Give reason:  First period in the modern periodic table contains only two elements.

D) both B and C

C) atomic numbers

Correct option is  B) 6th

Henry Moseley in 1913, studied X-ray spectra of large number of elements. He observed that the frequency of X-ray emitted from an element is related to atomic number (Z) of an...

Hydrogen (H) : 1s2, Helium (He) : 2s2 Since helium has a complete duplet i.e., two electrons in its valence shell, it has the stable complete electronic configuration.

i. Filling of electrons in the second period: In the second period, electrons are filled in the second shell i.e., n = 2. This shell can accommodate a maximum of eight electrons...

The fourth period corresponds to the filling of fourth shell, n = 4. Therefore, it begins with filling of 4s subshell. The first two elements of the fourth period are K...

The fifth period accommodates 18 elements as a result of successive filling of electrons in the 5s, 4d and 5p subshells.

i. First period  ii. f-Block

The Modern periodic table removed various anomalies of Mendeleev’s periodic table as follows:  i. All isotopes of the same element have different atomic masses but same atomic number. Therefore, they occupy...