Here's 20 elements on the periodic table.
1 H Hydrogen
2 He Helium
3 Li Lithium
4 Be Beryllium
5 B Boron
6 C Carbon
7 N Nitrogen
8 O Oxygen
9 F Fluoride
10 Ne Neon
11 Na Sodium
12 Mg Magnesium
13 Al Aluminum
14 Sl Silicon
15 P Phosphorus
16 S Sulfur
17 Cl Chlorine
18 Ar Argon
19 K Potassium
20 Ca Calcium
Your welcome ;)