You need the following information to solve this problem:

1. The molar mass of neon is 20.18 grams per mole. (From the Periodic Table.)

2. One mole of a substance contains 6.02 x 10^23 particles.

You can find the mass of a given number of neon atoms by multiplying it by conversion factors expressing the above relationships. The conversion factors we will use are:

1 mole/6.02 x 10^23 atoms

and

20.18 grams/1 mole

When the correct conversion factors are used all units cancel out except those that express the answer. In this case atoms and moles cancel out leaving units of grams:

(5. x10^9 atoms)(1 mole/6.02x10^23 atoms)(20.18 grams/1 mole)

= 1.8x10^-13 grams

Alternatively, you could break this down into two problems by finding the number of moles and then the mass:

(5.3x10^9 atoms)(1 mole/6.02x10^23 atoms) = 8.9 x 10^-15 moles

(8.9x10-^15 moles)(20.18 grams/mole) = 1.8x10^-13 grams