The correct answer is Both a and b. Explanation & Key Takeaways: • Atomic number: • The total number of protons in the nucleus of an atom gives us the atomic number of that atom. Hence, statement a is correct. • It is represented with the letter ‘Z.’ • All the atoms of a particular element have the same number of protons, and hence the same atomic number. • Atoms of different elements have different atomic numbers. • For example, all carbon atoms have an atomic number of 6, whereas all atoms of Oxygen have 8 protons in their nucleus. • Mass Number: • The number of protons and neutrons combined give us the mass number of an atom. Hence, statement b is correct. • It is represented using the letter ‘A.’ • As both protons and neutrons are present in the nucleus of an atom, they are together called nucleons. • For example, an atom of carbon has 6 protons and 6 neutrons. Thus, its mass number is 12. • While the number of protons remains the same in all atoms of an element, the number of neutrons can vary. Thus, atoms of the same element can have different mass numbers, and these are called isotopes. • The weight of an electron is almost negligible. Thus, the atomic mass of an atom is almost the same as its mass number. Important Notes: Related Concepts: • Atomic Structure • The atomic structure of an element refers to the constitution of its nucleus and the arrangement of the electrons around it. • Primarily, the atomic structure of matter is made up of protons, electrons, and neutrons. • The protons and neutrons make up the nucleus of the atom, which constitute its mass number. • The atomic number of an element describes the total number of protons in its nucleus. • Neutral atoms have equal numbers of protons and electrons. However, atoms may gain or lose electrons in order to increase their stability and the resulting charged entity is called an ion. • Atoms of different elements have different atomic structures because they contain different numbers of protons and electrons. This is the reason for the unique characteristics of different elements. • Discovery • Electron was discovered by J.J. Thomson in 1897. • Proton was discovered by Ernest Rutherford in 1917. • Neutron was discovered by James Chadwick in 1932.