Why are Electrons Important? C hemistry Unit 4

Why are Electrons Important?Chemistry Unit 4

Why are Electrons Important?

Light, a form of electronic radiation, has characteristics of both a wave and a particle

Wavelike properties of electrons help relate atomic emission spectra, energy states of atoms, and atomic orbitals.

Atomic emission spectra corresponds to the release of energy from an electron changing atomic energy levels.

A set of three rules determines the arrangement in an atom.

Main Ideas

4:1 Light and Quantized Energy

• Compare the wave and particle natures of light.

• Define a quantum of energy, and explain how it is related to an energy change of matter.

• Contrast continuous electromagnetic spectra and atomic emission spectra.

Light and Quantized Energy

Objectives:

• Recall that in Rutherford's model, the atom’s mass is concentrated in the nucleus and electrons move around it.

• The model doesn’t explain how the electrons were arranged around the nucleus.

• The model doesn’t explain why negatively charged electrons aren’t pulled into the positively charged nucleus.

The Atom and Unanswered Questions

The Atom and Unanswered Questions

• In the early 1900s, scientists observed certain elements emitted visible light when heated in a flame.

• Analysis of the emitted light revealed that an element’s chemical behavior is related to the arrangement of the electrons in its atoms.

• In order to understand this relationship and the nature of atomic structure, it will be helpful to first understand the nature of light.

Wave Nature of Light

Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space.

• Visible light• Microwaves• X-rays• Radio waves

Wave Nature of Light

• The wavelength (λ) is the shortest distance between equivalent points on a continuous wave. (crest to crest, trough to trough)

• The frequency (ν) is the number of waves that pass a given point per second.• Hertz- SI unit for frequency= one wave/sec• Energy increases with increasing frequency

All waves can be described by several characteristics.

Wave Nature of Light

• The amplitude is the wave’s height from the origin to a crest.• Independent of wavelength and frequency

All waves can be described by several characteristics.

Wave Nature of Light

Wave Nature of LightThe speed of light (3.00 x 108 m/s) is the

product of it’s wavelength and frequency c = λν.

Wave Nature of Light• All electromagnetic waves, including visible

light travels at 3.00 x 108m/s in a vacuum.• Speed is constant but wavelengths and

frequencies vary.• Sunlight contains a continuous range of

wavelengths and frequencies.• A prism separates sunlight into a continuous

spectrum of colors.

Wave Nature of Light• The electromagnetic spectrum includes all

forms of electromagnetic radiation.• Not just visible light.

Wave Nature of Light

Particle Nature of Light

The wave model of light cannot explain all of light’s characteristics.

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

• A quantum is the minimum amount of energy that can be gained or lost by an atom.

Particle Nature of Light

Max Planck (1858-1947) – matter can gain or lose energy only in small amounts.• E=hv• Planck’s constant has a value of

6.626 x 10–34 J ● s.• Energy can only be emitted or absorbed in

whole number multiples of h.

Particle Nature of Light• The photoelectric effect is when electrons

are emitted from a metal’s surface when light of a certain frequency shines on it.

Particle Nature of Light• Albert Einstein proposed in 1905 that light

has a dual nature. Nobel prize in 1921. • A beam of light has wavelike and particlelike

properties.• A photon is a mass-less particle of

electromagnetic radiation with no mass that carries a quantum of energy.

Ephoton = hν Ephoton represents energy.h is Planck's constant.ν represents frequency.

Example Problem 1

Microwaves are used to cook food and transmit information. What is the wavelength of a microwave that has a frequency of 3.44 x 109 Hz?

Example Problem 2

Microwaves are used to cook food and transmit information. What is the wavelength of a microwave that has a frequency of 3.44 x 109 Hz? What is the amount of energy in this wavelength?

Example Problem 2

While an FM radio station broadcasts at a frequency of 94.7 MHz, an AM station broadcasts at a frequency of 820 kHz. What are the wavelengths of the two broadcasts?

