Q: Which of the following is the correct formula for calculating the mass number $$A of an $$atom?

$$A = \text{number of protons} + \text{number of neutrons}$$

Question 1

Which experiment provided the first direct evidence for the existence of atoms by explaining the random motion of pollen particles in water?

Accepted Answer

Brownian motion observations by Robert Brown

Question 2

According to Dalton’s atomic theory, which statement is INCORRECT?

Accepted Answer

Atoms of one element can change into atoms of another element during a chemical reaction.

Question 3

A sample of water is decomposed into 16.0 g of oxygen and 2.0 g of hydrogen. What law does this observation best illustrate?

Accepted Answer

Law of Definite Proportions

Question 4

Which of the following best describes the result of Rutherford’s gold foil experiment?

Accepted Answer

Most alpha particles passed through, but some were deflected at large angles, indicating a dense nucleus.

Question 5

Which law states that when two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in ratios of small whole numbers?

Accepted Answer

Law of Multiple Proportions

Question 6

If an isotope of neon has 10 protons and 12 neutrons, what is its mass number?

Accepted Answer

$$A = 22$$

Question 7

Which subatomic particle was discovered by J.J. Thomson using the cathode ray tube experiment?

Accepted Answer

Electron

Question 8

A sodium atom has an atomic number $$Z = 11$$ and a mass number $$A = 23. $$How many neutrons does it have?

Accepted Answer

$$12$$

Question 9

Which of the following statements about isotopes is TRUE?

Accepted Answer

Isotopes of an element have different numbers of neutrons.

Question 10

Which scientist is credited with discovering the neutron?

Accepted Answer

James Chadwick

Question 11

A sample contains two compounds: CO and CO$$_2. If 1 g of $$carbon combines with 1.33 g of oxygen in CO and 2.67 g of oxygen in CO$$_2$$, what is the ratio of the masses of oxygen that combine with 1 g of carbon?

Accepted Answer

$$2:1$$

Question 12

Which of the following is NOT a postulate of Dalton’s atomic theory?

Accepted Answer

Atoms are composed of even smaller particles called electrons, protons, and neutrons.

Question 13

Which of the following best explains why the mass of a helium atom is about four times that of a hydrogen atom?

Accepted Answer

Helium has two protons and two neutrons, while hydrogen has one proton and no neutrons.

Question 14

Which of the following statements about the structure of the atom is correct according to the nuclear model?

Accepted Answer

Most of the atom’s mass is concentrated in the nucleus.

Question 15

Which of the following chemical symbols is based on the Latin name of the element?

Accepted Answer

Fe for iron

Question 16

A neutral atom has 17 protons, 18 neutrons, and 17 electrons. What is its mass number?

Accepted Answer

$$35$$

Question 17

Which of the following best describes the charge and mass of a neutron?

Accepted Answer

No charge, mass nearly equal to a proton

Question 18

Which experiment determined the charge of a single electron?

Accepted Answer

Millikan’s oil drop experiment

Question 19

Which of the following is the correct formula for calculating the mass number $$A of an $$atom?

Accepted Answer

$$A = 	ext{number of protons} + 	ext{number of neutrons}$$

Question 20

Which statement best explains why atoms are electrically neutral?

Accepted Answer

They contain equal numbers of protons and electrons.

Atoms and Elements: Foundations of Modern Atomic Theory

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