2.2.2 Examining Atomic Structure

Question 1

atomic structure

Answer 1

• Isotopes of an element have the same number of protons, but different numbers of neutrons.
• The atomic mass unit (amu) is a convenient unit for describing the masses of atoms.
• The average atomic mass expresses the weighted average of the masses of all naturally occurring isotopes of an element.
• The difference in mass between a nucleus and its constituent nucleons is released as the binding energy, E = mc^2

Question 2

isotope

Answer 2

• Isotopes of an element have the same number of protons, but different numbers of neutrons.
• The number of protons in the nucleus of an atom determines the type of element. For example, every isotope of carbon has 6 protons in the nucleus. The number of protons is designated by a subscript number before the element symbol.
• The total number of protons and neutrons (the number of nucleons) in the nucleus of an atom is designated by a superscripted number before the element symbol. For
  example, carbon-12 (12C) has a total of 12 nucleons. Since 6 of these are protons, the other 6 have to be neutrons. Similarly, carbon-13 (13C) has a total of 13 nucleons—6 protons and 7 neutrons.

Question 3

kilogram

Answer 3

• The atomic mass unit (amu) is a convenient unit for describing the masses of atoms.
• The kilogram is too large of a unit to be convenient when dealing with atomic masses. For example, a single atom of carbon-12 has a mass of 1.9924 x 10–26 kg.
• The atomic mass unit (amu) is defined as one twelfth of the mass of an atom of carbon-12.
• The mass of an atom in atomic mass units is approximately the same as the total number of nucleons. Carbon-12 has a mass of 12 amu (by definition), while hydrogen-1 (1H) has a mass of approximately 1 amu.

Question 4

note

Answer 4

• The average atomic mass expresses the weighted average of the masses of all naturally occurring isotopes of an element. The average atomic mass is useful, because it takes into account the fact that a single sample of an element will usually consist of a mixture of isotopes of that element.
• Three isotopes of neon exist in nature—neon-20 (
  20Ne), neon-21 (21Ne), and neon-22 (22Ne). The average atomic mass of neon is found by multiplying the mass of each isotope of neon (in amu) by the relative abundance of that isotope, and then adding these numbers together.

Question 5

subatomic particles

Answer 5

• The mass of an atom is slightly lower than the masses of the subatomic particles of which it is composed. For example, helium-4 (4He) has a mass of 6.646482 x 10^–27 kg, while the sum of the masses of the subatomic particles of which it is composed is 6.696926 x 10^–27 kg. The actual mass is 5.044599 x 10^–29 kg less than the mass of its components!
• Einstein explained this mystery by showing that matter and energy are interconvertible. The difference in mass between a nucleus and its constituent nucleons is released as the binding energy, E = mc^2, where E is the binding energy, m is the change in mass, and c is the speed of light. However, during a chemical reaction, the mass of a given nucleus will remain constant.

Question 6

The _______________ defines an element.

Answer 6

atomic number

Question 7

On the periodic table, the atomic number given for an element

Answer 7

equals the number of protons in the element.

Question 8

Isotopes are atoms of the same element with

Answer 8

the same number of protons and a varying number of neutrons.

Question 9

The mass of a single atom of carbon-12 is 1.99 × 10−26 kg. What is the conversion factor to convert mass in atomic mass units to mass in kilograms, or what is the mass of 1.00 amu expressed in kilograms?

Answer 9

1.66 × 10^−27 kg

Question 10

When dealing with chemical reactions, what assumption is made?

Answer 10

The integrity of the nucleus of each reactant is maintained and mass is conserved.

Question 11

True or false?

The number of nucleons in an element defines the name of the element.

Answer 11

false

Question 12

Looking at this entry from the periodic table, which of the arrows points to the atomic number?

Answer 12

1

Question 13

There are two isotopes of chlorine. Below is a list of each and their mass and abundance. Calculate the atomic mass of chlorine.

Isotope Mass (amu) % Abundance
Chlorine-35 34.96885 75.771%
Chlorine-37 36.96590 24.229%

Answer 13

35.453 amu

Question 14

An atomic number represents the number of:

Answer 14

Protons

Question 15

Which of the following statements defines atomic mass?

Answer 15

Atomic mass is the weight of a single atom.