What does the number 84 in the name krypton-84 represent?

The number 84 is the atomic mass of krypton-84.

Explanation:

There are 31 isotopes of krypton. Out of these 31 only six isotopes are stable.

Stable isotopes are:

Kr⁷⁸ Kr⁸⁰ Kr⁸² Kr⁸³ Kr⁸⁴ Kr⁸⁶

The atomic number of krypton is 36.

All isotopes have same atomic number but different atomic mass due to the different number of neutrons present.

The sum of of neutrons and protons is mass number while the number of electrons are number of protons are atomic number of an element.

Isotope:

The atoms of an element having same atomic number but different atomic mass due to different number of protons are called isotopes.

The krypton-84 is the isotope of krypton with atomic mass 84.

the answer is barium and copper bromide