What is the Difference Between Atomic Number and Atomic Weight?

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The difference between atomic number and atomic weight lies in their definitions and the properties they represent:

  • Atomic Number: The atomic number is the number of protons in an atom's nucleus. It is also equal to the number of electrons surrounding the nucleus. The atomic number is used to define the element and classify it. Elements with the same atomic number are considered to be of the same element.
  • Atomic Weight: Also known as relative atomic mass, atomic weight is the average mass of an element's atom, which includes the mass of both protons and neutrons in the nucleus. Atomic weight is typically expressed in atomic mass units (amu) and is used to classify different isotopes of the same element. The stability of a nucleus, and the radioactivity of an atom, is heavily dependent on the number of neutrons it contains.

In summary, atomic number represents the number of protons and electrons in an atom and identifies the element, while atomic weight represents the average mass of an element's atom, including the mass of protons and neutrons.

Comparative Table: Atomic Number vs Atomic Weight

Here is a table comparing atomic number and atomic weight:

Property Atomic Number Atomic Weight
Definition The number of protons in an atom's nucleus, which defines the element. The weighted average of the masses of all isotopes of an element, taking into account their abundance.
Symbol Z A
Units None Atomic mass units (amu)
Role in the Periodic Table Determines the position of an element in the periodic table. Determines the relative mass of an element in the periodic table.
Isotopes Isotopes have the same atomic number but differ in the number of neutrons. The atomic weight takes into account the relative abundance of different isotopes in a sample.

In summary, the atomic number represents the number of protons in an atom's nucleus and defines the element, while the atomic weight is the average mass of all isotopes of an element, weighted according to their natural abundance.