the number of protons in an element is represented by which number

HESI A2

Chemistry HESI A2 Quizlet

1. Which number represents the number of protons in an element?

A. Atomic mass
B. Mass number
C. Atomic number
D. Proton number

Correct answer: C

Rationale: The correct answer is C: Atomic number. The atomic number corresponds to the number of protons in an element. Protons are positively charged subatomic particles found in the nucleus of an atom. Each element has a unique atomic number, which defines its identity based on the number of protons it contains. Choice A, Atomic mass, is incorrect as it refers to the average mass of an atom of an element. Choice B, Mass number, represents the total number of protons and neutrons in an atom's nucleus. Choice D, Proton number, is not a commonly used term in chemistry to indicate the number of protons.

2. You contain two odorous gases in vials with porous plugs. Gas A has twice the mass of Gas B. Which observation is most likely?

A. You will smell Gas A before you smell Gas B.
B. You will smell Gas B before you smell Gas A.
C. You will smell Gas A but not Gas B.
D. You will smell Gas B but not Gas A.

Correct answer: A

Rationale: The rate of effusion of a gas is inversely proportional to the square root of its molar mass. Since Gas A has twice the mass of Gas B, Gas A will effuse more slowly than Gas B. Therefore, you will likely smell Gas A before you smell Gas B as Gas A will escape and diffuse through the porous plug at a slower rate compared to Gas B. Choice A is correct because Gas A, with its higher molar mass, will take longer to effuse through the porous plug, causing you to smell it first. Choices B, C, and D are incorrect as they do not consider the relationship between molar mass and effusion rate.

3. Which classification best describes B, Si, As, Te, At, Ge, and Sb that form a staircase pattern on the right side of the periodic table?

A. Metals
B. Semimetals
C. Nonmetals
D. Ultrametals

Correct answer: B

Rationale: B, Si, As, Te, At, Ge, and Sb are located in a staircase pattern on the periodic table's right side. Elements in this region are known as metalloids or semimetals because they exhibit properties of both metals and nonmetals. They possess characteristics of both metallic and non-metallic elements, making them valuable semiconductors with diverse applications in electronics. Choice A is incorrect because these elements are not considered true metals. Choice C is incorrect as these elements do not display typical nonmetal properties exclusively. Choice D, 'Ultrametals,' is not a recognized classification in chemistry and is therefore incorrect.

4. What is the product of the decomposition of water?

A. Hydrogen and oxygen
B. Carbon dioxide
C. Nitrogen and hydrogen
D. Methane

Correct answer: A

Rationale: The correct answer is A: Hydrogen and oxygen. When water undergoes decomposition, it breaks down into hydrogen and oxygen gases through a process known as electrolysis. This reaction is represented by 2Hâ‚‚O â†’ 2Hâ‚‚ + Oâ‚‚. Choice B, carbon dioxide, is incorrect as it is not a product of water decomposition. Choice C, nitrogen and hydrogen, is incorrect as water decomposes into hydrogen and oxygen, not nitrogen. Choice D, methane, is incorrect as methane is not a product of water decomposition.

5. What is stoichiometry?

A. The study of energy changes in chemical reactions
B. The study of the mass relationships in chemical reactions
C. The study of atomic structure
D. The study of molecular geometry

Correct answer: B

Rationale: Stoichiometry is the branch of chemistry that deals with the quantitative relationships between reactants and products in chemical reactions. It focuses on the calculation of the amounts of substances consumed and produced in a reaction based on the balanced chemical equation. Choice A is incorrect because the study of energy changes in chemical reactions falls under thermodynamics. Choice C is incorrect as atomic structure is related to the arrangement of atoms within molecules. Choice D is incorrect as molecular geometry deals with the spatial arrangement of atoms within molecules.