Example Problem 2

While an FM radio station broadcasts at a frequency of 94.7 MHz, an AM station broadcasts at a frequency of 820 kHz. What are the wavelengths of the two broadcasts? What is the associated energy for each of these broadcasts?

Example Problem 3

Every object gets its color by reflecting a certain portion of visible light. The color is determined by the wavelength of the reflected photons, and therefore their energy. The blue color in some fireworks occurs when copper (I) chloride is heated to approximately 1500K and emits blue light of wavelength 4.50 x 102 nm. How much energy does one photon of this light carry?

Atomic Emission Spectrum

The atomic emission spectrum of an element is the set of frequencies of the electromagnetic waves emitted by the atoms of the element.

• Emission lines are specific to an element and can be used for identification.

Atomic Emission Spectrum

Absorption Spectra

The absorption spectra of an element is the set of frequencies of the electromagnetic waves absorbed by the atoms of the element.

Absorption lines are specific to an element and can be used for identification.

Emission vs. Absorption

Question?

What is the smallest amount of energy that can be gained or lost by an atom? A. electromagnetic photon B. beta particle C. quanta D. wave-particle

What is a particle of electromagnetic radiation with no mass called? A. beta particle B. alpha particle C. quanta D. photon

Question?

4:2 Quantum Theory of the Atom

Quantum Theory of the Atom

• Compare the Bohr and quantum mechanical models of the atom.

• Explain the impact of de Broglie's wave article duality and the Heisenberg uncertainty principle on the current view of electrons in atoms.

• Identify the relationships among a hydrogen atom's energy levels, sublevels, and atomic orbitals.

Objectives:

Bohr’s Model of the Atom

Bohr correctly predicted the frequency lines in hydrogen’s atomic emission spectrum.

• The lowest allowable energy state of an atom is called its ground state.

• When an atom gains energy, it is in an excited state.

Bohr’s Model of the Atom

• Bohr suggested that an electron moves around the nucleus only in certain allowed circular orbits.

• The smaller the electron’s orbit the lower the atom’s energy state or level

Bohr’s Model of the Atom

• Bohr suggested that an electron moves around the nucleus only in certain allowed circular orbits.

• The larger the electron’s orbit the higher the atom’s energy state or level.

Bohr’s Model of the Atom

• Each orbit was given a number, called the quantum number. The orbit closest to the nucleus is n=1

Bohr’s Model of the Atom

• Example: Hydrogen’s single electron is in the n = 1 orbit in the ground state. Atom does not radiate energy.

• When energy is added, the electron moves to the n = 2 orbit. Atom is excited.

• When electron moves from an excited state to ground state, a photon is emitted.

Electron States

Quantum Mechanical Model

The Quantum Mechanical Model of the Atom – this model progressed through a series of scientific findings:

• Louis de Broglie (1892–1987) hypothesized that particles, including electrons, could also have wavelike behaviors.• Like vibrating guitar strings – multiples of half

waves. • Orbiting electron – whole number of

wavelengths.

Quantum Mechanical Model

Quantum Mechanical Model

• The de Broglie equation predicts that all moving particles have wave characteristics.

λ represents wavelengthsh is Planck's constant.m represents mass of the particle.v represents velocity.

Example Problem

Why do we not notice the wavelengths of moving objects such as automobiles?

Quantum Mechanical Model

Heisenberg showed it is impossible to take any measurement of an object without disturbing it.

• The Heisenberg uncertainty principle states that it is fundamentally impossible to know precisely both the velocity and position of a particle at the same time.• Means that it is impossible to assign fixed

paths for electrons like the circular orbits as previously thought.

Quantum Mechanical Model

Heisenberg showed it is impossible to take any measurement of an object without disturbing it.

• The Heisenberg uncertainty principle states that it is fundamentally impossible to know precisely both the velocity and position of a particle at the same time.• The only quantity that can be known is the

probability for an electron to occupy a certain region around the nucleus.

Quantum Mechanical Model

Schrödinger expanded on the de Broglie wave particle theory and created the quantum mechanical model that we know today.

