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Modern Atomic Theoryand the Periodic Table Chapter 10

Modern Atomic Theoryand the Periodic Table Chapter 10

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Chapter 10 - Modern Atomic Theory and the Periodic Table

10.1 A Brief History

10.2 Electromagnetic Radiation

10.3 The Bohr Atom

10.5 Atomic Structures of the First 18 Elements

10.6 Electron Structures and the Periodic Table

10.4 Energy Levels of Electrons

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A B r ie f H is to ry o f A to m ic T h eo ry

W hile th ese m od e ls w o rk re a so na b ly w e llth e ir lim ita ton s h av e le d to m o re m o de rn the or ies

a s to th e n a tu re o f the a to m .

L im ita tio ns o f D a lton 's m od e lle d to th e T h om pson an d R u th e rfo rd

m o de ls o f the a to m .

T h e ir e xpe rim en ts led toD a lto n 's A to m ic T h eo ry

E a rly ch e m is tsp e rfo rm ed expe r im en ts

G re eks w e re th e f i r st to su gg e stth a t m a tte r is m a d e u p o f a to m s

A Brief HistoryA Brief History

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light from the sun x-rays microwaves radio waves television waves radiant heat

All show wavelike behavior.

Each travels at the same speed in a vacuum.

3.00 x 108 m/s

Electromagnetic RadiationElectromagnetic Radiation

Examples

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Wavelength (λ)

Characteristics of a WaveCharacteristics of a WaveCharacteristics of a WaveCharacteristics of a Wave

wavelength(measured from peak to peak)

wavelength(measured from trough to trough)

10.1

Light has the properties of a wave.

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Frequency () is the number of wavelengths that pass a particular point per second.

10.1

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Speed (v) is how fast a wave moves through space.

10.1

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• Light also exhibits the properties of a particle. Light particles are called photons.

• Both the wave model and the particle model are used to explain the properties of light.

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10.2

visible light is part of the electromagnetic

spectrum

X-rays are part of the electromagnetic

spectrum

Infrared light is part of the

electromagnetic spectrum

The Electromagnetic Spectrum

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• At high temperatures or voltages, elements in the gaseous state emit light of different colors.

• When the light is passed through a prism or diffraction grating a line spectrum results.

The Bohr AtomThe Bohr Atom

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Line spectrum of hydrogen. Each line corresponds to the wavelength of the energy emitted when the electron of a hydrogen atom, which has absorbed energy falls back to a lower principal energy level.

These colored lines indicate that light is being emitted only at certain wavelengths.

Each element has its own unique set of spectral emission lines that distinguish it from other elements.

10.3

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Niels Bohr, a Danish physicist, in 1912-1913 carried out researchon the hydrogen atom.

Niels BohrNiels BohrNiels BohrNiels Bohr

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Electrons revolve around the nucleus in orbits that are located at fixed distances from the nucleus.

10.4

An electron has a discrete energy when it occupies an orbit.

The Bohr Atom

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When an electron falls from a higher energy level to a lower energy level a quantum of energy in the form of light is emitted by the atom.

10.4

The color of the light emitted corresponds to one of the lines of the hydrogen spectrum.

The Bohr Atom

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Different lines of the hydrogen spectrum correspond to different electron energy level shifts.

10.4

The Bohr Atom

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Light is not emitted continuously. It is emitted in discrete packets called quanta.

10.4

The Bohr Atom

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An electron can have one of several possible energies depending on its orbit.

E2 E3E1

10.4

The Bohr Atom

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Bohr’s calculations succeeded very well in correlating the experimentally observed spectral lines with electron energy levels for the hydrogen atom.

• Bohr’s methods did not succeed for heavier atoms.

• More theoretical work on atomic structure was needed.

The Bohr Atom

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In 1924 Louis De Broglie suggested that all objects have wave properties.

– De Broglie showed that the wavelength of ordinary sized objects, such as a baseball, are too small to be observed.

– For objects the size of an electron the wavelength can be detected.

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In 1926 Erwin Schröedinger created a mathematical model that showed electrons as waves.

– Schröedinger’s work led to a new branch of physics called wave or quantum mechanics.

– Using Schröedinger’s wave mechanics, the probability of finding an electron in a certain region around the atom can be determined.

– The actual location of an electron within an atom cannot be determined.

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Based on wave mechanics it is clear that electrons are not revolving around the nucleus in orbits.

• Instead of being located in orbits, the electrons are located in orbitals.

• An orbital is a region around the nucleus where there is a high probability of finding an electron.

