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Chapter 3: Electron Structure

and the Periodic Law

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PERIODIC LAW •  This is a statement about the behavior of the elements

when they are arranged in a specific order. •  In its present form the statement is: Elements with similar

chemical properties occur at regular (periodic) intervals when the elements are arranged in order of increasing atomic numbers.

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PERIODIC TABLE •  A periodic table is a tabular arrangement of the elements

based on the periodic law. •  In a modern periodic table, elements with similar

chemical properties are found in vertical columns called groups or families.

group/family

period

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PERIODIC TABLE GROUP OR FAMILY •  A vertical column of elements that have similar chemical

properties. •  Traditionally designated by a Roman numeral and a letter

(either A or B) at the top of the column. •  Designated only by a number from 1 to 18 in a modern but

as yet not universally-used designation.

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PERIODIC TABLE PERIOD •  A horizontal row of elements arranged according to

increasing atomic numbers. •  Periods are numbered from top to bottom of the periodic

table.

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APPEARANCE OF A MODERN PERIODIC TABLE •  In a modern table, elements 58-71 and 90-103 are not

placed in their correct periods, but are located below the main table.

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ELEMENTS AND THE PERIODIC TABLE •  Each element belongs to a group and period of the

periodic table. EXAMPLES OF GROUP AND PERIOD LOCATION FOR ELEMENTS

• Calcium, Ca, element # 20: group IIA, period 4 • Silver, Ag, element # 47: group IB, period 5 • Sulfur, S, element # 16: group VIA, period 3

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THE BOHR THEORY OF ELECTRON BEHAVIOR IN HYDROGEN ATOMS

•  Bohr proposed that the electron in a hydrogen atom moved in any one of a series of circular orbits around the nucleus.

•  The electron could change orbits only by absorbing or releasing energy.

•  This model was replaced by a revised model of atomic structure in 1926

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THE QUANTUM MECHANICAL MODEL OF ELECTRON BEHAVIOR IN ATOMS

•  According to the quantum mechanical model of electron behavior, the precise paths of electrons moving around the nucleus cannot be determined accurately.

•  Instead of circular orbits, the location and energy of electrons moving around the nucleus is specified using the three terms shell, subshell and orbital.

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SHELL •  The location of electrons in a shell is indicated by

assigning a number n to the shell and all electrons located in the shell.

•  The value of n can be 1, 2, 3, 4, etc. •  The higher the n value, the higher is the energy of the shell

and the contained electrons.

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SUBSHELL •  Each shell is made up of one or more subshells that are

designated by a letter from the group s, p, d, or f. •  The number of the shell to which a subshell belongs is

combined with the letter of the subshell to clearly identify subshells.

•  For example, a p subshell located in the third shell (n = 3) would be designated as a 3p subshell.

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•  The number of subshells located in a shell is the same as the number of the shell. Thus, shell number 3 (n = 3) contains three subshells, designated 3s, 3p, and 3d.

•  Electrons located in a subshell are often identified by using the same designation as the subshell they occupy. Thus, electrons in a 3d subshell are called 3d electrons.

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ATOMIC ORBITALS •  The description of the location and energy of an electron

moving around a nucleus is completed in the quantum mechanical model by specifying an atomic orbital in which the electron is located.

•  Each subshell consists of one or more atomic orbitals, which are specific volumes of space around the nucleus in which electrons move.

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•  Atomic orbitals are designated by the same number and letter used to designate the subshell to which they belong. Thus, an s orbital located in a 2s subshell would be called a 2s orbital.

•  All s subshells consist of a single s orbital. •  All p subshells consist of three p orbitals. •  All d subshells consist of five d orbitals. •  All f subshells consist of seven f orbitals.

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•  According to the quantum mechanical model, all types of atomic orbitals can contain a maximum of two electrons.

•  Thus, a single d orbital can contain a maximum of 2 electrons, and a d subshell that contains five d orbitals can contain a maximum of 10 electrons.

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ATOMIC ORBITAL SHAPES •  Atomic orbitals of different types have different shapes.

