What determines an atom’s identity?
A. The number of neutrons
B. The valence number
C. The number of protons
D. The number of electrons
C. The number of protons
C. The number of protons
The correct answer is C. The number of protons.
To determine an atom's identity, we look at the number of protons it has. Protons are positively charged particles found in the nucleus of an atom. Each element on the periodic table is defined by the number of protons it possesses. This number is called the atomic number and is unique to each element. For example, hydrogen has one proton, oxygen has eight protons, and gold has 79 protons.
On the other hand, the number of neutrons (option A) can vary within an element. Isotopes are atoms of the same element that have different numbers of neutrons. However, they still have the same atomic number and therefore the same identity.
The valence number (option B) refers to the number of electrons in the outermost energy level of an atom. While the valence number is important for understanding an atom's reactivity, it does not determine the atom's identity.
The number of electrons (option D) is equal to the number of protons in a neutral atom. In ions, the number of electrons can vary, but it does not determine an atom's identity since different elements can have the same number of electrons in their outer energy shells.