Give an atomic number, atomic symbol, atomic mass of the first 20 elements.

The atomic number of an atom is equal to the number of protons in the nucleus of an atom or the number of electrons in an electrically neutral atom.
Atomic number = number of protons.

Chemical symbols are the abbreviations used in chemistry for chemical elements. Element symbols for chemical elements normally consist of one or two letters from the English alphabet and are written with the first letter capitalised.

Atomic mass refers to the mass of an atom. It depicts how many times an atom of an element is heavier than one-twelth (1/12th) the mass of one atom of carbon-12 of mass of one carbon atom. It is measured in a unit called amu (atomic mass unit).

Element	Atomic number and mass number
Hydrogen (H)	1- 1
Helium(He)	2-4
Lithium(Li)	3-7
Beryllium(Be)	4-9
Boron(B)	5-11
Carbon(C)	6-12
Nitrogen(N)	7-14
Oxygen(O)	8 -16
Flourine(F)	9-19
Neon(Ne)	10-20
Sodium(Na)	11-23
Magnesium(Mg)	12-24
Aluminium(Al)	13-27
Silicon(Si)	14-28
Phosphorous(P)	15-31
Sulfur(S)	16-32
Chlorine(Cl)	17-35.5
Argon(Ar)	18-40
Potassium(K)	19-39
Calcium(Ca)	20-40