Periodic Classification and Elements

The total number of electrons present in the outermost shell of an atom represents the group number of the element.

There is increase in atomic no. as well as nuclear charge of the atoms of the elements when we go from left to right in a period. But, no new shell is added, which means more number of protons attracts the electrons present at the same distance from them. So there is a decrease in radius or atomic size of the atoms.

Number of period is equal to the number of shells occupied by the electrons of an atom in its ground state. So, the elements having same no. of electronic shells when their electronic configuration is represented in their respective ground states are placed in the same period.

Periods 2 and 3 are called short periods, according to the no. of elements contained in them (8).

As we move from left to right, decrease in atomic size and increase in nuclear charge of an atom occurs. So, the electrons present in the same number of shells (same period) are pulled more effectively towards the nucleus, and hence increase in electro negativity.

Fluorine (atomic no. 9) has only two shells in its atomic state with 7 electrons in its outermost shell. So, it it tends to gain one electron to become stable. The 7 protons present in the nucleus and small size of the atom make it possibly the most electronegative element of the periodic table.

Li occurs at the top of the alkali metals group. Number of shells increases down a group. So, Li is smallest with least no. of shells, with Na and K following the order.

On moving from left to right, the atomic size decreases due to increase in effective nuclear charge. From amongst the given elements, these occur in period 2 of periodic table in the order- beryllium, boron, nitrogen and fluorine (atomic numbers 4, 5, 7 and 9 respectively). Thus, atomic size decreases from beryllium to fluorine. Hence, beryllium has largest atomic size amongst the given elements.

The atoms with atomic numbers 8(2, 6) and 16 (2, 8, 6) have six electrons in their respective outermost shells. So, they belong to the same group, i.e., 6^th group.

Non-metallic character refers to the tendency of an atom to gain one or more electrons. On the left side of the periodic table, the elements having 1, 2 or 3 electrons in their outermost shell tend to lose them, thus exhibiting metallic character. On the right of it, the elements have 5, 6, or 7 electrons in their outermost shell, having a tendency to gain one or more electrons to be most stable, which is being non-metal. So, non-metallic character increases from left to right in a period.

Element with atomic number 13 has electronic configuration 2, 8, 3. It tends to lose the 3 electrons present in its outermost shell. The sulphate ion carries two negative charges over it. So, three ions of E must combine with two ions of sulphate ion to make a neutral compound. So, the compound must have a formula E₂(SO₄)_3.

Replace ‘as per’ by ‘by’. The element is characterised ‘by’.

The human body is composed primarily of oxygen (approx 65%), followed by carbon (approx 18%), and nitrogen (approx 3%). Therefore, the descending order is oxygen, carbon, nitrogen, which corresponds to 1, 3, 2.

Tritium is a radioactive isotope of hydrogen, containing one proton and two neutrons in its nucleus.

Hydrogen is the first element in the periodic table and has the lowest atomic mass, making its atom the lightest.