What does the number 18.9984 represent in the image?
a) the atomic number of fluorine
b) the number of protons in fluorine
c) the number of electrons in fluorine
d) the average atomic mass of fluorine

The element symbol in chemistry comprises atomic number and mass number.

Atomic number is written on the top of element and mass number is written at the bottom.

For eg:

If Y is an element that is having atomic number a and mass number x. Then its denotion will be:

ᵃYₓ.

So here ⁹F₁₈.₉₉

Here 18.99 will denote mass number while 9 will denote atomic number.

it should have an internal chamber and two thermometers sticking out of the top. the internal chamber will also be closed off from the outside.