Which one of the following statements is correct?

The correct statement is that the atomic number of an element is equal to the number of protons in its nucleus. This is a fundamental definition in atomic chemistry [2]. Option 1 is incorrect because Rutherford’s alpha-particle scattering experiment led to the discovery of the atomic nucleus, not the electron. The electron was discovered earlier by J.J. Thomson in 1897 [3]. Option 2 is incorrect because Thomson proposed the 'plum pudding' model, which suggested a sphere of positive charge with embedded electrons; he did not identify a nucleus containing protons [3]. Option 4 is incorrect because the mass number is the sum of protons and neutrons in the nucleus, whereas the number of electrons only equals the number of protons in a neutral atom [3]. Rutherford's 1911 experiment specifically identified the nucleus as a small, dense, positively charged core.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 3: Metals and Non-metals > 3.3 HOW DO METALS AND NON-METALS REA ALS REACT? > p. 46
2. [2] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 12: Major Crops and Cropping Patterns in India > Glossary > p. 100
3. [3] https://www.britannica.com/science/Rutherford-model