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According to Bohr the energies of electrons

in an atom are quantized.

The wave-mechanical model of the atom also predicts discrete principal energy levels within the atom

Energy Levels of ElectronsEnergy Levels of Electrons

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The first four principal energy levels of the hydrogen atom.

As n increases, the energy of the electron increases.

10.7

Each level is assigned a principal quantum number n.

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Each principal energy level is subdivided into sublevels.

10.7, 10.8

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Within sublevels the electrons are found in orbitals.

An s orbital is spherical in shape.

The spherical surface encloses a space where there is a 90% probability that the electron may be found.

10.10

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An electron can spin in one of two possible directions represented by ↑ or ↓.

The two electrons that occupy an atomic orbital must have opposite spins.

This is known as the Pauli Exclusion Principal.

10.10

An atomic orbital can hold a maximum of two electrons.

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Each p orbital has two lobes.

Each p orbital can hold a maximum of two electrons.

A p sublevel can hold a maximum of 6 electrons.

10.10

A p sublevel is made up of three orbitals.

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The three p orbitals share a common center.

10.10

pxpy

pz

The three p orbitals point in different directions.

The P orbitals

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The five d orbitals all point in different directions.

Each d orbital can hold a maximum of two electrons.

A d sublevel can hold a maximum of 10 electrons.

10.11

A d sublevel is made up of five orbitals.

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n=1 1s

n=2 2s 2p 2p 2p

n = 3 3s 3p 3p 3p 3d 3d 3d 3d 3d

n = 4 4s 4p 4p 4p 4d 4d 4d 4d 4d 4f 4f 4f 4f 4f 4f 4f

Distribution of Subshells by Principal Energy Level

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The Hydrogen Atom

In the ground state hydrogen’s single electron lies in the 1s orbital.

Hydrogen can absorb energy and the electron will move to excited states.

10.12

• The diameter of hydrogen’s nucleus is about 10-13 cm.

• The diameter of hydrogen’s electron cloud is about 10-8 cm.

• The diameter of hydrogen’s electron cloud is about 100,000 times greater than the diameter of its nucleus.

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To determine the electronic structures of atoms, the following guidelines are used.

Atomic Structure of the Atomic Structure of the First 18 ElementsFirst 18 Elements

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1. No more than two electrons can occupy one orbital

10.10

Pauli exclusion principle

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1 s orbital

2. Electrons occupy the lowest energy orbitals available. They enter a higher energy orbital only after the lower orbitals are filled.

3. For the atoms beyond hydrogen, orbital energies vary as s<p<d<f for a given value of n.

2 s orbital

10.10

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4. Each orbital in a sublevel is occupied by a single electron before a second electron enters. For example, all three p orbitals must contain one electron before a second electron enters a p orbital.

10.10

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Nuclear makeup and electronic structure of each principal energy level of an atom.

number of electronsin each sublevel

number of protons and neutrons in the nucleus

10.13

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Electron Configuration

Arrangement of electrons within their respective sublevels. 2p6Principal

energy levelType of orbital

Number of electrons in

sublevel orbitals

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In the following diagrams boxes represent orbitals.

• Electrons are indicated by arrows: ↑ or ↓.– Each arrow direction represents one of

the two possible electron spin states.

Orbital FillingOrbital FillingOrbital FillingOrbital Filling

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↑ 1s2↓

H ↑ 1s1

Hydrogen has 1 electron. It will occupy the orbital of lowest energy which is the 1s.

He

Helium has two electrons. Both helium electrons occupy the 1s orbital with opposite spins.

Filling the 1s Sublevel

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Li

1s22s2

The 1s orbital is filled. Lithium’s third electron will enter the 2s orbital.

↑↓ ↑

↓

1s 2s

↑

1s22s1

Be ↑↓

The 2s orbital fills upon the addition of beryllium’s third and fourth electrons.

1s 2s

Filling the 2s Sublevel

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B 1s22s22p1

1s 2s 2p

↑↓ ↑↓ ↑

Boron has the first p electron. The three 2p orbitals have the same energy. It does not matter which orbital fills first.

C1s 2s 2p

↑↓ ↑↓

The second p electron of carbon enters a different p orbital than the first p electron so as to give carbon the lowest possible energy.

1s22s22p2↑ ↑

Filling the 2p Sublevel

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↑ 1s22s22p4↑ ↑

1s 2s 2p

↑↓ ↑↓O

There are four electrons in the 2p sublevel of oxygen. One of the 2p orbitals is now occupied by a second electron, which has a spin opposite to that of the first electron already in the orbital.

