Question

Valency of elements___________ in a period from left to right.
(A) increases
(B) decreases
(C) remains constant
(D) increases in the beginning and then decreases

Answer 1

The periodic table is made in such a form that it can show the variations of several properties of the atoms both along the period and down the group. This is because it is arranged based on the atomic number and valency also depends on the atomic number itself.

Complete step by step solution:
-There are 7 periods and 18 groups in periodic tables where atoms are arranged in the increasing order of their atomic number because the atomic number gives us the number of electrons of the atom. Electrons only decide the chemical properties of an atom as they are loosely held by the atom.
-Along the period, the atomic number increases and so does the number of electrons without altering the number of shells of atoms. So the number of electrons keep on adding as we go from left to right across a period.
-To comment about the trend in valency, we need to know what exactly valency means. Valency is the number of the electrons present in the outermost shell of an atom which is also called the valence shell of an atom. Those electrons are important to be studied as they are the most loosely held electrons of the atoms.
-Every atom tries to reach the electronic configuration of the noble gases to become stable like them. The configuration of noble gases is of the form $n{{s}^{2}}n{{p}^{6}}$ .This configuration is reached either by sharing of electrons or losing of electrons.
-So, valency is the number of outermost electrons of the atom. It states the combining capacity of the atom. It can be found if we know the atomic number of an atom as it tells us the total number of electrons present in the atom.
Atomic no. = No. of proton = No. of electron
-The number of electrons to be filled in each shell depends on Aufbau’s principle and Hund’s rule. Valency tells us only the total number of electrons present in the outermost shell of the atom and so it is different from the atomic number.
-Along with the group, the atomic number increases by 1 and so the last electron always enters the outermost shell of the atom. Thus we can say that the outermost electrons called the valence electrons actually increases from left side to right side across a period.
-Valency does not always represent the number of outermost electrons. This remains valid only when the number of outermost electrons lies in the range of 1 to 4. If the outermost electrons are more than 4, then the valency is found by subtracting those electrons from 8.
Eg, if the outermost electrons are 7, then the valency will be 8-7=1, not 7, if the outermost electrons are 6, then the valency will be 8-6=2, not 6

Therefore the correct answer is (D). increases in the beginning and then decreases

Note: Down the group, the number of shells increases but the valence electrons remain the same. Atomic size increases down the group. This increases the distance between the nucleus of an atom to the valence electrons and leads to changes in many properties like electronegativity, electron affinity, ionization potential.