Group 16 Elements – Trends in the Properties|Chemistry notes for class 12
Chemistry notes for class 12. In these notes, we will learn Trends in Atomic and Physical Properties of Group 16 Elements from unit 7 the p-block of NCERT chemistry class 12.
TRENDS IN THE PROPERTIES OF GROUP 16 ELEMENTS
Group 16 elements are known as oxygen family or chalcogens. They include elements oxygen (O), sulphur (S), selenium (Se), tellurium (Te), and the radioactive element polonium (Po).
TRENDS IN ATOMIC AND PHYSICAL PROPERTIES
Atomic and Ionic Radii: The atomic and ionic radius of the group 16 elements increases as one move from oxygen to polonium (top to bottom).As the number of shells increases on going down the group, atomic and ionic radii increase from top to bottom in the group. However, the size of oxygen atoms is exceptionally small.
Ionization Enthalpy: Ionisation enthalpy of the elements decrease down the group, as there is an increase in the size of the atoms down the group. However, the element of group 16 has lower ionisation enthalpy values as compared to those of group 15 in the corresponding periods. This is due to the fact that group 15 element have extra stable half-filled p orbitals electronic configurations.
Electron Gain Enthalpy: The energy which is released by an atom in gaining an electron from another atom or ion to form negative ion (or anion) is called electron gain enthalpy. The electron gain enthalpy is negative energy is released by the atom while taking in the electron. The electron gain enthalpy decreases with an increase in the size of the atom, on moving down the group. This is the increase in atomic size and nuclear charge. The effect of atomic size is more prominent than that of nuclear charge, therefore, the force of attraction between the nucleus and the added electron decreases and thus enthalpy becomes less negative.
Electronegativity: It is the property, that defines the tendency of an atom to attract a shared pair of electrons towards itself. The electronegativity decreases as we move down the group (oxygen has the highest electronegativity, polonium has least) due to the increase in nuclear size.
Melting and Boiling Points: The atomic size increases from oxygen to tellurium, therefore the melting and boiling points also increase. The greater difference between the melting and boiling points of oxygen and sulphur may be explained on the basis of their atomicity.
Oxygen exists as diatomic molecules (O2), whereas sulphur exists as a polyatomic molecule (S8).
Metallic Character: Oxygen and sulphur are non-metal, Selenium and tellurium metalloids, while Polonium is a metal. All these element exhibit allotropies.
Oxidation States: An oxidation state is a number that is given to an element when it is bonded. This number represents the number of electrons that an atom can gain, lose, or share when it is chemically bonded to an atom of another element. The terms “oxidation state” and “oxidation number” are used interchangeably. The group 16 elements have a configuration of ns2 np4 in their outer shell, they may accomplish noble gas configuration either by the gain of two electrons, forming ions as M-2 or by sharing two electrons, in this manner, it forms two covalent bonds.
Thus, these elements show both negative and positive oxidation states. The regular oxidation states showed by the elements of group 16 are -2, +2, +4, and + 6.
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CHEMICAL PROPERTIES
The group sixteen elements react with hydrogen to form hydrides of the sort H2E, where E could be any element- oxygen, sulphur, selenium, tellurium or polonium.
The Physical Properties of Hydrides of Group 16 Elements
Water (hydride of oxygen) is an odourless and colourless liquid, but the hydrides of the various elements of this group are poisonous gases which are colourless with disagreeable smells.
The boiling point of these hydrides amazingly diminishes from water to hydrogen sulphide, and after that increases. Water has an extraordinarily high boiling point since its particles (hydrogen and oxygen) are bonded with each other by the hydrogen bonds in both its liquid as well as solid states.
Acidic Character of Hydrides
The bond between hydrogen and group 16 atoms are covalent, so the hydrides of group 16 elements are covalent molecules. There is an increase in the acidic nature of hydrides from H2O to H2Te. This is because of the electron density on the central atom decreases and hence its tendency to donate a pair of electrons decreases, thus acidic strength increases. Except for water, the different hydrides act as reducing agents. The reducing property of group 16 hydrides increases from H2S to H2Te, as there is in a decrease in ionization energy decreases from oxygen to polonium.
Every element of group 16 reacts with oxygen to form mono oxides (like SO) except Se, dioxides (like SO2) and trioxides. The dioxides and trioxides are acidic in nature.
Elements of group 16 forms different types of halides of the kind EX6, EX2, and EX4, where E is the element of group 16 elements and X is a halogen. The compounds of oxygen and fluorine are called oxyfluorides (example OF2) because fluorine is more electronegative than oxygen.
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Abnormal Molecular Mass and Vant’s Hoff Factor
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Important Tags – Group 16 Elements – Trends & Properties, Chemistry notes for class 12, The p-Block Elements, Atomic and Physical Properties, Acidic Character of Hydrides, group 16
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