17, 2. 35, 3. Chlorine, 4. 37 would be it mass number and we would call it ¹⁷Cl²⁰

You're very close! Let's go through them.

1. ✅ Correct. The atomic number is the number of protons.

   • The atomic number is $\boxed{\text{17}}$.

2. ✅ Correct. The mass number is the sum of protons and neutrons.

   $$\text{Mass Number} = 17 \text{ protons} + 18 \text{ neutrons} = 35$$
   • The mass number is $\boxed{\text{35}}$.

3. ✅ Correct. An element with an atomic number of 17 is Chlorine.

   • The element is $\boxed{\text{Chlorine}}$.

4. Partially correct.

   • If the isotope has 20 neutrons, its mass number would be: $$\text{Mass Number} = 17 \text{ protons} + 20 \text{ neutrons} = 37$$ So, the mass number is $\boxed{\text{37}}$.
   • The notation for an isotope is typically written as $^{A}_{Z}\text{X}$, where A is the mass number, Z is the atomic number, and X is the element symbol. Alternatively, it's written as Element-MassNumber.
   • So, this isotope would be called Chlorine-37.
   • Your notation $^{17}\text{Cl}^{20}$ is incorrect. The superscript on the left should be the mass number (37), and the subscript on the left should be the atomic number (17). The superscript on the right is not standard for neutrons.
   • The correct notation would be $\boxed{^{37}_{17}\text{Cl}}$.