• treated electrons as waves in a model called the quantum mechanical model of the atom.

• Schrödinger’s equation applied equally well to elements other than hydrogen.

Quantum Mechanical Model

• Both models limit an electron’s energy to certain values. Unlike the Bohr model, the quantum mechanical model makes no attempt to describe the electron’s path around the nucleus.

• Electrons are located around the nucleus at a position that can be described only by a probability map. A boundary surface is chosen to contain the region that the electron can be expected to occupy 90% of the time.

Quantum Mechanical Model

• The wave function predicts a three-dimensional region around the nucleus called the atomic orbital.

Quantum Numbers and the Revised Model

The revised model defines the relationship between an electron’s energy level, sublevel and atomic orbitals.Four quantum numbers make up

the identification of each electron in an atom.

Atomic Orbitals

• Principal quantum number (n) indicates the relative size and energy of atomic orbitals. n specifies the atom’s major energy levels, called the principal energy levels.

Atomic Orbitals• Energy sublevels (s,p,d or f) are

contained within the principal energy levels.

Atomic Orbitals

• n= # of sublevels per principal energy levels.

Atomic Orbitals• Each energy sublevel relates to orbitals of different

shape.

Atomic Orbitals• Each orbital can contain 2 electrons

Atomic Orbitals

Question?

Which atomic suborbitals have a “dumbbell” shape? A. s

B. f

C. p

D. d

Question

Who proposed that particles could also exhibit wavelike behaviors? A. Bohr B. Einstein C. Rutherford D. de Broglie

4:3 Energy and Wavelength Relationships

Atomic EmissionAtomic emission spectra corresponds to the release of energy from an electron changing atomic energy levels. Hydrogen’s emission spectrum comprises three

series of lines. One series of lines are ultraviolet (Lyman series)

and one series is infrared (Paschen series). Visible wavelengths comprise the Balmer series.

Bohr’s Model of the Atom

Atomic Emission

The Bohr atomic model attributes these spectral lines to transitions from higher-energy states with larger electron orbits (ni) to lower-energy states with smaller electron orbits (nf).The change in energy of this jump can

be calculated with:ΔE = -2.178 x 10-18 J (1/n2

final – 1/n2initial)

Example Problems

For the Balmer series, electron orbit transition occur from larger orbits to the n=2 orbit (nfinal = 2). Calculate the change in energy and wavelengths for the following electron transitions. ninitail = 3,4,5, and 6.

4:4 Electron Configuration

Electron ConfigurationObjectives:

• Apply the Pauli exclusion principle, the aufbau principle, and Hund's rule to write electron configurations using orbital diagrams and electron configuration notation.

• Define valence electrons, and draw electron-dot structures representing an atom's valence electrons.

Electron ConfigurationThe arrangement of electrons in the atom is called

the electron configuration.

Electron ConfigurationThree rules/ principals define how electrons can

be arranged in atom’s orbitals.

1. The aufbau principle states that each electron occupies the lowest energy orbital available.

Electron Configuration

Electron Configuration2. The Pauli exclusion principle states that a

maximum of two electrons can occupy a single orbital, but only if the electrons have opposite spins.

• Electrons in orbitals can be represented by arrows in boxes and each electron has an associated spin.

Electron Configuration3. Hund’s rule states that single

electrons with the same spin must occupy each equal-energy orbital before additional electrons with opposite spins can occupy the same energy level orbitals.

• Electrons don’t like roommates.

Electron Arrangement

-Electron arrangement can be represented by two common different methods. • Orbital Diagram – boxes labeled with principle

energy level and sublevel associated with each orbital. Arrows are drawn up and down in the box to represent electrons and their spins.

Orbital Diagram

Orbital Diagram

Examples:

Electron Arrangement-Electron arrangement can be

represented by two common different methods. • Electron Configuration Notation- lists the

following in order: Principle energy number, sublevel, superscript of number of electrons in the sublevel. Electron distribution follows the main three rules. • Noble Gas Notation – abbreviated electron

configuration by substituting noble gas symbols for a long series of notation.