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Learning Check

Indicate the number and type of orbitals in each of the following: A. 4s sublevel

B. 3d sublevel

C. n = 3

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Solution

A. 4s sublevel one 4s orbital

B. 3d sublevel five 3d orbitals

C. n = 3 one 3s orbital, three 3p orbitals,

and five 3d orbitals

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Learning Check

The number of A. electrons that can occupy a p orbital is 1) 1 2) 2 3) 3 B. p orbitals in the 2p sublevel is 1) 1 2) 2 3) 3 C. d orbitals in the n = 4 energy level is 1) 1 2) 3 3) 5 D. electrons that can occupy the 4f sublevel are 1) 2 2) 6 3) 14

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Solution

The number of A. electrons that can occupy a p orbital is 2) 2 B. p orbitals in the 2p sublevel is 3) 3 C. d orbitals in the n = 4 energy level is 3) 5 D. electrons that can occupy the 4f sublevel are 3) 14

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THE ENERGY OF ELECTRONS IN ATOMS •  Electron energy increases with increasing n value. Thus, an

electron in the third shell (n = 3) has more energy than an electron in the first shell (n = 1).

•  For equal n values but different orbitals, the energy of electrons in orbitals increases in the order s, p, d and f. Thus, a 4p electron has more energy than a 4s electron.

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RELATIONSHIPS BETWEEN SHELLS, SUBSHELLS, ORBITALS AND ELECTRONS

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ELECTRONS AND CHEMICAL PROPERTIES •  The valence shell of an atom is the shell that contains

electrons with the highest n value. •  Atoms with the same number of electrons in the valence

shell have similar chemical properties.

Members of Group IIA(2)

magnesium calcium strontium 24

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Valence Electrons

The valence electrons § Determine the chemical properties of the elements. § Are the electrons in the highest energy level. § Are related to the Group number of the element. Example: Phosphorus has 5 valence electrons

5 valence electrons

P Group 5A(15) 1s2 2s2 2p6 3s2 3p3

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All the elements in a group have the same number of valence electrons. Example: Elements in Group 2A(2) have two (2)

valence electrons. Be 1s2 2s2

Mg 1s2 2s2 2p6 3s2

Ca [Ar] 4s2 Sr [Kr] 5s2

Groups and Valence Electrons

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ELECTRON OCCUPANCY OF SHELLS •  What do magnesium and calcium have in common?

•  What predictions can be made about the number of electrons in strontium’s valence shell?

•  What other element on this chart has similar properties to Mg, Ca, and Sr?

2 electrons in valence shell

• Sr has similar chemical properties to Mg and Ca, so it likely has 2 electrons in its valence.

Beryllium 27 27

Periodic Table and Valence Electrons

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State the number of valence electrons for each: A. O 1) 4 2) 6 3) 8

B. Al 1) 13 2) 3 3) 1

C. Cl 1) 2 2) 5 3) 7

Learning Check

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State the number of valence electrons for each. A. O 2) 6

B. Al 2) 3

C. Cl 3) 7

Solution

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State the number of valence electrons for each.

A. Calcium 1) 1 2) 2 3) 3

B. Group 6A (16)

1) 2 2) 4 3) 6 C. Tin 1) 2 2) 4 3) 14

Learning Check

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State the number of valence electrons for each. A. Calcium

2) 2 B. Group 6A (16)

3) 6 C. Tin

2) 4

Solution

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State the number of valence electrons for each. A. 1s2 2s2 2p6 3s2 3p1 B. 1s2 2s2 2p6 3s2

C. 1s2 2s2 2p5

Learning Check

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State the number of valence electrons for each. A. 1s2 2s2 2p6 3s2 3p1 3 B. 1s2 2s2 2p6 3s2 2

C. 1s2 2s2 2p5 7

Solution

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An electron configuration §  Lists the sublevels filling with electrons in order of

increasing energy. §  Uses superscripts to show the number of electrons

in each sublevel. §  For neon is as follows: number of electrons

sublevel 1s2 2s2 2p6

Electron Configuration

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THE ORDER OF SUBSHELL FILLING •  Electrons will fill subshells in the order of increasing energy

of the subshells. Thus, a 1s subshell will fill before a 2s subshell.

•  The order of subshell filling must obey Hund's rule and the Pauli exclusion principle.

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HUND'S RULE •  According to Hund's rule, electrons will not join other

electrons in an orbital of a subshell if an empty orbital of the same energy is available in the subshell.

