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Chapter 6

The Periodic Table

6.1 Organizing the Elements

6.2 Classifying the Elements

6.3 Periodic Trends

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Although the weather changes from day to day, the weather you experience is related to your location on the globe.

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CHEMISTRY & YOU

How are trends in the weather similar to trends in the properties of elements?

1. How is knowing the trends in weather for a specific region helpful?

2. Consider how knowing trends in elemental properties might be helpful to scientists.

Bellringer

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Independent Activity

Activity: In your NOTES section…

1. Create a circle map for the term: Atomic Radius. See page 174-175

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Trends in Atomic Size

What are the trends among the elements for atomic size?

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Trends in Atomic Size

One way to think about atomic size is to look at the units that form when atoms of the same element are joined to one another.

These units are called molecules.
Because the atoms in each molecule shown below are identical, the distance between the nuclei of these atoms can be used to estimate the size of the atoms.

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Trends in Atomic Size

This size is expressed as an atomic radius.

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The atomic radius is one-half of the distance between the nuclei of two atoms of the same element when the atoms are joined.

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Trends in Atomic Size

The distances between atoms in a molecule are extremely small.

The atomic radius is often measured in picometers (pm).

Recall that there are one trillion, or 10¹², picometers in a meter.

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Trends in Atomic Size

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The distance between the nuclei in an iodine molecule is 280 pm.

Because the atomic radius is one-half the distance between the nuclei, a value of 140 pm (280/2) is assigned to the radius of the iodine atom.

Distance between the nuclei

280 pm

140 pm

Atomic radius

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Trends in Atomic Size

In general, atomic size increases from top to bottom within a group, and decreases from left to right across a period.

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The atomic radius within these groups increases as the atomic number increases.

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Group Trends in Atomic Size

Atomic radius (pm)

Atomic number

Interpret Graphs

This increase is an example of a trend.

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Trends in Atomic Size

As the atomic number increases within a group, the charge on the nucleus increases and the number of occupied energy levels increases.

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Group Trends in Atomic Size

These variables affect atomic size in opposite ways.

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Trends in Atomic Size

The increase in positive charge draws electrons closer to the nucleus.

The increase in the number of occupied orbitals shields electrons in the highest occupied energy level from the attraction of protons in the nucleus.
The shielding effect is greater than the effect of the increase in nuclear charge, so the atomic size increases.

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Group Trends in Atomic Size

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Trends in Atomic Size

Across a period, the electrons are added to the same principal energy level.

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Periodic Trends in Atomic Size

Atomic radius (pm)

Atomic number

The shielding effect is constant for all elements in a period.
The increasing nuclear charge pulls the electrons in the highest occupied energy level closer to the nucleus, and the atomic size decreases.

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Trends in Atomic Size

The figure below summarizes the group and period trends in atomic size.

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What are the trends for atomic size?

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What are the trends for atomic size?

In general, atomic size increases from top to bottom within a group and decreases from left to right across a period.

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Ions

How do ions form?

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Ions

Some compounds are composed of particles called ions.

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An ion is an atom or group of atoms that has a positive or negative charge.

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Ions

An atom is electronically neutral because it has equal numbers of protons and electrons.

For example, an atom of sodium (Na) has 11 positively charged protons and 11 negatively charged electrons.

The net charge on a sodium atom is zero [(+11) + (-11) = 0].

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Ions

Positive and negative ions form when electrons are transferred between atoms.

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Atoms of metals, such as sodium, tend to form ions by losing one or more electrons from their highest occupied energy levels.

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Ions

In the sodium ion, the number of electrons (10) is not equal to the number of protons (11).

Because there are more positively charged protons than negatively charged electrons, the sodium ion has a net positive charge.

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Ions

An ion with a positive charge is called a cation.

The charge for a cation is written as a number followed by a plus sign.
If the charge is 1, the number in 1+ is usually omitted from the symbol for the ion.

For example, Na¹⁺ is written as Na⁺.

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Ions

Atoms of nonmetals, such as chlorine, tend to form ions by gaining one or more electrons.

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In a chloride ion, the number of electrons (18) is not equal to the number of protons (17).
Because there are more negatively charged electrons than positively charged protons, the chloride ion has a net negative charge.

