which element would you expect to be least reactive

HESI A2

Chemistry HESI A2 Practice Test

1. Which element would you expect to be least reactive?

A. Li
B. Cr
C. Nd
D. Xe

Correct answer: D

Rationale: Xenon (Xe) is a noble gas located in Group 18 of the periodic table. Noble gases are known for their stable and unreactive nature due to having a full outer shell of electrons. Therefore, xenon would be expected to be the least reactive among the given elements. Choice A (Li) is a metal in Group 1 of the periodic table, which tends to be reactive, especially with water. Choice B (Cr) is a transition metal known for its reactivity in some forms. Choice C (Nd) is a rare earth element, which can exhibit varying reactivity depending on the compound or environment.

2. What can stop the penetration of gamma radiation?

A. Aluminum foil
B. Glass
C. Several feet of concrete
D. Piece of paper

Correct answer: C

Rationale: Gamma radiation is highly penetrative and requires dense materials to block it effectively. While aluminum foil and glass are not sufficient to stop gamma radiation, several feet of concrete is needed due to its high density and ability to absorb gamma radiation effectively. A piece of paper is too thin and lacks the density required to block gamma radiation, making it an ineffective shield.

3. If gas A has four times the molar mass of gas B, you would expect it to diffuse through a plug ___________.

A. at half the rate of gas B
B. at twice the rate of gas B
C. at a quarter the rate of gas B
D. at four times the rate of gas B

Correct answer: A

Rationale: When comparing the diffusion rates of two gases, according to Graham's law of diffusion, the rate of diffusion is inversely proportional to the square root of the molar mass. If gas A has four times the molar mass of gas B, the square root of the molar masses ratio (4:1) is 2. This means that gas A would diffuse through a plug at half the rate of gas B. Therefore, the correct answer is A, at half the rate of gas B. Choices B, C, and D are incorrect because they do not reflect the correct relationship between the molar masses and the rates of diffusion according to Graham's law.

4. How many electrons are in a neutral atom of neon?

A. 9
B. 10
C. 11
D. 12

Correct answer: B

Rationale: The atomic number of neon is 10, which represents the number of protons in its nucleus. In a neutral atom, the number of electrons is equal to the number of protons to maintain electrical neutrality. Therefore, a neutral atom of neon contains 10 electrons, matching the 10 protons within the nucleus. Choice A (9 electrons) is incorrect as it doesn't correspond to the atomic number of neon. Choices C (11 electrons) and D (12 electrons) are also incorrect as they do not align with the correct atomic number of neon.

5. Which compound contains a polar covalent bond?

A. O
B. F
C. Br
D. H₂O

Correct answer: D

Rationale: The compound 'H₂O' (water) contains a polar covalent bond. In a water molecule, the oxygen atom is more electronegative than the hydrogen atoms. As a result, the electrons in the O-H bonds are unevenly shared, leading to a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms. This unequal sharing of electrons creates a polar covalent bond in water. Choices A, B, and C are incorrect because they represent individual elements, not compounds, and do not involve the concept of polar covalent bonds.