Mnemonic Device:

• Happy Henry Lives Beside Boron Cottage, Near Our Friend Nelly Nancy MgAllen. Silly Patrick Stays Close. Arthur Kisses Carrie.
• Here He Lies Beneath Bed Clothes, Nothing On, Feeling Nervous, Naughty Margret Always Sighs, ” Please Stop Clowning Around ” (18 elements)
• How He Likes Bear By Cups Not Overflowing. Although Science Presents Some Clues, Are Kittens Cats?

Explanation:

Happy. – H Hydrogen
Henry – He Helium
Lives – Li Lithium
Beside – Be Beryllium
Born – B Boron
Cottage – C Carbon
Near – N Nitrogen
Our – O Oxygen
Friend – F Fluorine.
Nelly – Ne Neon
Nancy – Na Sodium
Mg – Mg Magnesium
Allen – Al Aluminum
Silly – Si Silicon
Patrick – P Phosphorus
Stays – S Sulphur
Close – Cl Chlorine
Arthur – Ar Argon
Kisses – K Potassium
Carrie – Ca Calcium

Additional Information: the first twenty elements, of the periodic table, with their atomic number

1. hydrogen H
2. helium He
3. lithium Li
4. beryllium Be
5. boron B
6. carbon C
7. nitrogen N
8. oxygen O
9. flourine F
10. neon Ne
11. sodium Na
12. magnesium Mg
13. aluminum Al
14. silicon Si
15. phosphorus P
16. sulfur S
17. chlorine Cl
18. argon Ar
19. potassium K
20. calcium Ca

Find the average atomic mass:

Rules to find the average atomic mass, if the atomic number (Z) is an:

Even number except Be (9), Ar (40),

• multiply the atomic number by 2. For example,
  • 1) Helium atomic number (Z) is the EVEN #2, hence to find the average atomic mass (A) is (Atomic number) multiply by 2 which is 2 x 2 = 4

Odd number except H (1), N (14 ), Cl (35.5),

• multiply the atomic number by 2, then add 1. For example,
  • 1) Lithium atomic number (Z) is the ODD # 3, hence to find the average atomic mass (A ) is (Atomic number) multiply by 2 plus 1, which is 3 x 2 +1 = 7
  • 2) Boron atomic number (Z) is the ODD # 5, hence to find the average atomic mass (A ) is (Atomic number ) multiply by 2 plus 1, which is 5 x 2 +1 = 11

Please remember the exception to the rules:

• Be, Ar, H, N, Cl

Song for the first 20 elements:

Many more mnemonics for remembering the elements of the periodic table

See https://en.wikipedia.org/wiki/Periodic_table for more information.