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Ions

An ion with a negative charge is called an anion.

The charge for an anion is written as a number followed by a minus sign.

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What type of element tends to form anions? What type tends to form cations?

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What type of element tends to form anions? What type tends to form cations?

Nonmetals tend to form anions. Metals tend to form cations.

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Trends in Ionization Energy

What are the trends among the elements for first ionization energy?

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Trends in Ionization Energy

Electrons can move to higher energy levels when atoms absorb energy. Sometimes the electron has enough energy to overcome the attraction of the protons in the nucleus.

The energy required to remove an electron from an atom is called ionization energy.

This energy is measured when an element is in its gaseous state.

The energy required to remove the first electron from an atom is called the first ionization energy.

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Trends in Ionization Energy

First ionization energy tends to decrease from top to bottom within a group and increase from left to right across a period.

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Ionization Energies of Some Common Elements
Symbol	First	Second	Third
H	1312
He (noble gas)	2372	5247
Li	520	7297	11,810
Be	899	1757	14,840
C	1086	2352	4619
O	1314	3391	5301
F	1681	3375	6045
Ne (noble gas)	2080	3963	6276
Na	496	4565	6912
Mg	738	1450	7732
S	999	2260	3380
Ar (noble gas	1520	2665	3947
K	419	3096	4600
Ca	590	1146	4941

Interpret Data

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Trends in Ionization Energy

Ionization energies can help you predict what ions an element will form.

It is relatively easy to remove one electron from a Group 1A metal atom, but it is difficult to remove a second electron.

This difference indicates that Group 1A metals tend to form ions with a 1+ charge.

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Group Trends in Ionization Energy

First ionization energy (kJ/mol)

Atomic number

Interpret Graphs

Look at the data for noble gases and alkali metals.

In general, first ionization energy generally decreases from top to bottom within a group.

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Trends in Ionization Energy

The atomic size increases as the atomic number increases within a group.

As the size of the atom increases, nuclear charge has a smaller effect on the electrons in the highest occupied energy level.

Less energy is required to remove an electron from this energy level, and the first ionization energy is lower.

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Group Trends in Ionization Energy

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Trends in Ionization Energy

In general, the first ionization energy of representative elements increases from left to right across a period.

This trend can be explained by the nuclear charge and the shielding effect.

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Periodic Trends in Ionization Energy

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Trends in Ionization Energy

The nuclear charge increases across the period, but the shielding effect remains constant.

As a result, there is an increase in the attraction of the nucleus for an electron.
Thus, it takes more energy to remove an electron from an atom.

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Periodic Trends in Ionization Energy

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Trends in Ionization Energy

The figure below summarizes the group and period trends for the first ionization energy.

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Trends in Ionic Size

What are the trends among the elements for ionic size?

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Trends in Ionic Size

During reactions between metals and nonmetals, metal atoms tend to lose electrons and nonmetal atoms tend to gain electrons.

This transfer of electrons has a predictable effect on the size of the ions that form.
Cations are always smaller than the atoms from which they form.
Anions are always larger than the atoms from which they form.

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Trends in Ionic Size

Ionic size tends to increase from top to bottom within a group. Generally, the size of cations and anions decreases from left to right across a period.

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Trends in Ionic Size

For each of these elements, the ion is much smaller than the atom.

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Group Trends in Ionic Size

The radius of a sodium ion (95 pm) is about half the radius of a sodium atom (191 pm).
When a sodium atom loses an electron, the attraction between the remaining electrons and the nucleus is increased. As a result, the electrons are drawn closer to the nucleus.

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Trends in Ionic Size

Metals that are representative elements tend to lose all their outermost electrons during ionization.

Therefore, the ion has one fewer occupied energy level.

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Group Trends in Ionic Size

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Trends in Ionic Size

The trend is the opposite for nonmetals, like the halogens in Group 7A.

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Group Trends in Ionic Size

For each of these elements, the ion is much larger than the atom.

For example, the radius of a fluoride ion (133 pm) is more than twice the radius of a fluorine atom (62 pm).
As the number of electrons increases, the attraction of the nucleus for any one electron decreases.

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Trends in Ionic Size

From left to right across a period, two trends are visible—a gradual

decrease in the

size of the positive

ions (cations),

followed by a

gradual decrease

in the size of the

negative ions

(anions).

