Q: Which of the following equations correctly represents the calculation of the average atomic mass of an element with two isotopes?

$$\text{Average atomic mass} = (\text{mass}_1 \times \text{fraction}_1) + (\text{mass}_2 \times \text{fraction}_2)$$

Question 1

Which statement best summarizes the Law of Definite Composition as proposed by John Dalton?

Accepted Answer

A chemical compound always contains exactly the same proportion of elements by mass.

Question 2

According to Dalton’s Atomic Theory, which of the following statements is INCORRECT based on modern understanding?

Accepted Answer

Atoms of a given element are identical in mass and properties.

Question 3

Which experiment led to the discovery of the electron?

Accepted Answer

Cathode ray tube experiment by J.J. Thomson

Question 4

What is the charge and approximate relative mass of a proton?

Accepted Answer

Charge: $$+1$$, Relative mass: $$1$$

Question 5

Which scientist is credited with discovering the neutron, and what is the significance of the neutron in the atom?

Accepted Answer

James Chadwick; neutrons add mass to the nucleus without adding charge.

Question 6

An isotope of carbon has 6 protons and 8 neutrons. What is its nuclear symbol and mass number?

Accepted Answer

$$^{14}_{6}\mathrm{C}$$; mass number = 14

Question 7

Which of the following best describes the structure of the atom according to the nuclear model proposed by Rutherford?

Accepted Answer

A dense, positively charged nucleus surrounded by electrons in mostly empty space.

Question 8

A neutral atom of sodium has an atomic number of 11 and a mass number of 23. How many protons, neutrons, and electrons does it have?

Accepted Answer

11 protons, 12 neutrons, 11 electrons

Question 9

Which of the following equations correctly represents the calculation of the average atomic mass of an element with two isotopes?

Accepted Answer

$$	ext{Average atomic mass} = (	ext{mass}_1 	imes 	ext{fraction}_1) + (	ext{mass}_2 	imes 	ext{fraction}_2)$$

Question 10

Which of the following statements about ions is correct?

Accepted Answer

A cation is formed when an atom loses one or more electrons.

Question 11

A sample of magnesium contains three isotopes: $$^{24}\mathrm{Mg} (78.99$$%), $$^{25}\mathrm{Mg} (10.00$$%), and $$^{26}\mathrm{Mg} (11.01$$%). What is the average atomic mass of magnesium? (Round to two decimal places)

Accepted Answer

$$24.32\ \mathrm{u}$$

Question 12

Which of the following best explains why atoms are electrically neutral?

Accepted Answer

The number of protons equals the number of electrons.

Question 13

Which scientist’s experiment involved the deflection of alpha particles and led to the discovery of the atomic nucleus?

Accepted Answer

Ernest Rutherford

Question 14

If an atom has 17 protons, 18 neutrons, and 17 electrons, what is its nuclear symbol?

Accepted Answer

$$^{35}_{17}\mathrm{Cl}$$

Question 15

Which of the following is a correct statement about isotopes?

Accepted Answer

Isotopes of an element have the same number of protons but different numbers of neutrons.

Question 16

Calculate the molecular mass of carbon dioxide ($$\mathrm{CO}_2) $$using the following atomic masses: C = $$12.01\ \mathrm{u}$$, O = $$16.00\ \mathrm{u}.$$

Accepted Answer

$$44.01\ \mathrm{u}$$

Question 17

Which of the following best describes the difference between a molecule and a formula unit?

Accepted Answer

A molecule refers to a covalently bonded group of atoms, while a formula unit refers to the simplest ratio of ions in an ionic compound.

Question 18

Which of the following best explains why the atomic mass of an element on the periodic table is not a whole number?

Accepted Answer

It is a weighted average of all naturally occurring isotopes of that element.

Question 19

Which of the following is the correct formula for calculating the number of neutrons in an atom?

Accepted Answer

$$	ext{Number of neutrons} = 	ext{Mass number} - 	ext{Atomic number}$$

Atoms and Elements: Foundations of Atomic Theory and Structure

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