Q: What is the average atomic mass of an element with the following isotopic abundances: $$95.00\% at$$ $$31.972\ \mathrm{u}$$, $$0.76\% at$$ $$32.971\ \mathrm{u}$$, and $$4.22\% at$$ $$33.967\ \mathrm{u}$$?

$$(0.9500 \times 31.972) + (0.0076 \times 32.971) + (0.0422 \times 33.967) = 32.07\ \mathrm{u}$$

Q: Calculate the formula mass of $$\mathrm{CCl}_4$$ given the atomic masses: $$\mathrm{C} = 12.011\ \mathrm{u}$$, $$\mathrm{Cl} = 35.453\ \mathrm{u}.$$

$$12.011 + 4 \times 35.453 = 153.823\ \mathrm{u}$$

Question 1

Which statement best describes the Law of Definite Proportions as demonstrated by the mass ratio data for carbon and hydrogen in three different samples of a compound?

Accepted Answer

Each sample contains carbon and hydrogen in a fixed mass ratio, regardless of the sample size.

Question 2

Which experiment provided the first evidence for the existence of subatomic particles?

Accepted Answer

J.J. Thomson's cathode ray tube experiment

Question 3

A neutral atom of titanium (Ti) has an atomic number of 22. How many electrons does a Ti$$^{2+}$$ ion have?

Accepted Answer

$$20$$

Question 4

Which of the following statements about isotopes is correct?

Accepted Answer

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

Question 5

Which scientist is credited with organizing the periodic table by increasing atomic mass and predicting the properties of undiscovered elements?

Accepted Answer

Dmitri Mendeleev

Question 6

What is the average atomic mass of an element with the following isotopic abundances: $$95.00\% at$$ $$31.972\ \mathrm{u}$$, $$0.76\% at$$ $$32.971\ \mathrm{u}$$, and $$4.22\% at$$ $$33.967\ \mathrm{u}$$?

Accepted Answer

$$(0.9500 	imes 31.972) + (0.0076 	imes 32.971) + (0.0422 	imes 33.967) = 32.07\ \mathrm{u}$$

Question 7

Which of the following best explains why most alpha particles passed through the gold foil in Rutherford’s experiment?

Accepted Answer

Atoms are mostly empty space.

Question 8

Which group on the periodic table is known as the 'salt formers'?

Accepted Answer

Group VII (Halogens)

Question 9

A lead sample has 124 neutrons. If the atomic number of lead is 82, what is the mass number of this isotope?

Accepted Answer

$$206$$

Question 10

Which of the following best describes the difference between a cation and an anion?

Accepted Answer

A cation is formed by the loss of electrons, while an anion is formed by the gain of electrons.

Question 11

Which of the following is a metalloid and acts as a semiconductor material?

Accepted Answer

Silicon (Si)

Question 12

If an atom has 15 protons, 16 neutrons, and 18 electrons, what is its charge?

Accepted Answer

$$3-$$

Question 13

Which of the following best describes the periodic law as defined by Henry Moseley?

Accepted Answer

When elements are arranged by increasing atomic number, elements with similar properties appear at regular intervals.

Question 14

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

Accepted Answer

Atoms can change into other atoms as a result of a chemical reaction.

Question 15

Calculate the formula mass of $$\mathrm{CCl}_4$$ given the atomic masses: $$\mathrm{C} = 12.011\ \mathrm{u}$$, $$\mathrm{Cl} = 35.453\ \mathrm{u}.$$

Accepted Answer

$$12.011 + 4 	imes 35.453 = 153.823\ \mathrm{u}$$

Question 16

Which of the following best explains why the identity of an atom changes if the atomic number changes?

Accepted Answer

The atomic number is the number of protons, which defines the element.

Question 17

Which of the following is a property of metals?

Accepted Answer

Malleable and good conductors of electricity

Question 18

A nitrogen ion has 10 electrons. What is its charge?

Accepted Answer

$$3-$$

Question 19

Which of the following best describes the difference between the modern atomic model and Rutherford’s atomic model?

Accepted Answer

Electrons are found in areas of probability (clouds) rather than fixed orbits.

Atoms, Elements, and the Development of Atomic Theory

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