Atomic Theory

Question 1

What is an atom?

Answer 1

An atom is the smallest unit of matter that can undergo a chemical change.

Question 2

What is a proton?

Answer 2

A proton is a subatomic particle with a positive charge located in the nucleus of an atom.

A proton has a mass of approximately 1 amu.

Question 3

What is a neutron?

Answer 3

A neutron is a subatomic particle with no charge located in the nucleus of an atom.

A neutron has a mass of approximately 1 amu.

Question 4

What is an electron?

Answer 4

An electron is a subatomic particle with a negative charge located in orbitals outside the nucleus of an atom.

An electron has a mass of 1/1836 amu or approximately 0 amu.

Question 5

What is atomic number?

Answer 5

The atomic number is the number of protons in an atom.

The atomic number defines the element.

Question 6

What is mass number?

Answer 6

Mass number is the number of protons and neutrons in an atom.

Note that mass number is NOT found in the Periodic Table.

Question 7

What is an ion?

Answer 7

An ion is a charged atom formed by gaining or losing electron(s).

Question 8

What is a cation?

Answer 8

A cation is a positive ion.

A cation is formed when an atom loses electron(s). Because electrons are lost, a cation has a smaller radius than the neutral atom.

Question 9

What is an anion?

Answer 9

An anion is a negative ion.

An anion is formed when an atom gains electrons. Because electrons are gained, the radius of an anion is bigger than the neutral atom.

Question 10

What are isotopes?

Answer 10

Isotopes are atoms that have the same number of protons (same atomic number) but different numbers of neutrons (different mass numbers).

Question 11

What is atomic mass?

Answer 11

Atomic mass is the weighted average mass of all the naturally occurring isotopes of an element.

Question 12

Given the isotopic notation, how do you calculate the number of neutrons for that isotope?

Answer 12

The number of neutrons is the mass number (top left number in isotopic notation) minus the atomic number (bottom left number).

Question 13

How does an atom produce light?

Answer 13

An atom produces light when excited state electrons move to lower energy states.

Question 14

How do you calculate the atomic mass of an element?

Answer 14

The atomic mass of an element = (natural abundance of isotope 1)*(mass of isotope 1) + (natural abundance of isotope 2)*(mass of isotope 2) + …

Natural abundance is in decimals for the above formula; if using percentages, divide the total by 100.

If the actual mass of the isotope is not given, use the mass number as its mass.

Question 15

What did Rutherford conclude from his observation that most of the alpha particles passed straight through the gold foil?

Answer 15

Rutherford concluded that the atom is made up of mostly empty space.

Question 16

What did Rutherford conclude from his observation that some of the alpha particles were greatly deflected upon hitting the gold foil?

Answer 16

Rutherford concluded that the atom has a small, dense, positively-charged nucleus.

Question 17

What is the ground state of an atom?

Answer 17

The ground state is the state of an atom when the electrons are in the lowest available energy levels.

Question 18

What is the excited state of an atom?

Answer 18

Atoms are in the excited state when at least one electron is at a higher energy level.

Question 19

What is an orbital?

Answer 19

An orbital is a three-dimensional region around the nucleus of the atom which describes the electron’s most probable location.

Question 20

What is electron configuration?

Answer 20

The electron configuration is the arrangement of the electrons (into the energy levels) of an atom.

Question 21

What are valence electrons?

Answer 21

Valence electrons are the electrons in the outermost energy level of an atom.

Each atom can have from 0 to 8 valence electrons. In an electron configuration, the valence electrons are the last number.

Question 22

What are Lewis dot diagrams?

Answer 22

Lewis dot diagrams are diagrams that depict the valence electrons as dots around the symbol of an element.

Question 23

Is the electron configuration 2-8-13-1 a ground state or an excited state configuration?

Answer 23

2-8-13-1 is a ground state electron configuration.

Do determine whether an electron configuration is ground or excited, add the electrons in the configuration (24 in this question). Then look for the electron configuration for the element with that atomic number. If the electron configurations match, it is ground state; if they don’t match, it is excited state.

Question 24

The diagram below shows bright-line spectra of selected gases. Identify the two gases in the unknown mixture.

Answer 24

The mixture is made up of Gas A and Gas D. All of the spectral lines in Gas A and Gas D are present in the mixture.

Question 25

An atom has an atomic number of 7 and an electron configuration of 2-3-2. Explain why the number of electrons in the second and third shells shows that this atom is in an excited state.

Answer 25

The third shell has one electron before the second shell is completely filled.

or

The electron configuration is not 2-5, which is the ground state for an atom with atomic number 7.

Question 26

Argon has three isotopes: ³⁶Ar, ³⁸Ar, and ⁴⁰Ar. Based on argon’s atomic mass of 39.948 u, which isotope is most abundant?

Answer 26

Argon’s most abundant isotope is ⁴⁰Ar.

Since atomic mass is a weighted average, the atomic mass is generally closest to the mass of the most abundant isotope.