Modern Periodic Law And The Present Form Of The Periodic Table

Dmitri Mendeleev arranged elements in his periodic table in the order of increasing atomic weights. He also grouped elements with similar chemical properties together, which led to some elements being placed out of strict atomic weight order to preserve the pattern. Later, Modern Periodic Law replaced atomic weight with atomic number, but Mendeleev's table was originally based on atomic weight.

The correct answer is option (A): atomic weight

Newlands arranged the known elements in increasing order of their atomic masses and proposed the Law of Octaves. According to him, every eighth element showed properties similar to the first. At the time of his classification, only 56 elements were known, and he included all of them in his periodic table.

The correct option is (C):

Correct answer: 3

Explanation:
Elements in the VA group (Group 15) of the modern periodic table have 5 valence electrons in their outermost shell. To achieve a stable octet configuration, they typically gain 3 electrons.

Hence, their common valency is:
Valency = 8 - number of valence electrons = 8 - 5 = 3
So, the valency of Group VA elements like nitrogen, phosphorus, arsenic, etc., is usually 3.

In the periodic table, the VIA group elements are commonly referred to by two names. These elements include oxygen, sulfur, selenium, tellurium, and polonium. They are known as:

• Chalcogens: This term is derived from Greek, where "chalcos" means bronze or ore, and "genes" means born of or produced. This name is used because these elements are commonly found in ore compounds.
• Oxygen Family: This name is used because the family starts with the element oxygen, which is one of the most abundant and significant elements on Earth.

Therefore, the group VIA elements are called both chalcogens and the oxygen family.

Valency is defined as the ability of an atom to combine with other atoms when forming chemical compounds or molecules. It is often determined by the number of electrons in an atom's outermost shell, which is known as the valence shell. When you move down a group in the periodic table, the elements have the same number of electrons in their outermost shell.

Therefore, the valency of elements in a group remains constant as you move from top to bottom.
For example, in Group 1 (Alkali metals), all elements have 1 electron in their outermost shell, leading to a consistent valency of 1.

Hence, the correct answer is: No change