Chemical Substances - Their Nature and Behaviour - 5...

Sixth period is the longest period in the modern periodic table.

19 is the atomic number of potassium, which is an alkali metal. So, option 3 is the possible answer.

Eight elements are present in the 3^rd period of the modern periodic table.

Atomic numbers 11 and 19 belong to the elements of first group of the periodic table.

The element with electronic configuration 2, 8, 8, 1 will be most electropositive as after the loss of an electron, it will acquire stable configuration.

The number of electrons in the valence shell of an element indicates the group to which an element belongs.

Atomic radius is generally stated as being the total distance from an atom’s nucleus to the outermost orbital of electron. In simpler terms, it can be defined as something similar to the radius of a circle, where the center of the circle is the nucleus and the outer edge of the circle is the outermost orbital of electron. As you begin to move across or down the periodic table, trends emerge that help explain how atomic radii change. So, we see that Cs has the largest atomic size.

Electron affinity decreases down a group because due to increase in atomic size, the force of attraction between the nucleus and the electrons to be added becomes less.

The elements present in the 1^st group having one electron in the valence shell are known as alkali metals and are highly active in nature and exist in solid state.

Ionisation energy goes on decreasing down a group. It is due to increase in the distance between the valence electrons and the nucleus. As the atomic size increases down a group, the force of attraction between the nucleus and the valence electrons decreases. Therefore, the energy required to remove the electrons, i.e. ionisation energy, goes on decreasing.

Atomic radii decrease on moving from left to right along a period in the periodic table.

Metallic character increases down a group due to decrease in ionisation energy.

Fluorine is the most reactive element of group 17.
Fluorine is much more reactive because it has the highest electronegativity of all the elements (4.0). It wants an electron the most; so it will practically rip off electrons from other elements just to fill its shell.
This also leads to why fluorine is never seen in its elemental form naturally because of its extreme reactivity.

Be would show the maximum metallic character as it is present on the extreme left of the periodic table.

The horizontal rows in the periodic table are called periods.