Electrons in Atoms.ppt

8/10/2019 Electrons in Atoms.ppt

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Section 5-1The Atom and Unanswered Questions

Recall that in Rutherford's model, theatoms mass is concentrated in the nucleusand electrons move around it.

The model doesnt explain how the electronswere arranged around the nucleus.

The model doesnt explain why negativelycharged electrons arent pulled into the

positively charged nucleus.


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Section 5-1The Atom and Unanswered Questions

In the early 1900s, scientists observedcertain elements emitted visible light whenheated in a flame.


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Section 5-1The Wave Nature of Light

Visible light is a type of electromagneticradiation, a form of energy that exhibitswave-like behavior as it travels throughspace.

All waves can be described by severalcharacteristics.


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Section 5-1The Wave Nature of Light (cont.)

The wavelength() is the shortestdistance between equivalent points on acontinuous wave.

The frequency() is the number of wavesthat pass a given point per second.

The amplitudeis the waves height from theorigin to a crest.


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The speed of light (3.00 108

m/s) is theproduct of its wavelength and frequencyc = .


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Sunlight contains a continuous range ofwavelengths and frequencies.

A prism separates sunlight into a continuousspectrum of colors.

The electromagnetic spectrumincludes allforms of electromagnetic radiation.


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Section 5-1The Particle Nature of Light

The wave model of light cannot explain allof lights characteristics.

Matter can gain or lose energy only in small,specific amounts called quanta.

A quantumis the minimum amount of energythat can be gained or lost by an atom.

Plancks constanthas a value of6.626 1034J s.


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Section 5-1The Particle Nature of Light (cont.)

The photoelectric effectis when electronsare emitted from a metals surface whenlight of a certain frequency shines on it.


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Section 5-1The Particle Nature of Light (cont.)

Albert Einstein proposed in 1905 that lighthas a dual nature.

A beam of light has wavelike and particlelikeproperties.

A photonis a particle of electromagneticradiation with no mass that carries a quantumof energy.

Ephoton= h Ephotonrepresents energy.his Planck's constant.represents frequency.


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Section 5-1Atomic Emission Spectra (cont.)


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Section 5-1Atomic Emission Spectra (cont.)

The atomic emission spectrumof anelement is the set of frequencies of theelectromagnetic waves emitted by theatoms of the element.

Each elements atomic emission spectrum isunique.


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Electrons move around the nucleus infixed paths or orbi tsmuch like the

planets move around the sun

Orbit positions, labeled with the integern, have specific potential energy

The lowest energy state of an atom is

called the ground state(an electron withn= 1 for a hydrogen atom)

Bohrs Model Of The Atom


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Atomic Emission Spectra

The excited atoms emit light to release

energy.


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Absorption And Emission

Electrons thatabsorbenergy areraised to a higher

energy level A particular

frequency of light isemittedwhen anelectron falls to alower energy level


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Bohr's Model of the Atom

Bohr correctly predicted the frequency linesin hydrogens atomic emission spectrum.

The lowest allowable energy state of an atomis called its ground state.

When an atom gains energy, it is in anexcited state.


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Bohr's Model of the Atom (cont.)

Bohr suggested that an electron movesaround the nucleus only in certain allowedcircular orbits.


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Each orbit was given a number, called thequantum number.


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Hydrogens single electron is in the n= 1orbit in the ground state.

When energy is added, the electron moves tothe n= 2 orbit.


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Bohrs model explained the hydrogensspectral lines, but failed to explain anyother elements lines.

The behavior of electrons is still not fully

understood, but it is known they do not movearound the nucleus in circular orbits.


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Wavelike properties of electrons help relate atomicemission spectra, energy states of atoms, andatomic orbitals.


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The Quantum Mechanical Model of the Atom

Louis de Broglie (18921987)hypothesized that particles, includingelectrons, could also have wavelikebehaviors.


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The Quantum Mechanical Model of the Atom(cont.)

