Final answer:
The true statement among the options provided is option a, which states that the number of protons will tell you what element it is, as the number of protons corresponds to an element's atomic number.
Step-by-step explanation:
The number of protons in an atom's nucleus defines the atomic number (Z), which determines the identity of the element. For example, an atom with six protons is always carbon, with an atomic number of 6. A neutral atom has an equal number of protons and electrons. The mass number (A) is the sum of the number of protons and neutrons. Therefore, we can find the number of neutrons by subtracting the atomic number from the mass number (A - Z = number of neutrons). This informs us that option a, 'The number of protons will tell you what element it is,' is the true statement. Options b, c, d, and e are incorrect because the atomic mass minus the atomic number gives the number of neutrons, an ion has a different number of electrons not neutrons, elements in the same vertical column are called groups or families, not periods, and the three groups of the periodic table are metals, non-metals, and metalloids, not gases.