In the symbol 4 2he, the superscript 4 is the for helium and the subscript 2 is the for helium.
a. atomic number; mass number
b. mass number; atomic number
c. atomic number; atomic mass
d. atomic mass; atomic number

Answer is: b. mass number; atomic number.

The superscript is mass number and the subscript is atomic number.

Atomic number(Z) is the number of protons, which is characteristic of a chemical element..  

Mass number (A) is the total number of protons and neutrons in a nucleus.

Helium (symbol: He) is a chemical element with two protons, two electrons and two neutrons.

Option (b) is the correct answer.

Explanation:

Mass number means the total number of protons and total number of neutrons present in an atom. Mass number is written at the top left of the symbol of atom.

Whereas atomic number means the total number of protons present in the atom. Atomic number is written at the bottom left of the symbol of atom.

For example, in , the number 12 denotes mass number of carbon atom whereas 6 denotes the atomic number of carbon atom.

Thus, we can conclude that in the symbol , the superscript 4 is the mass number for helium and the subscript 2 is the atomic number for helium.

nacl or sodium chloride is an ionic compound. this is because an ionic compound pertains to a compound that consists of positively and negatively charged ions. in this case, na (sodium) has a tendency to lose one electron when it reacts with with other atoms, so the resulting sodium ion contains 11 protons and 10 electrons, thus making it positively charged (na+), when chlorine reacts, it usually gains one electron to form an ion called chloride, thus making it negatively charged ( since the ions are oppositely charged, they attract.