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Electronic Structure and Periodic Properties

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Elements in the same group exhibit similar chemical and physical properties. Alkali Metals: ... The properties of elements depend on their electronic structure. ... – PowerPoint PPT presentation

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Title: Electronic Structure and Periodic Properties


1
Electronic Structure and Periodic Properties
  • Wave Nature of Light
  • Models of the Atom
  • Bohr Model
  • Quantum Mechanical Model
  • Atomic Orbitals
  • Electron Configurations
  • Periodic Properties of Elements

2
Electronic Structure of Atoms--Introduction
  • Elements in the same group exhibit similar
    chemical and physical properties.
  • Alkali Metals
  • soft
  • very reactive
  • metal
  • Noble Gases
  • gases
  • inert (unreactive)
  • Why???

3
Electronic Structure of Atoms--Introduction
  • When atoms react, their electrons interact.
  • The properties of elements depend on their
    electronic structure.
  • the arrangement of electrons in an atom
  • number of electrons
  • distribution of electrons around the atom
  • energies of the electrons

4
Electronic Structure of Atoms--Introduction
  • Understanding the nature of electrons and the
    electronic structure of atoms is the key to
    understanding the reactivity of elements and the
    reactions they undergo.
  • Much of our knowledge of the electronic structure
    of atoms came from studying the ways elements
    absorb or emit light.

5
The Wave Nature of Light
  • Light is a type of electromagnetic radiation
  • a form of energy with both electrical and
    magnetic components

Wavelength (l) the distance between successive
peaks Frequency (u) the number of complete
wavelengths that pass a given point in 1 sec
6
The Wave Nature of Light
The electromagnetic spectrum
7
The Wave Nature of Light
  • Different types of electromagnetic radiation have
    different properties because they have different
    u and l.
  • Gamma rays
  • wavelength similar to diameter of atomic nuclei
  • Hazardous
  • Radio waves
  • wavelength can be longer than a football field

8
Quantized Energy and Photons
  • Classical physics (mechanics) suggests that both
    electromagnetic radiation and matter can have any
    energy

A car rolling down a hill can have any potential
energy (energy of position) depending on its
position on the hill.
9
Quantized Energy and Photons
  • Classical mechanics is not correct, however.
  • Max Planck suggested that energy is transferred
    in packets called quanta (plural).
  • Quantum the smallest quantity of energy that
    can be emitted or absorbed as electromagnetic
    energy

10
Quantized Energy and Photons
  • Planck proposed that the energy of a single
    quantum is directly proportional to its
    frequency
  • E hu
  • where E energy
  • u frequency
  • h Plancks constant (6.63x10-34 J-s)

11
Quantized Energy and Photons
  • According to Plancks theory, energy is always
    emitted or absorbed in whole number multiples of
    hu (i.e hu, 2hu, 3hu)
  • According to Plancks theory, the energy levels
    that are allowed are quantized.
  • restricted to certain quantities or values

12
Quantized Energy and Photons
  • In order to understand quantized energy levels,
    compare walking up (or down) a ramp versus
    walking up (or down) stairs
  • Ramp continuous change in height
  • Stairs quantized changed in height
  • You can only stop on the stairs, not between them

13
Quantized Energy and Photons
  • If Plancks quantum theory is correct, why dont
    we notice its effects in our daily lives?
  • Plancks constant is very small (6.63 x 10-34
    J-s).
  • A quantum of energy (E hu) is very small.
  • Gaining or losing such a small amount of energy
    is
  • insignificant on macroscopic objects
  • very significant on the atomic level

14
Quantized Energy and Photons
  • In 1905 Einstein used Plancks quantum theory to
    explain the photoelectric effect.
  • Light shining on a clean metal surface causes the
    surface to emit electrons.
  • The light must have a minimum
  • frequency in order for electrons
  • to be emitted.

15
Quantized Energy and Photons
  • Einstein explained these results by assuming that
    the light striking the metal is a stream of tiny
    energy packets of radiant energy (photons).
  • The energy of each photon is proportional to its
    frequency.
  • E hu

16
Quantized Energy and Photons
  • When a photon strikes a metal surface
  • Energy is transferred to the electrons in the
    metal
  • If the energy is great enough, the electron can
    overcome the attractive forces holding it to the
    metal.
  • Any extra energy above the amount required to
    free the electron simply increases the kinetic
    energy of the electron.

