Question

Two students collect data on three unknown samples in the lab. Their data is presented in the table below.

Element X Element Y Element Z
Number of Protons 19 20 unknown
Number of Neutrons 21 20 unknown
Number of Electrons 19 20
19

Appearance solid, dull gray solid, dull light gray solid, dull gray
Reacts with acid? yes yes yes
Mass Number 40 40 40
Student 1 turned in his lab sheet stating that all three samples must be the same element, because their appearance, reactivity with acid, and mass number were all the same.

Student 2 disagreed and wrote that Element X and Z were the same, while Element Y was different. She argued that the similar appearance, identical number of electrons, and identical mass number means that Element X and Z are the same element.

Which of the following would most disprove the second student's argument?

A. After long-term exposure to air, Element Z also turns light gray


B. New data determines that Element Z has 20 neutrons.


C. A repeat experiment reveals that an atom of the sample of Element Z actually has 20 electrons.


D. A repeat experiment reveals that the sample of Element Z actually has a mass number of 41.

Answer 1

Answer choice B, which provides data on neutrons, would most disprove Student 2's argument by indicating that Element Z might have a different atomic number, and therefore may be a different element or an isotope.

Step-by-step explanation:

The student's question concerns whether Element X, Y, and Z are the same, based on various properties such as appearance, reactivity with acid, and mass number. According to the concept that the atomic number (Z) defines the identity of an element, which is the number of protons in the nucleus, answer choice B would most disprove Student 2's argument. Determining that Element Z has 20 neutrons would mean its mass number is 40 (20 protons + 20 neutrons), which is the same as Element X and Y. However, since the atomic number (number of protons) is unknown for Element Z, it cannot be definitively stated that X and Z are the same element based solely on their mass number. To confirm whether Element Z is the same as Element X or different like Element Y, the number of protons must be identified. Isotopes of the same element have the same atomic number but a different number of neutrons, so without knowing the atomic number of Element Z, the assertion that X and Z are the same cannot be confirmed.