Questions

Electronegativity is the tendency for an atom to attract electrons. There are general trends on the periodic table which you can use to determine the relative electronegativity for different elements, but otherwise you can also look at their valence shell and effective nuclear charge to determine their relative electronegativity.

Here from the given options, Lithium has least tendency to attract electrons and thus has a lower electronegativity.

RADAR - RAdio Detection And Ranging.

A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna (often the same antenna is used for transmitting and receiving) and a receiver and processor to determine properties of the objects. 

Radar is an object-detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain.

Radio waves from the transmitter reflect off the object and return to the receiver, giving information about the object's location and speed.

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In chemistry, Weak bonds are those forces of attraction that, do not take a large amount of energy to break.

For example, hydrogen bonds are broken by energies in the order of 4 - 5 kcal/mol.

Ionic bonds, hydrogen bonds and van der Waals interactions are considered weak bonds.

Weak bonds are transient and easily reversible.