Periodic Classification and Elements

An element with atomic number 13 has electronic configuration as 1s², 2s², 2p⁶, 3s², 3p¹. So it has 3 electrons in the valence shell.

The number of electrons in the outer most shell of elements of the same group is always similar.

The horizontal rows of elements, in the periodic table are called periods.

Ac, Th, Pa are elements belonging to actinides series with atomic numbers 89, 90 and 91, respectively.

Fluorine, chlorine and iodine are halogens and belong to group 17 of the periodic table.

In this electronic configuration, 3^rd energy shell is the highest energy level. So, this element belongs to the 3^rd period of the periodic table.

₂₆Mn²⁺has the electronic configuration of 3d⁵4s⁰. 3d⁷4s⁰ is the electronic configuration of ₂₇Co²⁺.

Correct answer is (1). 

Uranium is the heaviest naturally occurring element found in significant quantities on Earth, often cited as the most complex natural element.

An element is defined by the number of protons in the nucleus of its atoms, but its chemical reactivity is determined by the number of electrons in its outer shell property fundamental to the organization of the periodic table of the elements. So, atomic number is good to defined the different properties of the elements.

Both gold and mercury have completely filled 5d orbital. Gold (₇₉Au): [Xe] 5d¹⁰ 6s¹ Mercury (₈₀Hg): [Xe] 5d¹⁰ 6s²

Both niobium and molybdenum have half filled 5s orbitals. Niobium (₄₁Nb): [Kr] 4d⁴ 5s¹ Molybdenum (₄₂Mo): [Kr] 4d⁵ 5s¹

Scandium (Z = 21) and zinc (Z = 30) do not exhibit variable oxidation states. Scandium shows only +3 oxidation state in its compounds. Zinc shows only +2 oxidation state in its compounds.

The modern periodic table is organized into 7 horizontal rows, which are called periods.

Sodium (Na) comes from the Latin 'natrium', which does not match its English name. While Tungsten (W) and Potassium (K) also have symbols not matching their English names, Sodium is the lightest among the options provided (Atomic number 11).