Q: Which of the following is the correct formula for calculating the weighted average atomic mass of an element with two isotopes?

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

Question 1

Which experiment led to the discovery of the electron, and what was the key observation that supported this discovery?

Accepted Answer

The Cathode Ray Experiment; the observation that cathode rays were deflected by electric and magnetic fields, indicating they were negatively charged particles.

Question 2

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

Accepted Answer

11 protons, 12 neutrons, 11 electrons

Question 3

Which of the following best describes why the mass of a helium atom is not exactly twice the mass of a hydrogen atom?

Accepted Answer

Helium contains two protons, but the rest of the mass comes from neutrons, which are not present in hydrogen-1.

Question 4

Calculate the average atomic mass of chlorine given the following isotopic abundances: $^{35}\mathrm{Cl}$ (75.77%, mass = 34.97 g/mol) and $^{37}\mathrm{Cl}$ (24.23%, mass = 36.97 g/mol).

Accepted Answer

$35.45\ \mathrm{g/mol}$

Question 5

Which statement about metals is correct?

Accepted Answer

Metals are malleable, ductile, and tend to lose electrons to form cations.

Question 6

A sample contains $79\mathrm{Br}$ and $81\mathrm{Br}$ isotopes. If the average atomic mass of bromine is $79.90\ \mathrm{amu}$, what is the percent abundance of $79\mathrm{Br}$? ($79\mathrm{Br}$ mass = $78.9183371\ \mathrm{amu}$, $81\mathrm{Br}$ mass = $80.9162906\ \mathrm{amu}$)

Accepted Answer

50.7%

Question 7

Which of the following is a property of noble gases?

Accepted Answer

They have eight valence electrons and are generally unreactive.

Question 8

Which experiment disproved the Plum Pudding Model and led to the discovery of the atomic nucleus?

Accepted Answer

Gold Foil Experiment

Question 9

Which of the following best describes an isotope?

Accepted Answer

Atoms with the same number of protons but different numbers of neutrons.

Question 10

A student finds a shiny, malleable element that conducts electricity and reacts vigorously with water. To which group does this element most likely belong?

Accepted Answer

Alkali Metals (Group 1A)

Question 11

Which of the following statements about the three states of matter is correct?

Accepted Answer

Gases are compressible because their particles are far apart.

Question 12

Which of the following elements is a metalloid and a semiconductor?

Accepted Answer

Silicon (Si)

Question 13

A chlorine ion has 17 protons and 18 electrons. What is its charge?

Accepted Answer

$-1$

Question 14

Which of the following best explains why most of the mass of an atom is concentrated in the nucleus?

Accepted Answer

The nucleus contains protons and neutrons, which are much more massive than electrons.

Question 15

Which of the following is the correct formula for calculating the weighted average atomic mass of an element with two isotopes?

Accepted Answer

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

Question 16

Which of the following is a real-world application of the Gold Foil Experiment's findings?

Accepted Answer

Development of nuclear medicine and cancer treatments using radioactive isotopes.

Question 17

Which group on the periodic table contains elements that exist as diatomic molecules and tend to gain one electron to form anions?

Accepted Answer

Halogens

Question 18

A student is asked to identify an unknown element. It is a poor conductor of electricity, tends to gain electrons, and is found on the right side of the periodic table. What type of element is it?

Accepted Answer

Nonmetal

Question 19

Which of the following best describes the Octet Rule?

Accepted Answer

Atoms tend to lose or gain electrons to achieve a full set of eight valence electrons.

Atoms, Elements, and the Structure of Matter: CHEM131 Study Guide

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