Match the Numbers with the letters so the letters define the numbers 1. Atomic mass
2. groups
3. periods
4. atomic number
5. valence
6. negligible
7. electron shell

A. the outermost electrons of an atom, or those on residing in the outermost energy shell of an atom or ion

B. horizontal rows of the periodic table

C. vertical columns of the periodic table

D. a group of electrons at a similar energy level

E. extremely small and unimportant

F. generally the sum of the protons and neutrons

G. the number of protons in every atom of an element