Here's why:
* Atomic weight is determined by the number of protons and neutrons in an atom's nucleus. This number remains constant regardless of whether the atom gains or loses electrons.
* Anions are negatively charged ions formed when an atom gains electrons. The gain of electrons increases the atom's electron count, but does not affect the number of protons or neutrons in the nucleus.
* Cations are positively charged ions formed when an atom loses electrons. The loss of electrons decreases the atom's electron count, but does not affect the number of protons or neutrons in the nucleus.
Therefore, the atomic weight of an element remains the same whether it is in its neutral state, an anion, or a cation.
Example:
* Sodium (Na) has an atomic weight of 22.99 amu.
* Sodium cation (Na+) is formed when a sodium atom loses one electron. The atomic weight of Na+ remains 22.99 amu.
* Chlorine (Cl) has an atomic weight of 35.45 amu.
* Chloride anion (Cl-) is formed when a chlorine atom gains one electron. The atomic weight of Cl- remains 35.45 amu.
Important Note: The mass number of an element can change when it forms an ion. This is because the mass number is the total number of protons and neutrons. If an element gains or loses neutrons, its mass number will change. However, the atomic weight, which is based on the average mass of all isotopes of an element, remains the same.
