Atoms, Isotopes, and Ions

WHY?
Atoms are the fundamental building blocks of all substances. To begin to understand the properties

of atoms and how they combine to form molecules, you must be familiar with their composition

and structure.

LEARNING OBJECTIVES

• Understand the composition and structure of atoms, isotopes, and ions

• Understand how atomic symbols and names identify the number of particles composing an

atom, isotope, or ion

SUCCESS CRITERIA

• Use atomic symbols to represent different isotopes and ions

• Given one or more of the following items, determine the others: name, atomic symbol, atomic

number, mass number,- neutron number, and electron number

• Calculate the percent of the atomic mass that is located in the nucleus of an atom

• Compare the size of an atom to the size of the atomic nucleus

PREREQUISITES

• Calculation of percent

• Activity 01-1: Units of Measurement

• Activity 01-2: Unit Analysis

INFORMATION

Matter, which is anything that has mass and occupies space, is composed of substances and

mixtures of substances.

A substance, or more explicitly, a pure substance, is a variety of matter that has uniform and

constant composition. For example, pure water is a substance.

Mixtures are composed of two or more substances. For example, salt water is a mixture, even

though it is uniform, because the amount of salt in the water (the composition) can vary.

An element is a substance that cannot be decomposed into two or more other substances by

chemical or physical means. In nuclear reactions, however, one element can be converted into one

or more other elements. Only about 118 different elements are known to exist.

An atom is the smallest part of an element that can exist either alone or in combination with other

atoms.

Isotopes are atoms that have the same number of protons but different numbers of neutrons.

An ion is an atom or molecule with a positive or negative charge.

A cation is an ion with a positive charge.

An anion is an ion with a negative charge.

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The diagrams below show representations of sodium. Note that the diameter of an atom 10,000 times larger

than the diameter of the atomic nucleus.

Table 1 Subatomic Particles

Particle Mass (amu)* Charge

Proton 1.0073 +1

Neutron 1.0087 0

Electron 0.0005 -1

* Atomic mass unit (amu) is a unit of mass equal to 1.66054 x 10-27 kg.

KEY QUESTIONS

1. What are the three particles that comprise a sodium atom?

2. Which particles contribute most of the mass to the atom, and where are these particles located?

3. Which particles contribute most to the volume or size of the atom, and where are these particles
located?

4. What information is provided by the atomic number, Z, which is the subscript in the atomic

symbol?

5. What information is provided by the mass number, A, which is the superscript in the atomic

symbol?

6. What notation is used in the atomic symbol to indicate the charge of an atom or ion?

7. Given the definition of mass number and the information in Table I regarding the masses of

protons, neutrons, and electrons, why is the mass number approximately, but not exactly equal to,

the mass of an atom in amu?

8. How is the charge of the atom or ion determined from the number of protons, neutrons, and

electrons present?

9. What do all atoms and ions of sodium have in common?

10. In general, what feature of an atom identifies it as a particular element?

11. In general, how do isotopes of the same element differ?

12. How many isotopes of any particular element could there be? What might prevent all of these

isotopes from occurring naturally?

Exercises
1. Insert the missing information in the following table. The first row is completed for you to

provided as an example.

Name Symbol Z A

Number of

Neutrons

Number of

Electrons

Boron-10
10
5

B + 5 10 5 4

40
20

Ca 18

Oxygen – 16 8

U 146 92

9 19 9

17 18 18

39 20 19

2. Show how to calculate the mass of a proton, neutron, and electron in kilograms using the the

Table 1 and the equality statement: 1 amu = 1.66054 x 10-27 kg

The Periodic Table of the Elements

Why?
Substances that contain only atoms with the same number of protons are called elements. The

Periodic Table lists all the known elements in order of their atomic number and in columns that

depend on similarities in their chemical and physical properties. The Periodic Table is a useful tool

for both students and professionals to identify the properties of the elements and understand the

properties of molecules.

