The cause of periodicity is the recurrence of similar outer most electronic configurations.

Electron affinity of elements increases on moving from left to right in the periodic table. Chlorine has highest electron affinity.

Electronegativity decreases on moving down the group. This is due to increase in the size of the atom on moving down the group.

Sixth period of the periodic table contains maximum number of elements.

A halogen atom has 7 valence electrons, i.e. its electronic structure is ns²np⁵. Hence, the given element will belong to the group of halogens. 

The elements arranged in two rows at the bottom of the periodic table are f-block elements.

The element whose configuration is 1s², 2s², 2p⁶, 3s² is a metal as it can lose 2 electrons to attain stable noble gas configuration.

Electronegativity increases on moving from left to right in the periodic table in a period.

All the elements of the same groups will have same number of valence electrons.

Alkali metals have greatest tendency to lose electrons. So potassium will have greatest tendency to lose electrons.

Oxygen needs two electrons to complete its octet. So, electron affinity is positive when O^2- is formed from O.

Atomic size decreases on moving from lest to right along a row in the periodic table and it increases on moving from top to bottom  in a period.So oxygen has the smallest size of the given options.

Atomic numbers 11 and 12 represents s block elements.

Fluorine is the most electronegative elements in the periodic table.

Element with atomic number 15 is phosphorus which is non metal.

For isoelectronic ions, size decreases with increase in the atomic charge on the ion.

Lithium is the lightest metal in the periodic table.

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

Copper is a transition element and transition elements show variable valencies.

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

In a period, as we proceed from left to right, the metallic character of elements decreases. This is due to decrease in size and increase in the s- character.

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

Nitrogen has stable half filled p subshell. So it has the maximum value of first ionization energy.

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

La, Ce, Pr are the elements belonging to the Lanthanide series.

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

Both sodium and potassium belongs to the alkali metals and hence they have similar properties.

Sulfur has the highest negative value of electron gain enthalpy.

Cesium is an alkali metal which becomes liquid at 15⁰ C

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.