Explanation

A neutron has mass ≈ 1.675 × 10⁻²⁷ kg, very close to a proton’s mass.

An electron is much lighter (≈ 1/1836 of a proton), so not comparable.

Explanation

An Angstrom (Å) measures very small distances, especially wavelengths of light and atomic-scale lengths.

It is equal to 10^-10 meters, or 0.1 nanometers. 

Therefore, among the given options, angstrom is used to measure the length of light waves.

It is also used to measure other microscopic dimensions, such as: 

• The size of atoms and molecules
• The lengths of chemical bonds
• The arrangement of atoms in crystals

Explanation

Hydrogen has one electron in its outer shell and needs one more to complete its duet.

Therefore, its valency is 1, meaning it can form one covalent bond.

Explanation

Explanation

The atomic weight of Mercury (Hg) is 200.59 unified atomic mass units (u), as listed on the periodic table.

Key Information:

• Symbol: Hg (from Latin Hydrargyrum)
• Atomic Number: 80
• Standard Atomic Weight: 200.592 u (often rounded to 200.59 u).

Explanation

Sodium hydroxide is commonly known as caustic soda.

It is a strong base used in soap making and other chemical industries.

Explanation

Primary pollutants are emitted directly into the air from sources like vehicles and industries.

Examples include carbon monoxide (CO), sulfur dioxide (SO₂), and nitrogen dioxide (NO₂).

Explanation

• Deuterium, also known as heavy hydrogen.
• It has 1 proton and 1 neutron in its atomic nucleus.

Explanation

Hydrogen is present in the atmosphere in trace amounts, making it the least common among the listed gases.

Nitrogen (~78%), Oxygen (~21%), and Argon (~0.93%) are much more abundant.

Explanation

Graphite crystallizes in a hexagonal crystal system with layers of carbon atoms arranged in hexagons.

Substances like diamond belong to the cubic system.