LEARNING OBJECTIVES

• Become familiar with the organization of the Periodic Table

• Appreciate both the diversity and commonalities in the chemical and physical properties of the

elements

SUCCESS CRITERIA

• Identify groups and periods in the Periodic Table

• Use the Periodic Table to provide information about the elements

PREREQUISITES

• Activity 02-1: Atoms, Isotopes and Ions

• Activity 02-2: Mass Spectrometry and Masses of Atoms

INFORMATION

Dmitri Mendeleev (1834 – 1907), a Russian scientist, constructed the first Periodic Table by listing

the elements in horizontal rows in order of increasing atomic mass. He started new rows whenever

necessary to place elements with similar properties in the same vertical column. Mendeleev found

that the correlations in properties between some elements in the columns were not perfect. These

observations led him to predict the existence of undiscovered elements and to wonder how the

table might be better organized. Later H.G.J. Moseley used x-ray spectra to refine the ordering and

show that atomic numbers rather than atomic masses should be used to order the elements.

In the Periodic Table, elements with similar properties occur in vertical columns called groups.

Two numbering conventions are used to label the groups. The older convention numbers the groups

using Roman numerals I through VIII followed by a letter A or B; the other convention numbers

each column 1 through 18. The A groups are known as the main group elements. The B groups are

called the transition elements. The group numbers IA through VIIIA in the older convention tells

you how many valence electrons an element has. The valence electrons are the outer electrons that

are most important in determining the chemical bonding and other properties of the element.

The horizontal rows of the table are called periods and are numbered 1 through 7 starting with the

row that only contains H and He.

Foundations of Chemistry

INFORMATION

There are three categories of elements in the Periodic Table: metals, nonmetals, and metalloids.

The metals are located in the left and center. They are good conductors of heat and electricity. The

nonmetals are in the upper right-hand comer. They are poor conductors of heat and electricity.

The metals and nonmetals are separated by the metalloids, which are six elements on a diagonal

line. These elements are B, Si, Ge, As, Sb, and Te. The metalloids are also called semimetals or

semiconductors because their conductivity is between that of metals and nonmetals. Metals readily

lose electrons to form positive ions, called cations, and nonmetals readily gain electrons to form

negative ions, called anions.

KEY QUESTIONS

1. What information about an element is provided in each box for that element in the Periodic Table

in the model?

2. What determines the sequence of the elements from the first to the last?

3. What determines where one row stops and another begins?

4. Where are the metals, nonmetals, and metalloids located?

5. What is the difference between a group and a period?

6. How can you determine the total number of electrons that an atom has from the Periodic Table?

7. How can you determine the number of valence electrons that atoms in groups 1, 2 and 13 through

18 have?

8. What are the five other elements like helium that are gases and are not very reactive?

EXERCISES

1. Write the name, symbol, atomic number, and average mass for the Group 2 metal in Period 3.

2. Write the name, symbol, average mass, and number of protons for the Group 16 nonmetal in

Period 2.

3. Write the name, symbol, average mass, and number of electrons for the Group 15 metalloid in

Period 4.

4. Write the name and symbol of the element that has 48 electrons.

5. Name two elements that have properties similar to sodium, Na. How many valence electrons do

each of these three elements have?

6. Name two elements that have properties similar to bromine, Br. How many valence electrons do

each of these three elements have?

7. Using atomic symbols, list the elements in Period 2 in order of increasing number of electrons.

8. Using atomic symbols, list the elements in Group 14 in order of increasing number of protons.

9. Using atomic symbols, list the elements in Group 12 in order of increasing atomic mass.

1. Boron10Row1:
2. 540 20 Ca:
3. 1040 20 Ca:
4. 540 20 Ca_2:
5. 40 20 CaOxygen 16:
6. 5Oxygen 16:
7. 10Oxygen 16:
8. 5Oxygen 16_2:
9. Oxygen 16Row1:
10. 5U:
11. 10U:
12. Oxygen 16Row2:
13. URow1:
14. Oxygen 16Row3:
15. URow2:
16. 1917:
17. Oxygen 16Row4:
18. URow3:
19. 17Row1:
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