Question

In the nuclear symbol of deuterium ${}_1^2H$ , what is the atomic number and the mass number?

Answer 1

Mass number is defined as the mass of the atom, primarily the nucleus which contains the protons and neutrons, hence mass number is the number of protons and neutrons present in an atom. Atomic number is the no. of protons present in the atom. Often this is equal to the no. of electrons.

Complete answer:
In the representation of an element in the periodic table, every element is given as ${}_Z^AX$ where A is the mass number and Z is the atomic number.
The compound given to us is Deuterium. Deuterium is the isotope of Hydrogen. Isotopes are the compounds that have the same atomic number but different mass number. They differ in the no. of neutrons present in the nucleus. By saying isotope, we clearly mean that the atomic number of both Hydrogen and Deuterium is the same. Hydrogen atoms are given as ${}_1^1H$ the same atomic number as for deuterium. Hence the atomic number of deuterium will be one.
Isotopes differ in the no. of neutrons, and number of neutrons can be given as = $A - Z$
The no. of protons in Hydrogen will be equal to no. of electrons, which will be equal to one. Hence the atomic mass will be the sum of Neutrons and protons $= 1 + 1 = 2$ . Therefore, the atomic mass of deuterium is 2.

Note:
Hydrogen is the only element with two isotopes having different common names. One isotope is Deuterium, also known as Hydrogen-2 ( ${}_1^2H$ ), and the second one is Tritium also known as Hydrogen-3 $({}_1^3H)$ . The symbols D and T are used sometimes to represent Deuterium and Tritium respectively.