Platinum metals

The group of transition metals ruthenium (Ru) osmium (Os), rhodium (Rh), iridium (Ir), palladium (Pd), and platinum (Pt).

Platinum-iridium

An alloy of platinum containing upto 30% iridium. It is used in jewellery, electrical contacts, and hypodermic needles.

Platinum (II) chloride

(PtCl₂), a grey brown powder prepared by the partial decomposition of platinum (IV) chloride. It may also be prepared by passing chlorine over heated platinum.

Polar bond

A covalent bond in which the bonding electrons are not shared equally between the two atoms. A bond between two atoms of different Electronegativity is said to be polarized in the direction of the more electronegative atom, i.e., the electrons are drawn preferentially towards the atom. This leads to a small separation of charge and the development of a bond dipole moment as in, for example, hydrogen fluoride, represented as H6+--F6-(F is more electronegative).

Polar covalent bond

A covalent bond in which more than half of the bond’s negative charge is concentrated around one of the two atoms.

Polarisation

1. the restriction of the vibrations in a transverse wave so that the vibration occurs in a single plane. Electromagnetic radiation, for instance, is a transverse wave motion. It can be though of as an oscillating magnetic field both at right angles to the direction of propagation and at tight angles to each other. Usually, the electric vector is considered since it is the electric field that interacts with charged particles of matter and causes the effect. In ‘normal’ unpolarized radiation, the electric field oscillates in all possible directions perpendicular to the wave direction. On reflection or on transmission through certain substances (e.g., Polaroid), the field in confined to a single plane. The radiation is then said to be plane polarized.