↓

↑ ↑ ↑N1s 2s 2p

↑ ↓↓ ↑

The third p electron of nitrogen enters a different p orbital than its first two p electrons to give nitrogen the lowest possible energy.

1s22s22p3

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↑↓ ↑↓ ↑↓

2p

Ne1s 2s

↑↓ ↑↓

There are 6 electrons in the 2p sublevel of neon, which fills the sublevel.

1s22s22p6

↑↓ ↑↓ ↑

2p

F1s 2s

↑↓ ↑↓

There are five electrons in the 2p sublevel of fluorine. Two of the 2p orbitals are now occupied by a second electron, which has a spin opposite to that of the first electron already in the orbital.

1s22s22p5

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Na 1s22s22p63s1

1s 2s 2p 3s

↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑

The 2s and 2p sublevels are filled. The next electron enters the 3s sublevel of sodium.

Mg1s 2s 2p 3s

↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑

The 3s orbital fills upon the addition of magnesium’s twelfth electron.

1s22s22p63s2↓

Filling the 3s Sublevel

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Electron Filling Order

Sublevel energy level order:

1s < 2s < 2p < 3s < 3p <

4s < 3d < 4p < 5s < 4d <

5p < 6s < 4f < 5d < 6p <

7s < 5f < 6dYou can memorize this sequence or....

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In 1869 Dimitri Mendeleev of Russia and Lothar Meyer of Germany independently published periodic arrangements of theelements based on increasing atomic masses.

Mendeleev’s arrangement is the precursor to the modern periodic table.

Electron Structures and the Electron Structures and the Periodic TablePeriodic Table

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Horizontal rows are called periods.

Period numbers correspond to the highest occupied energy level.

10.14

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Elements in the A groups are designated

representative elements.

10.14

Elements in the B groups are designated transition

elements.

Elements with similar properties are organized in groups or families.

Groups are numbered with Roman numerals.

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10.15

For A family elements the valence electron configuration is the same in each column.

The chemical behavior and properties of elements in a family are associated with the electron configuration of its elements.

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10.15

With the exception of helium which has a filled s orbital, the nobles gases have filled p orbitals.

To write an electron configurationusing a noble-gas core:

2. Write the elemental symbol of the noble gas in square brackets, followed

by the remaining configuration

1. Find the highest atomic-numberednoble gas (Group 8A element)less than the atomic numberof the element for which theconfiguration is being written

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The electron configuration of any of the noble gas elements can be represented by the symbol of the element enclosed in square brackets.

B 1s22s22p1 [He]2s22p1

Cl 1s22s22p63s23p5 [Ne]3s23p5

Na 1s22s22p63s1 [Ne]3s1

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The electron configuration of argon is

Ar 1s22s22p63s23p6

Ca 1s22s22p63s23p6 4s2 [Ar]4s2

K 1s22s22p63s23p64s1 [Ar]4s1

The elements after argon are potassium and calcium Instead of entering a 3d orbital, the valence electrons of these elements enter the 4s orbital.

Exceptions to the conventional filling order:

1. d4 configurations generally do not exist

Chromium (Z = 24):Systematic prediction:

Cr: [Ar]4s23d4

But d4 is not likely,so promote an electron from the 4s sublevel:

Cr: [Ar]4s13d5

2. d9 configurations generally do not exist

Copper (Z = 29):Systematic prediction:

Cu: [Ar]4s23d9

But d9 is not likely,so promote an electron from the 4s sublevel:

Cu: [Ar]4s13d10

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10.16

d orbital filling d orbital numbers are 1 less

than the period number

Arrangement of electronsaccording to sublevel being filled.

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10.16

f orbital filling f orbital numbers are 2 less

than the period number

Arrangement of electronsaccording to sublevel being filled.

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10.17

Period number corresponds with the highest energy level occupied by

electrons in that period.

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10.17

The group numbers for the representative elements are equal to the total number of

outermost electrons in the atoms of the group.

The elements of a family have the same outermost electron configuration except that the electrons are in different energy levels.

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Chapter 10 - Modern Atomic Theory and the Periodic Table 10.1 A Brief History

10.2 Electromagnetic Radiation

10.3 The Bohr Atom – Niels Bohr description of the atom (electron orbitals).

10.4 Energy Levels of Electrons – Electron configuration (from the periodic table), s, p, d, and f orbitals.

10.5 Atomic structures of the First 18 Elements – Valence electrons, Representatives and Transition elements, Families names.

10.6 Electron Structures and the Periodic table – Relationship between group number and valence electrons.