Question

A nitrogen atom with 7 protons and 8 neutrons has a mass number of 15 amu,. However, in the periodic table, the atomic mass for nitrogen is 14.01. How is this possible?

Answer 1

The nitrogen is the chemical element present in the periodic table which is denoted by N. The nitrogen is present in the form of two stable isotopes which are ${}^{14}N$ and ${}^{15}N$ where the superscript is the mass number.

Complete step by step answer:
Each atom is formed of a nucleus or one or more than one electrons surrounding the nucleus. The nucleus is formed of one or more protons and the number of neutrons.
The number of protons in the nucleus of the atom is equal to the atomic number (Z). The mass number of the atom is defined as the sum of the number of protons and neutrons present in the nucleus.
The nitrogen is the chemical element which is denoted by the symbol N. The atomic number of nitrogen is 7.
A nitrogen atom with 7 protons and 8 neutrons has a mass number of 15 amu,. However, in the periodic table, the atomic mass for nitrogen is 14.01. This is possible as the nitrogen exists in different forms of isotope.
When the number of protons is 7 and the number of neutrons is 7, then the isotope will be ${}^{14}N$, where the mass number is 14.
When the number of protons is 7 and the number of neutrons in 8, then the isotope will be ${}^{15}N$, where the mass number is 15.
The atomic mass of nitrogen is the weighted average of the individual mass number of isotopes. Thus, In periodic table, the atomic mass for the nitrogen is 14.01

Note:
The number of neutrons present in the nucleus is equal to the difference between the mass number of atom (M) and the atomic number (Z). The isotopes are defined as those species which contain the same atomic number but different mass number.