Final answer:
As you move across a period on the Periodic Table, each element has one more proton than the previous one, which determines the element's identity and influences its chemical properties.
Step-by-step explanation:
This happens because as you move across a period each element has one more proton in its nucleus than the last one. The correct answer is A) Proton.
When we look at the Periodic Table of the Elements, we can observe a pattern known as the periodic trend. As we move from left to right across a period, each successive element has an additional proton and electron, which is significant because the number of protons in an atom's nucleus determines the identity of the element.
For instance, the chemical properties of an element are directly influenced by the number of protons and electrons within the atom.
Isotopes, on the other hand, are variations of elements that have the same number of protons but different numbers of neutrons. While changing the number of neutrons in an atom does not change its chemical properties, it does affect the atomic mass. This distinction is crucial when discussing isotopes, but for identifying the element itself, the number of protons is what matters.