•  Thus, the second electron entering a p subshell will go into an empty p orbital of the subshell rather than into the orbital that already contains an electron.

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THE PAULI EXCLUSION PRINCIPLE •  Electrons behave as if they spin on an axis. •  According to the Pauli exclusion principle, only electrons

spinning in opposite directions (indicated by ↑ and ↓) can occupy the same orbital within a subshell.

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FILLING ORDER FOR THE FIRST 10 ELECTRONS •  When it is remembered that each orbital of a subshell can

hold a maximum of two electrons, and that Hund's rule and the Pauli exclusion principle are followed, the following filling order for the first 10 electrons in atoms results.

H He Li Be B C N Ne

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Write the orbital diagrams for A. carbon

B. oxygen

C. magnesium

Learning Check

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Write the orbital diagrams for A. carbon

1s 2s 2p B. oxygen

1s 2s 2p C. magnesium

1s 2s 2p 3s

Solution

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FILLING ORDER FOR ALL SUBSHELLS IN ATOMS •  The filling order for any number

of electrons is obtained by following the arrows in the diagram.

•  Shells are represented by large rectangles.

•  Subshells are represented by small colored rectangles.

•  Orbitals within the subshells are represented by circles.

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AID TO REMEMBER SUBSHELL FILLING ORDER •  The diagram provides a compact way to remember the

subshell filling order. •  The correct order is given by

following the arrows from top to bottom of the diagram, going from the arrow tail to the head, and then from the next tail to the head, etc.

•  The maximum number of electrons each subshell can hold must also be remembered: s subshells can hold 2, p subshells can hold 6, d subshells can hold 10, and f subshells can hold 14.

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SUBSHELL FILLING ORDER AND THE PERIODIC TABLE

•  Notice the order of subshell filling matches the order of the subshell blocks on the periodic table, if the fill occurs in the order of increasing atomic numbers.

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EXAMPLES OF ELECTRON CONFIGURATIONS FOR ATOMS OF VARIOUS ELEMENTS

•  The following electronic configurations result from the correct use of any of the diagrams given earlier.

•  Magnesium, Mg, •  Silicon, Si, •  Iron, Fe, •  Galium, Ga,

12 electrons: 1s22s22p63s2 14 electrons: 1s22s22p63s23p2

26 electrons: 1s22s22p63s23p64s23d6

31 electrons: 1s22s22p63s23p64s23d104p1

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A. The correct electron configuration for nitrogen is 1) 1s2 2p5 2) 1s2 2s2 2p6 3) 1s2 2s2 2p3

B. The correct electron configuration for oxygen is 1) 1s2 2p6 2) 1s2 2s2 2p4 3) 1s2 2s2 2p6 C. The correct electron configuration for calcium is

1) 1s2 2s2 2p6 3s2 3p6 3d2

2) 1s2 2s2 2p6 3s2 3p6 4s2

3) 1s2 2s2 2p6 3s2 3p8

Learning Check

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A. The correct electron configuration for nitrogen is 3) 1s2 2s2 2p3

B. The correct electron configuration for oxygen is 2) 1s2 2s2 2p4

C. The correct electron configuration for calcium is

2) 1s2 2s2 2p6 3s2 3p6 4s2

Solution

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NOBLE GAS CONFIGURATIONS •  With the exception of helium, all noble gases (group VIIIA)

have electronic configurations that end with completely filled s and p subshells of the highest occupied shell. These configurations are called noble gas configurations.

•  Noble gas configurations can be used to write electronic configurations in an abbreviated form in which the noble gas symbol enclosed in brackets is used to represent all electrons found in the noble gas configuration.

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EXAMPLES OF THE USE OF NOBLE GAS CONFIGURATIONS

•  Magnesium:

•  Iron:

•  Galium:

[Ne]3s2. The symbol [Ne] represents the electronic configuration of neon, 1s22s22p6.

[Ar]4s23d6. The symbol [Ar] represents the electronic configuration of argon, 1s22s22p63s23p6.

[Ar]4s23d104p1. The symbol [Ar] represents the electronic configuration of argon, 1s22s22p63s23p6.

