Atomic number is equal to the number of protons in an atom.

Explanation:

The atomic number of an atom refers to the number of protons in the nucleus of that atom. It is denoted by the symbol "Z." The atomic number determines the chemical properties and identity of an element. Each element on the periodic table has a unique atomic number.

Key Points:
- The atomic number is specific to each element and does not change for a given element.
- It is the defining characteristic of an element and helps in organizing the elements in the periodic table.
- The atomic number also determines the number of electrons in an atom when the atom is neutral.
- In a neutral atom, the number of protons is equal to the number of electrons. This is because the positive charge of the protons is balanced by the negative charge of the electrons, resulting in a net neutral charge for the atom.
- The atomic number provides important information about the element, such as its position in the periodic table, its atomic mass, and its chemical behavior.
- The atomic number is crucial in identifying and differentiating between different elements.
- Increasing the atomic number by one corresponds to moving one place to the right on the periodic table, indicating a different element with distinct properties.
- For example, hydrogen has an atomic number of 1, which means it has one proton. Carbon has an atomic number of 6, indicating it has six protons in its nucleus.
- So, the correct answer is option 'b' - the atomic number is equal to the number of protons in an atom.

Conclusion:

The atomic number is a fundamental property of an element and is equal to the number of protons in an atom. It helps in distinguishing between different elements and plays a significant role in understanding their chemical behavior and organizing them in the periodic table.