Written in this form, carbon-13, what does the 13 represent and what is the atomic number?

Explanation:
Carbon has 3 isotopes. Carbon-12, Carbon-13, and Carbon-14
All of them have an atomic number of 6, which means all of them have 6 protons (if they had different numbers of protons, they'd be different elements).
Since they all have 6 protons, in their elemental forms, they all have 6 electrons, too.
Carbon-12:
Atomic number: 6
Mass number: 12
protons: 6
electrons: 6
neutrons: 6
Carbon-13:
Atomic number: 6
Mass number: 13
protons: 6
electrons: 6
neutrons: 7
Carbon-14:
Atomic number: 6
Mass number: 14
protons: 6
electrons: 6
neutrons: 8

All the elements are arranged by melting points on the periodic table. in row they go from metals to gasses. in the row from sodium to argon, sodium is more reactive than the one on its right, magnesium.