Which property determines an atom's identity as the element phosphorus?

• A. The number of protons in the nucleus
• B. The number of neutrons in the nucleus
• C. The atomic mass of the atom
• D. The number of electrons in the outer shell

Answer: (A)

Verified step by step guidance

1. Understand that an element is defined by its atomic number, which is the number of protons in the nucleus of its atoms.
2. Recognize that the number of protons is unique to each element and does not change for atoms of that element, regardless of isotopes or ions.
3. Note that the number of neutrons can vary within atoms of the same element, leading to different isotopes, but this does not change the element's identity.
4. Realize that the atomic mass is the sum of protons and neutrons and can vary due to isotopes, so it is not the defining property of the element.
5. Understand that the number of electrons in the outer shell can change when atoms form ions or bonds, so it also does not determine the element's identity.