[tex]^{20}_{10}\text{Ne}[/tex].
That's a neon-20 atom.
Here's the symbol of neon-20, showing both its atomic number and its mass number.
[tex]^{20}_{10}\text{Ne}[/tex].
How to name this atom?
What about the atomic number? The atomic number is unique for each element. It is implied in the first part of the name. For example, the atomic number of a "neon-20" atom is always going to be 10 since the atomic number of neon is 10. That number is printed on the periodic table. However, the mass number may vary between isotopes of each element. That number needs to be specified.
The name of this atom is neon-20.