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The atomic number is a fundamental concept in understanding elements, including isotopes. It represents the number of protons found in the nucleus of an atom. For magnesium, the atomic number is 12, meaning every magnesium atom has 12 protons.

Protons define not only the element's identity but also its position on the periodic table. If another atom has a different number of protons, it is considered a different element altogether.

• The atomic number is always a whole number.
• It is unique to each element.
• In a neutral atom, the atomic number also equals the number of electrons.

Because isotopes are variations of the same element, all magnesium isotopes share the atomic number 12. This commonality ensures they have similar chemical properties, as they have the same number of electrons, influencing their chemical behavior.

Protons and neutrons are the subatomic particles found in the nucleus of an atom. While protons have a positive charge, neutrons are neutral. Together, they significantly contribute to the atom's mass.

For isotopes, while the number of protons remains constant (ensuring the element's identity), the number of neutrons varies. This is what distinguishes different isotopes of the same element. In magnesium:

• Magnesium-24 contains 12 neutrons.
• Magnesium-25 contains 13 neutrons.
• Magnesium-26 contains 14 neutrons.

As a result, different isotopes have different mass numbers but the same atomic number. The varying number of neutrons does not affect the chemical behavior of the isotopes much but it does influence physical properties like stability and atomic weight.

Atomic weight, also known as atomic mass, is the total mass of an atom, consisting mostly of the protons and neutrons since electrons have negligible mass. It is usually expressed in atomic mass units (amu) and is not always a whole number.

For isotopes, the atomic weight varies depending on the number of neutrons. Therefore, each isotope of magnesium will not have the same atomic weight.

• Magnesium-24 has an atomic weight close to 24 amu.
• Magnesium-25 has an atomic weight close to 25 amu.
• Magnesium-26 has an atomic weight close to 26 amu.

Despite these differences, the average atomic weight often seen on the periodic table represents a weighted average accounting all of an element's isotopes and their relative abundance. In understanding isotopes, recognizing how these weights vary gives insight into isotope stability and natural occurrence.