The atomic number and mass number are two different properties associated with an atomic nucleus, and they provide different types of information about an element.
Atomic Number: The atomic number of an element represents the number of protons found in the nucleus of an atom. It is denoted by the symbol 'Z' and is a unique identifier for each element. The atomic number determines the element's position in the periodic table and is used to classify and identify elements. For example, hydrogen has an atomic number of 1, carbon has an atomic number of 6, and oxygen has an atomic number of 8.
Mass Number (Atomic Weight): The mass number, also referred to as the atomic weight, represents the sum of protons and neutrons present in the atomic nucleus of an element. It is denoted by the symbol 'A'. While the atomic number determines the element's identity, the mass number provides information about the atom's mass. This means that different isotopes of the same element may have different mass numbers, as they contain varying numbers of neutrons. Isotopes are atoms of the same element that have different mass numbers but the same atomic number. For example, carbon-12 and carbon-14 are isotopes of carbon, with mass numbers 12 and 14 respectively.
The differences between atomic number and mass number arise from the composition of an atom's nucleus. Protons, with a positive charge, contribute to the atomic number, as each element has a unique number of protons. Neutrons, on the other hand, have no electrical charge but contribute to the mass number by adding to the overall mass of the atom. Electrons, which orbit the nucleus, are not included in the mass number calculation since their mass is significantly smaller compared to protons and neutrons.
The atomic number and mass number were discovered and understood through advancements in experimental techniques and theoretical understanding in the field of atomic physics. The discovery of the atomic nucleus and its composition led to the development of the concept of atomic number by Henry Moseley in 1913. The concept of isotopes and the ability to measure the relative abundance of different isotopes using mass spectrometry allowed for the determination of atomic weights or mass numbers of elements.