsenic acid is a neutral compound so the sum of oxidation numbers is zero. Hence
$
  3 \times 1 + x + 4 \times ( - 2) = 0 \\
  x = 5 \\
 $.
So, the oxidation number of $As$ in the chemical formula for arsenic acid, ${H_3}As{O_4}$ is $ + 5$.

So the correct answer is option E

Note: For a given orbital maximum number of electrons it can hold is determined as $2(2l + 1)$ where $l$ is azimuthal quantum number. For $s$ the value of $l$ is zero, for $p$ the value of $l$ is one and so on. So the maximum number of electrons in $s$ orbitals is two.