⁕ Element
↪ Simple forms of matter which can't be decomposed into simpler substance by any chemical methods are called elements.
Eg: hydrogen, nitrogen, iron, etc.

⁕ Atom

↪ The smallest particle of an element which can take part in chemical reaction can't be broken into smaller unit is called an atom.

⁕ Compound

↪ A compound is a pure substance produced by the combination of two or more elements in definite proportion by weight. It is decomposed into simple substances by suitable chemical reactions.
Eg: Water, CO2, etc.

⁕ Symbol

↪ A symbol is short abbreviation for the full name of an element.
Full name of elements may be Latin, German, English, in honor scientists, planets, etc.

Symbols of a few elements with their Latin names are:

Symbols of a few familiar elements in honor of the Scientists are:

Symbols of some elements from names of planets:

↪ Qualitative Significance of Symbol
⇒ It represents the name of an element.

↪ Quantative Significance of Symbol
⇒ one atom of magnesium element.
⇒ one mole of magnesium.
⇒ one gram atom of magnesium element.
⇒ 24 parts by weight of magnesium.
⇒ 6.023×10²³ atoms of magnesium

Some More Terms

⁕ Nucleon

↪ A proton or neutron present inside the nucleus of an atom is called nucleon.

⁕ Atomic number (Z)

↪ It is the total number of protons present in an atom of an element.

⁕ Atomic mass number (A)

↪ It is the total number of proton and neutrons.
Atomic mass number (A) = Total number of p + Total number of n
                                              = Atomic number (Z) + number of n

⁕ Isotopes

↪ Different species of an element having same atomic number but with different atomic mass numbers.

⁕ Atomic Weight

↪ Atomic weight of an element is defined as how many times of an atom of an element is heavier than 1/12^th mass of one atom of C-12 isotopes.

Eg: Atomic weight of mg = 24 amu.

⁕ Molecular weight 

↪ Molecular weight of a substance is defined as how many times of a molecules of a substance is heavier than the 1/12th mass of one atom of C-12 isotope. Eg: molecular weight of CO2 = 44 amu.

⁕ Molecule

↪ The smallest particle of substance which is capable of independent existence and possess the properties of that substance during chemical reaction is called molecule.

⁕ Radical

↪ An atom or a group of atoms as an integral fragment of molecule having either positive, negative or no charge is called radical.

⁕ Valency

↪ It is combining capacity of an atom or group of atoms and it is measured as the number of H atoms that one radical can combine to form a molecule.

⁕ Variable Valency

↪ Some elements show two or more than two valency in their compounds. These elements are said to exhibit variable valency. Metal name ends in -ous for lower valency and -ic for higher valency.

⁕ Oxidation state of positive and negative radicals

↪ The positive and negative radicals in solution have their integral identity and called as cation and anion respectively. 
↪ Metals form cations and non-metals form anions. The valency of these radicles when expressed in their charge are termed as oxidation state.