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Period Trends in Ionic Size

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Trends in Ionic Size

The figure below summarizes the group and period trends in ionic size.

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What are the trends for ionic size?

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What are the trends for ionic size?

Ionic size tends to increase from top to bottom within a group. Generally, the size of cations and anions decreases from left to right across a period.

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Trends in Electronegativity

What are the trends among the elements for electronegativity?

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Trends in Electronegativity

There is a property that can be used to predict the type of bond that will form during a reaction.

This property is called electronegativity.
Electronegativity is the ability of an atom of an element to attract electrons when the atom is in a compound.

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Scientists use factors such as ionization energy to calculate values for electronegativity.

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Electronegativity Values for Selected Elements
H 2.1
Li 1.0	Be 1.5	B 2.0	C 2.5	N 3.0	O 3.5	F 4.0
Na 0.9	Mg 1.2	Al 1.5	Si 1.8	P 2.1	S 2.5	Cl 3.0
K 0.8	Ca 1.0	Ga 1.6	Ge 1.8	As 2.0	Se 2.4	Br 2.8
Rb 0.8	Sr 1.0	In 1.7	Sn 1.8	Sb 1.9	Te 2.1	I 2.5
Cs 0.7	Ba 0.9	Tl 1.8	Pb 1.9	Bi 1.9

Interpret Data

The data in this table is expressed in Pauling units.

This table lists electronegativity values for representative elements in Groups 1A through 7A.

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Trends in Electronegativity

In general, electronegativity values decrease from top to bottom within a group. For representative elements, the values tend to increase from left to right across a period.

Metals at the far left of the periodic table have low values.
By contrast, nonmetals at the far right (excluding noble gases) have high values.
Values among transition metals are not as regular.

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Trends in Electronegativity

The least electronegative element in the table is cesium, with an electronegativity of 0.7.

It has the least tendency to attract electrons.
When it reacts, it tends to lose electrons and form cations.

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Trends in Electronegativity

The most electronegative element is fluorine, with a value of 4.0.

Because fluorine has such a strong tendency to attract electrons, when it is bonded to any other element it either attracts the shared electrons or forms an anion.

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Trends in Electronegativity

This figure summarizes several trends that exist among the elements.

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What are the trends for electronegativity values?

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What are the trends for electronegativity values?

In general, electronegativity values decrease from top to bottom within a group. For representative elements, the values tend to increase from left to right across a period.

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You are familiar with using a weather map to identify trends in the weather. For example, certain areas are typically warmer than other areas. What trends in the properties of elements can you identify with the help of the periodic table?

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CHEMISTRY & YOU

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You are familiar with using a weather map to identify trends in the weather. For example, certain areas are typically warmer than other areas. What trends in the properties of elements can you identify with the help of the periodic table?

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CHEMISTRY & YOU

You can identify trends in atomic size, first ionization energy, ionic size, and electronegativity with the help of the periodic table.

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Key Concepts

In general, atomic size increases from top to bottom within a group and decreases from left to right across a period.

Positive and negative ions form when electrons are transferred between atoms.

First ionization energy tends to decrease from top to bottom within a group and decrease from left to right across a period.

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Key Concepts

Ionic size tends to increase from top to bottom within a group. Generally, the size of cations and anions decreases from left to right across a period.

In general, electronegativity values decrease from top to bottom within a group. For representative elements, the values tend to increase from left to right across a period.

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Glossary Terms

atomic radius: one-half the distance between the nuclei of two atoms of the same element when the atoms are joined
ion: an atom or group of atoms that has a positive or negative charge
cation: any atom or group of atoms with a positive charge
anion: any atom or group of atoms with a negative charge

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Glossary Terms

ionization energy: the energy required to remove an electron from an atom in its gaseous state
electronegativity: the ability of an atom to attract electrons when the atom is in a compound

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BIG IDEA

Electrons and the Structure of Atoms

Atomic size, ionization energy, ionic size, and electronegativity are trends that vary across periods and groups of the periodic table.
These trends can be explained by variations in atomic structure.
The increase in nuclear charge within groups and across periods explains many trends.
Within groups, an increase in electron shielding has a significant effect on these trends.

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