Question

The period number in the long form of the periodic table is equal to
A: Magnetic quantum number of any element of the period.
B: Atomic number of any element of the period.
C: Maximum principal quantum number of any element of the period.
D: Maximum azimuthal quantum number of any element of the period.

Answer 1

Hint : Each row of periodic table is called period. All elements which are present in the same period have the same number of atomic orbitals. Same atomic orbitals mean they have the same principal quantum number.

Complete step by step solution :
Atomic number of an element is the number of protons present in the nucleus of an atom. Magnetic quantum number distinguishes the orbital availability within a subshell. It is denoted by ${m_l}$ . It’s value ranges from $- l$ to $+ l$ . $l$ is orbital angular quantum number that determines shape of an orbital and it’s angular distribution. Number of angular nodes equal to the value of this quantum number can be calculated as $l = n - 1$ .
$n$ is the principal quantum number which designates the principal electron shell. Longer the number $n$ farther the electron is from the nucleus.
As we know, period represents the number of atomic orbital and principal quantum number represents number of shells which is equal to number of atomic orbitals. So the answer to this question is option C that is the maximum principal quantum number of any element of the period.
Additional information: Columns of periodic table represent groups there are a total $18$ groups and $7$ period in a periodic table. All elements having the same group have similar electronic configuration. By electronic configuration we mean there is the same number of outermost electrons in each element. They only differ by principal quantum number if they have the same electronic configuration. This means their physical and chemical properties will also be similar. Group $2$ and group $3$ elements of periodic table show diagonal relationship. By diagonal relationship we mean properties of lithium resemble magnesium, beryllium with aluminium, boron with silicon and properties of carbon resembling with properties of phosphorus.

Note : ${m_l}$ represents the magnetic quantum number, l represents orbital angular quantum number, n represents principal quantum number and ${m_s}$ represents spin quantum number.
These are the four quantum numbers. With the help of these four quantum numbers we can easily find out information about electrons. No two electrons can have all the four quantum numbers same.