### Theory:

Rules to be followed to write the symbol of an element:

1
. If an element has a single English letter as a symbol, it should be written in the upper case.
Example:
H - Hydrogen
B - Boron
2. If an element has a two-letter symbol, the first letter should be written in the upper case and the second letter in the lower case.
Example:
Ca - Calcium
Ni - Nickle
Therefore, the symbol of an element signifies,
• Name of the element.
• One atom of the element.
Example:
The symbol "O" refers to the element of oxygen, i.e., one atom of oxygen.
Rules to be followed to write chemical formula:
1. The charges ($$+$$, $$-$$) or valency (free electrons) on the ion must be balanced.

Ions must be balanced.

2. While writing a compound of metals and non-metal, the symbol of metal should be written first (left side) and followed  by the non-metal compound (right side).
Example:
$$NaCl$$: Here, Sodium $$Na$$ is a metal so that should be written on the left side, and chlorine $$Cl$$ is a non-metal that should be written on the right side.
3. Ions should be enclosed in a bracket before writing the number to indicate the ratio.

So now, we will see how to write a chemical formula with some examples to understand it clearly in the next topic.