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. What type of reaction involves atoms attempting to achieve stable electron configurations?

A. Chemical
B. Nuclear
C. Physical
D. Mechanical

Correct answer: A

Rationale: In a chemical reaction, atoms interact to achieve stable electron configurations through the formation of new chemical bonds or the breaking of existing ones. This process aims to reach a more stable state by filling or emptying electron orbitals, leading to the formation of new substances with more stable configurations. Choice B, nuclear reactions, involve changes in the atomic nucleus rather than electron configurations. Choice C, physical reactions, involve changes in physical state or appearance without changing the chemical makeup. Choice D, mechanical reactions, do not involve the rearrangement of electrons to achieve stable configurations.

3. What type of intermolecular force is a dipole attraction?

A. Strong
B. Weak
C. Medium
D. Very strong

Correct answer: B

Rationale: A dipole attraction is considered a weak intermolecular force. It occurs between molecules with permanent dipoles, where the positive end of one molecule is attracted to the negative end of another molecule. While dipole-dipole interactions are stronger than dispersion forces, they are weaker than hydrogen bonding or ion-dipole interactions. Therefore, the correct answer is 'Weak.' Choices A, C, and D are incorrect because dipole attractions are not classified as strong, medium, or very strong intermolecular forces, but rather fall into the category of weak intermolecular forces.

4. How many times more acidic is a substance with a pH of 3 compared to a substance with a pH of 5?

A. 8
B. 2
C. 100
D. 1,000

Correct answer: D

Rationale: The pH scale is logarithmic, indicating that each pH unit change reflects a 10-fold difference in acidity level. Going from pH 5 to pH 3 involves a difference of 2 units, which translates to a 100-fold increase in acidity level (10^2 = 100 for each unit). Therefore, a substance with a pH of 3 is 1,000 times more acidic than a substance with a pH of 5 (100 * 10 = 1,000). Choice A (8) is incorrect as it does not consider the logarithmic nature of the pH scale. Choice B (2) is incorrect because it represents the difference in pH units, not the increase in acidity level. Choice C (100) is incorrect as it miscalculates the increase in acidity level, which is 1,000 times and not 100 times.

5. If oxygen is in a compound, what would its oxidation number be?

A. 2
B. -2
C. 0
D. -1

Correct answer: B

Rationale: Oxygen typically has an oxidation number of -2 in compounds because it tends to gain electrons. This is due to its high electronegativity, which leads to oxygen attracting electrons towards itself in a chemical bond. Choice A (2) is incorrect because oxygen doesn't have a +2 oxidation number in compounds. Choice C (0) is incorrect as oxygen rarely has an oxidation number of 0 in compounds. Choice D (-1) is incorrect as oxygen's oxidation number in compounds is typically -2, not -1.