Nitrogen, Oxygen, small amounts of Argon and Carbon-di-oxide. Other gases such as Neon, Helium and Methane are present in traceable amounts

A chemical formula is a symbolic representation of one molecule of an element or a compound

Molecule is the smallest unit of compound.

Metals: Sodium, Bismuth, Silver, Iron, Copper.
Non-metals: Nitrogen, Carbon, Chlorine.
Metalloids: Silicon.

(i) Symbol for some elements as proposed by Dalton.

(ii) The following rules are followed while assigning symbol to an elements:
(a) Chemical symbols usually consist of one or two letters
(b) The symbols of most elements correspond to the first letter (which is capitalized) of their English name. For example, the symbol for oxygen is "O" and that for hydrogen is "H".

i) A compound is formed only when the constituent elements combine in a fixed proportion.
ii) Compound exhibits entirely different properties of their constituent elements.
iii) A compound cannot be broken down by physical methods.
iv) A compound can be separated into its constituent elements by chemical methods only.

When solid is heated, the particles gain energy and vibrate vigorously. The particles move slightly further apart from one another.
Hence, the volume increases due to the greater distance between the particles. But the size of the particles remains the same.

(i) The properties of a compound are different from those of its constituent elements.
(ii) A compound cannot be broken down by physical methods. This is because a compound is made up of different elements that are chemically combined. Sodium chloride cannot be separated by physical methods such as filtration

There are 118 known elements till date. 94 of these elements occur naturally while 24 elements have been created artificially in the laboratory

On earth the three most abundant atom are iron, oxygen, and silicon.