Q: Which of the following is the correct formula for calculating 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 experiment provided the first direct evidence for the existence of negatively charged subatomic particles, and what was the key observation?

Accepted Answer

Thomson's Cathode Ray Tube Experiment; observation of cathode rays deflected by electric and magnetic fields

Question 2

According to the Law of Definite Proportions, which statement is correct regarding the composition of a chemical compound?

Accepted Answer

The proportion by mass of each element in a compound is constant, regardless of the sample or source.

Question 3

Which of the following best describes an isotope?

Accepted Answer

Atoms of the same element with different numbers of neutrons

Question 4

A sample contains two naturally occurring isotopes of element X: X-35 (mass = 34.97 amu, abundance = 75.8%) and X-37 (mass = 36.97 amu, abundance = 24.2%). What is the average atomic mass of element X? (Round to two decimal places)

Accepted Answer

$$35.45$$

Question 5

Which statement about ions is correct according to modern atomic theory?

Accepted Answer

Cations are positively charged because they have fewer electrons than protons.

Question 6

Which law explains why the ratio of the masses of one element that combine with a fixed mass of another element are always simple whole numbers?

Accepted Answer

Law of Multiple Proportions

Question 7

If an atom has 17 protons, 18 neutrons, and 18 electrons, what is its charge and isotope notation?

Accepted Answer

Anion with $$-1$$ charge, $$^{35}_{17}X$$^{-}$$

Question 8

Which statement is NOT part of Dalton's Atomic Theory?

Accepted Answer

Atoms can be created or destroyed in chemical reactions.

Question 9

Which group of elements is most likely to form cations with a $$+1$$ charge?

Accepted Answer

Alkali Metals (Group 1)

Question 10

A main group nonmetal atom has 6 valence electrons. How many electrons must it gain to achieve the same electron configuration as the nearest noble gas?

Accepted Answer

It must gain 2 electrons.

Question 11

Which experiment led to the conclusion that atoms are mostly empty space with a small, dense nucleus?

Accepted Answer

Rutherford's Gold Foil Experiment

Question 12

Which of the following is the correct formula for calculating 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 13

Which statement about the periodic law is correct?

Accepted Answer

When elements are arranged by increasing atomic number, certain sets of properties recur periodically.

Question 14

A neutral atom of element Y has an atomic number of 12 and a mass number of 25. How many neutrons does it have?

Accepted Answer

$$13$$

Question 15

Which of the following is NOT a main group element?

Accepted Answer

Transition Metal

Question 16

If 1 mole of carbon-12 atoms has a mass of exactly $$12 g$$, how many atoms are present in $$6 g of $$carbon-12?

Accepted Answer

$$3.011 	imes 10$$^{23}$$

Question 17

Which statement about the modern atomic theory is FALSE?

Accepted Answer

Atoms of the same element always have the same mass.

Question 18

Which of the following best explains why chemical reactions do not change the identity of atoms?

Accepted Answer

Chemical reactions only involve the reorganization of atoms, not their conversion into other elements.

Question 19

A compound contains 40.0 g of element A and 60.0 g of element B. According to the Law of Definite Proportions, what is the mass fraction of element A in the compound?

Accepted Answer

$$0.40$$

Question 20

Which of the following statements about stoichiometry is correct?

Accepted Answer

Stoichiometry is the study of the amounts of elements and substances consumed and produced in chemical and physical processes.

Foundations of Atomic Theory and the Periodic Table: Key Laws, Experiments, and Concepts

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