Electron Configuration Notation

Examples:

Electron Configuration

Electron Configuration

Valence Electrons

• Valence electrons are defined as electrons in the atom’s outermost orbitals—those associated with the atom’s highest principal energy level.

• Electron-dot structure consists of the element’s symbol representing the nucleus, surrounded by dots representing the element’s valence electrons.

Electron Dot Structure

Electrons are placed one at a time on the four sides of the symbol and then paired until used up. Side order doesn’t matter.Example:

Na

Cl

Valence Electrons

4:5 Accumulating Content

Accumulating Content

Wavelengths are often measured in nm or Å. How do measurements like these affect calculations?

Accumulating Content

Gamma rays can be very harmful radiation. What did you learn about the magnetic spectrum that helps you understand why?

Accumulating Content

Light travels slower in water than it does in air due to the change in densities of the medium; however its frequency remains the same. How does the wavelength of light change as it travels from air to water?

Question?

In the ground state, which orbital does an atom’s electrons occupy? A. the highest available

B. the lowest available C. the n = 0 orbital D. the d suborbital

Question?

The outermost electrons of an atom are called what? A. suborbitals B. orbitals C. ground state electrons D. valence electrons

Study Guide Key Concepts• All waves are defined by their wavelengths, frequencies,

amplitudes, and speeds. c = λν

• In a vacuum, all electromagnetic waves travel at the speed of light.

• All electromagnetic waves have both wave and particle properties.

• Matter emits and absorbs energy in quanta.Equantum = hν

Study Guide Key Concepts

• White light produces a continuous spectrum. An element’s emission spectrum consists of a series of lines of individual colors.

Study Guide Key Concepts

• Bohr’s atomic model attributes hydrogen’s emission spectrum to electrons dropping from higher-energy to lower-energy orbits.

∆E = E higher-energy orbit - E lower-energy orbit = E photon = hν

• The de Broglie equation relates a particle’s wavelength to its mass, its velocity, and Planck’s constant. λ = h / mν

• The quantum mechanical model of the atom assumes that electrons have wave properties.

• Electrons occupy three-dimensional regions of space called atomic orbitals.

Study Guide Key Concepts

• The arrangement of electrons in an atom is called the atom’s electron configuration.

• Electron configurations are defined by the aufbau principle, the Pauli exclusion principle, and Hund’s rule.

• An element’s valence electrons determine the chemical properties of the element.

• Electron configurations can be represented using orbital diagrams, electron configuration notation, and electron-dot structures.

Chapter Questions

The shortest distance from equivalent points on a continuous wave is the: A. frequency B. wavelength C. amplitude D. crest

Chapter Questions

The energy of a wave increases as ____. A. frequency decreases B. wavelength decreases C. wavelength increases D. distance increases

Chapter Question

Atom’s move in circular orbits in which atomic model? A. quantum mechanical model B. Rutherford’s model C. Bohr’s model D. plum-pudding model

Chapter QuestionIt is impossible to know precisely both the location and velocity of an electron at the same time because: A. the Pauli exclusion principle B. the dual nature of light C. electrons travel in waves D. the Heisenberg uncertainty

principle

Chapter Assessment 5

How many valence electrons does neon have? A. 0

B. 1

C. 2

D. 3

Chapter Questions

Spherical orbitals belong to which sublevel? A. s

B. p

C. d

D. f

Chapter Questions

What is the maximum number of electrons the 1s orbital can hold? A. 10

B. 2

C. 8

D. 1

Chapter Questions

In order for two electrons to occupy the same orbital, they must: A. have opposite charges B. have opposite spins C. have the same spin D. have the same spin and charge

Chapter Questions

How many valence electrons does boron contain? A. 1

B. 2

C. 3

D. 5

Chapter Questions

What is a quantum? A. another name for an atom B. the smallest amount of energy

that can be gained or lost by an atom

C. the ground state of an atom D. the excited state of an atom

The End