17
Quantized Energy and Photons
  • Einsteins explanation of the photoelectric
    effect led to a dilemma.
  • Is light a wave or does it consist of particles?
  • Currently, light is considered to have both
    wave-like and particle-like properties.

Matter also has this same dual nature.
18
Models of Atomic Structure
  • Scientists initially thought of the atom as a
    microscopic solar system.
  • electrons orbiting the nucleus
  • Unit 2 suggested that the atom has a tiny
    positively charged nucleus with a diffuse cloud
    of electrons surrounding it.
  • need better understanding of the nature of this
    cloud of electrons.

19
Atomic Models
  • Two models are used to explain the behavior and
    reactivity of atoms and ions.
  • Bohr model
  • Quantum mechanical model

20
Bohr Model
  • Bohr developed an atomic model that explained the
    line spectrum observed for the hydrogen atom.
  • When an electrical current is passed thru a
    sample of H2 (g), energy is transferred to the H2
    molecules.
  • The molecules are broken up. The H atoms absorb
    energy and jump to a higher energy level.

21
The Bohr Model of the Atom
The H atoms relax back to their original energy
level by giving off the absorbed energy as
electromagnetic radiation.
High voltage
H2
22
The Bohr Model of the Atom
The light is analyzed in a spectrometer by
separating it into its different colors.
High voltage
H2
23
The Bohr Model of the Atom
The separated colors are recorded as spectral
lines.
High voltage
H2
Atomic spectrum
24
The Bohr Model of the Atom
  • The spectrum of atomic hydrogen consists of a
    series of discrete lines such as the ones shown
    previously.
  • Why would an atom emit only certain frequencies
    of light and not all of them?

25
The Bohr Model of the Atom
  • According to the Bohr Model of the atom
  • Electrons move in circular orbits around the
    nucleus.
  • Energy is quantized
  • only orbits of certain radii corresponding to
    certain definite energies are allowed
  • an electron in a permitted orbit has a specific
    energy (an allowed energy state)

26
The Bohr Model of the Atom
  • The allowed orbits have specific energies given
    by the formula
  • En (-RH) 1 where n 1, 2, 3
  • n2
  • RH Rydberg constant 2.18 x 10-18 J
  • n is called the principal quantum number

27
The Bohr Model of the Atom
  • Each orbit in an atom corresponds to a different
    value of n.
  • As n increases the radius of the orbit increases
    (i.e. the orbit and any electrons occupying it
    are further from the nucleus)
  • n1 is the closest to the nucleus
  • 0.529 Angstroms for the hydrogen atom

28
The Bohr Model of the Atom
  • The energy of the orbit is lowest for n1 and
    increases with increasing n.
  • Lower energy more stable
  • Lower energy more preferred state

29
The Bohr Model of the Atom
  • The lowest energy state of an atom is called the
    ground state.
  • n 1 for the electron in a H atom
  • When an electron has jumped to a higher energy
    orbit (i.e. n 2, 3, 4) it is considered to be
    in an excited state.

30
The Bohr Model of the Atom
  • To explain the line spectrum for hydrogen, Bohr
    assumed that an electron can jump from one
    allowed energy state to another.
  • Energy absorbed ? e- jumps to higher energy
    state
  • e- relaxes back to a lower energy state ?
    energy is emitted

31
The Bohr Model of the Atom
32
The Bohr Model of the Atom
  • Since the energies of the orbits in an atom are
    quantized, transitions from one allowed orbit to
    another involves only specific amounts of energy.
  • DE Ef - Ei

33
The Bohr Model of the Atom
  • Since E hu, the energy of the light emitted can
    have only specific values.
  • Therefore the u of the light can have only
    specific values as well.
  • So, the line spectrum for each element will be
    unique and will depend on the allowed energy
    levels in that element.

34
The Bohr Model of the Atom
  • The Bohr model effectively explains the line
    spectra of atoms and ions with a single electron
  • H, He, Li2
  • Another model is needed to explain the reactivity
    and behavior of more complex atoms or ions
  • Quantum mechanical model
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