The atomic number for an element represents the number of protons in the nucleus of its atom. Since Ga3+ refers to a Ga ion with a 3+ charge, it means that the ion has lost three electrons compared to a neutral Ga atom.
To determine the atomic number for Ga3+, we need to know the atomic number of Ga (gallium). The atomic number of gallium is 31. This means that a neutral gallium atom has 31 protons.
When Ga loses three electrons to form Ga3+, the number of protons remains the same, as the charge of an ion does not affect the number of protons. Therefore, Ga3+ still has 31 protons in its nucleus.
Hence, the atomic number for Ga3+ remains 31, just like in a neutral gallium atom.