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Electrons in Atoms

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Chapter 5 Electrons in Atoms What we know so far about the atom Atoms have a nucleus ( made of protons and neutrons) surrounded by fast moving electrons. – PowerPoint PPT presentation

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Title: Electrons in Atoms


1
Chapter 5
  • Electrons in Atoms

2
What we know so far about the atom
  • Atoms have a nucleus ( made of protons and
    neutrons) surrounded by fast moving electrons.
  • All atoms positive charge and virtually all its
    mass is concentrated in the nucleus.
  • Always the number of protons in the nucleus
    equals the number of electrons.

3
Light and quantized energy
  • Light , a form of electromagnetic radiation, has
    characteristics of both wave and particle.
  • An electromagnetic radiation is a form of energy
    that exhibits wavelike behavior as it travels
    through space.
  • Examples of electromagnetic radiation
    microwaves, X rays and waves that carry radio and
    television programs

4
Characteristics of waves
  • Wavelength ( represented by Greek letter lambda)
    is the shortest distance between 2 equivalent
    points.
  • Frequency ( represented by the Greek letter nu)
    is the number of waves that passes a given point
    per second.
  • The amplitude of a wave is the waves height from
    the origin to crest, or from the origin to
    trough. Wavelength and frequency do not affect
    the amplitude of a wave.

5
How we make electromagnetic waves
  • Electromagnetic waves
  • are formed when an
  • electric field (shown with
  • blue arrows) couples
  • with a magnetic field
  • (shown with red arrows).

6
Electromagnetic wave relationship
  • C ( speed of light in vacuum) wavelength X
    frequency

7
Electromagnetic waves have different wavelength
8
Electromagnetic spectrum
  • The EM spectrum includes all forms of
    electromagnetic radiation, with the only
    differences in the type of radiation being their
    wavelength and frequencies.

9
Practice problems
  • Independent work
  • Textbook page 140 1,2,3 and 4

10
The particle nature of light
  • As mentioned before light , a form of
    electromagnetic radiation, has characteristics of
    both wave and particle.
  • When objects are heated they emit glowing light.
    Max Planck discovered that matter can gain or
    lose energy in small amounts called quanta. A
    quantum is the minimum amount of energy that can
    be gained or lost by an atom.

11
The photoelectric effect
  • In the photoelectric effect electrons, called
    photoelectrons, are emitted from a metals
    surface when light of a certain frequency, or
    higher than a certain frequency shines on a
    surface.

12
Formulas Need to know
  • Energy of a
  • quantum Planck constant X frequency
  • A photon is a massless particle that carries a
    quantum of energy
  • Energy of a
  • photon Planck constant X frequency

13
Practice problems
  • Independent work
  • Textbook page 143 5,6,7

14
Atomic emission spectra
  • The atomic emission spectrum of an element is a
    set of frequencies of the electromagnetic waves
    emitted by atoms of the element.
  • Each elements atomic emission spectrum is unique
    and can be used to identify an element or
    determine whether the element is part of an
    unknown compound.

15
Atomic emission spectrum for hydrogen, barium
and iron
16
Comparison of atomic emission spectra
  • http//jersey.uoregon.edu/vlab/elements/Elements.h
    tml

17
Quantum theory and the atom
  • Scientists tried to explain the relationship
    between atomic structure, electrons and atomic
    emission spectra
  • Wavelike properties of electrons help relate
    atomic emission spectra, energy states of atoms
    and atomic orbitals
  • .

18
5.2 Bohrs model of the atom
  • The lowest allowable energy state of an atom is
    called its ground state.
  • When an atom gains energy, it is said to be in an
    excited state.

19
Example for the atom of hydrogen
  • Ground state Excited state

20
Bohrs description of the hydrogen atom
  • The electron in a hydrogen atom moves around the
    nucleus only in certain allowed orbits. The lower
    the electrons orbit, the higher the energy state
    or energy level. Due to the electrons attraction
    to the nucleus.

21
The hydrogen line spectrum
  • In order to complete his calculations Bohr
    assigned a number n, called a quantum number, to
    each orbit.

Atoms orbit Quantum number Orbit radius nm Corresponding atomic energy level Relative energy
First n1 0.0529 1 E1
Second n2 0.212 2 E24E1
Third n3 0.476 3 E39E1
Fourth n4 0.846 4 E416E1
Fifth n5 1.32 5 E525E1
Sixth n6 1.90 6 E636E1
Seventh n7 2.59 7 E749E1
22
The hydrogen line spectrum-cont.
  • Bohr suggested that the hydrogen atom is in the
    ground state, also called the first energy level,
    when its single electron is in n1.
  • When energy is added from an outside source the
    electron moves to a higher energy orbit, such as
    n2, so the atom is now in an excited state. When
    the atom is in an excited state, the electron can
    drop from the higher-energy orbit to a lower
    energy orbit, so as a results of the transition
    an photon is emitted.
  • E photonE higher-energy orbit - E lower-energy
    orbit

23
The hydrogen line spectrum
24
The hydrogen line spectrum
25
Limits of Bohrs model
  • The model explained the hydrogens spectral lines
    but not for other elements
  • Bohrs model did not account for the chemical
    behavior of the atom
  • In later experiments it was proven that electrons
    in atoms do not move around the nucleus in
    circular orbits

26
The quantum mechanical model of the atom
  • Louis De Broglie proposed that just as light had
    been shown to have both a wave and a particle
    aspect, so might matter.
  • His now famous wave equation, indicated that an
    electron in a Bohr orbit racing around the atom's
    nucleus would possess a wavelength of the right
    dimension to form standing waves.

27
De Broglies equation
  • wavelength planks constant / mass of particle
    x velocity

28
Heisenberg uncertainty principle
  • states that it is fundamentally impossible to
    know precisely both the velocity and the position
    of a particle at the same time.

"The more precisely the POSITION is
determined,the less precisely the MOMENTUM is
known"
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