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Using the periodic table, write the electron configuration and abbreviated configuration for each of the following elements: A. Cd

B. Sr

C. I

Learning Check

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A. Cd 1s2 2s2 2p6 3s2 3p6 4s2 3d10

[Kr] 4s2 3d10 B. Sr

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2

[Kr] 5s2 C. I

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p5

[Kr] 5s2 4d10 5p5

Solution

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Learning Check

Give the symbol of the element that has A. [Ar]4s2 3d6

B. Four 3p electrons

C. Two electrons in the 4d sublevel D. The element that has the electron configuration

1s2 2s2 2p6 3s2 3p6 4s2 3d2

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Solution

Give the symbol of the element that has A. [Ar]4s2 3d6 Fe

B. Four 3p electrons S

C. Two electrons in the 4d sublevel Zr D. Electron configuration Ti 1s2 2s2 2p6 3s2 3p6 4s2 3d2

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PERIODIC TABLE CLASSIFICATIONS OF THE ELEMENTS •  The periodic table can be used to classify elements in

numerous ways: • by Distinguishing Electron. • by status as Representative, Transition, or Inner-Transition

Element. • by status as Metal, Nonmetal, or Metalloid.

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CLASSIFICATION ACCORDING TO DISTINGUISHING ELECTRONS

•  The distinguishing electron is the last electron listed in the electronic configuration of the element.

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REPRESENTATIVE, TRANSITION AND INNER-TRANSITION ELEMENTS

•  Elements are, again, classified according to the type of distinguishing electron they contain.

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METALS, METALLOIDS AND NONMETALS

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PROPERTY TRENDS WITHIN THE PERIODIC TABLE •  Properties of elements change in a systematic way within

the periodic table.

METALLIC AND NONMETALLIC PROPERTIES

•  Most metals have the following properties: high thermal conductivity, high electrical conductivity, ductility, malleability and metallic luster.

•  Most nonmetals have properties opposite those of metals and generally occur as brittle, powdery solids or as gases.

The Elements of Group VA(15)

nitrogen phosphorous

arsenic antimony

bismuth

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•  Metalloids are elements that form a diagonal separation zone between metals and nonmetals in the periodic table. Metalloids have properties between those of metals and nonmetals, and often exhibit some characteristic properties of each type.

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TRENDS IN METALLIC PROPERTIES •  Elements in the same period of the periodic table become

less metallic and more nonmetallic from left to right across the period.

•  Elements in the same group of the periodic table become more metallic and less nonmetallic from top to bottom down the group.

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TRENDS IN THE SIZE OF ATOMS •  For representative elements in the same period, atomic

size decreases from left to right in the period. •  For representative elements in the same group, atomic

size increases from top to bottom down the group.

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SCALE DRAWINGS OF REPRESENTATIVE ELEMENT ATOMS

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TRENDS IN FIRST IONIZATION ENERGY •  The first ionization energy is the energy required to remove

one electron from a neutral gaseous atom of an element. •  For representative elements in the same period, the

general trend is an increase from left to right across the period.

•  For representative elements in the same group, the general trend is a decrease from top to bottom down the group.

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TRENDS IN CHEMICAL REACTIVITY •  Based on the photo, what is the trend for chemical

reactivity with ethyl alcohol in group 1A(1)?

•  As the atomic number increases in group 1A(1), the chemical reaction becomes more vigorous. The rate of gas formation and the size of the bubbles indicate that reactivity increases from top to bottom in this family.

potassium sodium lithium

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Electronegativity Trends Electronegativity, is a chemical property that

describes the tendency of an atom or a functional group to attract electrons (or electron density) towards itself.

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Electron Affinity Trends electron affinity describes the ability of an atom to accept an electron.

Unlike electronegativity, electron affinity is a quantitative measure that measures the energy change that occurs when an electron is added to a neutral gas atom

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Melting Point Trends

Melting points are the amount of energy required to break a bond(s) to change the solid phase of a substance to a liquid. Generally, the stronger the bond between the atoms of an element, the higher the energy requirement in breaking that bond. Since temperature is directly proportional to energy, a high bond dissociation energy correlates to a high temperature. Melting points are varied and don't generally form a distinguishable trend across the periodic table. However, certain conclusions can be drawn from the following graph.

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Melting Point Trends

Metals generally possess a high melting point. Most non-metals possess low melting points. The non-metal carbon possesses the highest boiling

point of all the elements. The semi-metal boron also possesses a high melting point.