Periodic Classification and Elements

Oxygen, sulphur, selenium, tellurium and polonium constitute the group 16 of the periodic table of elements. The first four elements are collectively called CHALCOGENS. The name derives from the Greek word for bronze and points to the association of sulphur and its congeners with copper. All these are non-metallic in character. The metallic character increases down the group. The last element of the family, polonium has metallic character and is a radioactive element with a very short life period.

An element is a pure substance consisting entirely of atoms that have the same number of protons in their atomic nuclei. Molecules and compounds can consist of different types of atoms.

The atomic number of an element is equal to the number of protons in its nucleus. In a neutral atom, the number of protons equals the number of electrons, so nitrogen with 7 electrons has an atomic number of 7.

Sodium chloride (NaCl) is a compound formed by the chemical bonding of sodium and chlorine. Carbon, oxygen, and potassium are all individual elements.

Atomicity is defined as the number of atoms present in one molecule of an element. For example, oxygen (O2) has an atomicity of 2.

Helium is a noble gas, which means its outer electron shell is full, making it stable enough to exist as a single atom. Hydrogen, nitrogen, and oxygen typically exist as diatomic molecules.

Sulphur (S8) has an atomicity of 8, which is higher than phosphorus (P4, atomicity 4), hydrogen (H2, atomicity 2), or ozone (O3, atomicity 3).

Chlorine exists as a diatomic molecule (Cl2) in its elemental state. Phosphorus (P4), Sulphur (S8), and Helium (monatomic) do not exist as diatomic molecules.

ANS 3.

A complex ion contains a metal atom or ion known as the central metal atom or ion. It is sometimes also called a nuclear atom.

The s-block of the periodic table of elements consists of the first two groups: the alkali metals and alkaline earth metals, plus hydrogen and helium.These elements are distinguished by the property that in the atomic ground state, the highest energy electron is in a s-orbital. Except in hydrogen and helium, these electrons are very easily lost to form positive ions.

The two elements in the first period are H & He.

The element with electronic configuration 2, 8, 7 is chlorine. It is placed in group 17, i.e. the halogen group.

The modern periodic table has 7 horizontal rows known as periods and 18 vertical columns known as groups.

Scandium(Sc) is a transition element.