The figure illustrates that electrons orbit thenucleus only in whole-numberwavelengths.


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The de Broglie equationpredicts that allmoving particles have wave characteristics.

represents wavelengthshis Planck's constant.

mrepresents mass of the particle.represents frequency.


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Heisenberg showed it is impossible to takeany measurement of an object withoutdisturbing it.

The Heisenberg uncertainty principle

states that it is fundamentally impossible toknow precisely both the velocity and positionof a particle at the same time.

The only quantity that can be known is theprobability for an electron to occupy a certainregion around the nucleus.


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Schrdinger treated electrons as waves ina model called the quantum mechanicalmodel of the atom.

Schrdingers equation applied equally well to

elements other than hydrogen.


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The wave function predicts a three-dimensional region around the nucleuscalled the atomic orbital.


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Hydrogen Atomic Orbitals

Principal quantum number(n) indicatesthe relative size and energy of atomicorbitals.

nspecifies the atoms major energy levels,

called the principal energy levels.


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Are a shorthand to describe characteristicsof an electrons position and to predict itsbehavior

n= pr inc ipal quantum number. All

orbitals with the same principle quantumnumber are in the same shel l

l = secondary quantum numberwhich

divides the orbitals in a shell into smallergroups called subshel ls

ml= magnetic quantum numberwhichdivides the subshells into individual

orbitals

Quantum Numbers


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n= roughly describes a distance of theelectrons from the nucleus. designated by integers: 1, 2, 3, 4, 5, 6,

l = describes the shape of the orbitals.

designated with numbers : 0, 1, 2, 3, 4, 5 or with letters: s, p, d, f, g, h

ml=describes the spatial orientation of the

orbital. designated by numbers specific to the

particular orbital

range fromlto +l

Quantum Numbers: What Do They Mean?


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Orbitals get larger as the principle quantum

number nincreases

Nodes, or regions of zero electron density,

appear beginning with the 2sorbital

.

s Orbitals And Nodes


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Possess a nodal plane that separates thelobes of high probability

Dot-density diagrams of the cross sectionof the probability distribution of a single (a)2pand (b) 3porbital showing the nodalplane and the size difference

p Orbitals


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Hydrogen Atomic Orbitals (cont.)

Each energy sublevel relates to orbitals ofdifferent shape.


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Hydrogen Atomic Orbitals (cont.)


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Ground-State Electron Configuration

The arrangement of electrons in the atomis called the electron configuration.

The aufbau principlestates that eachelectron occupies the lowest energy orbital

available.


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Electrons within

atoms interact with amagnet field in one oftwo ways:

clockwise (spin up)anti-clockwise (spin

down)

This gives rise to the

spin quantumnumber, ms

allowed values: + 1/2

or1/2

Electrons Behave Like Tiny Magnets

G d St t El t C fi ti


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Section 5-3Ground-State Electron Configuration (cont.)


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The Pauli exclusion principlestates thata maximum of two electrons can occupy asingle orbital, but only if the electrons haveopposite spins.

Hunds rulestates thatsingle electrons with thesame spin must occupy eachequal-energy orbital before

additional electrons withopposite spins can occupythe same energy levelorbitals.


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Ground-State Electron Configuration (cont.)


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Ground-State Electron Configuration (cont.)

Noble gas notation uses noble gassymbols in brackets to shorten innerelectron configurations of other elements.

G d St t El t C fi ti


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The electron configurations (for chromium,copper, and several other elements) reflectthe increased stability of half-filled andfilled sets of s and d orbitals.

V l El t


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Section 5-3Valence Electrons

Valence electronsare defined aselectrons in the atoms outermost orbitalsthose associated with the atoms highestprincipal energy level.

Electron-dot structureconsists of theelements symbol representing the nucleus,surrounded by dots representing theelements valence electrons.

Valence Electrons


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Section 5-3Valence Electrons (cont.)

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Electrons in Atoms.ppt