The Modern Atom
Emission Line Spectra
• When an electrical voltage is passed across a gas
in a sealed tube, a series of narrow lines is seen.
• These lines are the emission line spectrum. The
emission line spectrum for hydrogen gas shows
three lines: 434 nm, 486 nm, and 656 nm.
Wave Nature of Light
• Light is made of particles (photons) with no mass
carrying energy.
• Photons travels through space as a wave, similar to an
ocean wave.
• A wave has characteristics:
a) Wavelength is the distance light travels in one cycle.
b) Frequency is the number of wave cycles completed
each second.
c) Speed: Light has a constant speed: 3.00 × 108 m/s.
Wavelength vs. Frequency
• The longer the wavelength of light, the lower the
frequency.
• The shorter the wavelength of light, the higher the
frequency.
Energy and Frequency
• There is a relationship between the energy
and the frequency of photons: E = h x ,
where E is the energy, h is called Planck’s
constant (h= 6.62x 10-34 J.s), and , is the
frequency.
• The frequency of an electromagnetic
radiation determines the color of the light.
• Photons with high frequency carry more
energy than photons with low frequency.
Visible Spectrum
• Radiation composed of only one wavelength is called
monochromatic.
• White light is made of photons of different wavelengths.
• These photons can be separated into a continuous
spectrum of colors.
• The visible spectrum is the range of wavelengths between
400 and 700 nm.
• Radiant energy that has a wavelength lower than 400 nm
and greater than 700 nm cannot be seen by the human
eye.
Radiant Energy Spectrum
• The complete radiant energy spectrum is an
uninterrupted band, or continuous spectrum.
Bohr Model of the Atom
• Niels Bohr speculated that electrons orbit about
the nucleus in fixed energy levels.
• Electrons are found only in specific energy levels,
and nowhere else.
• The electron
energy levels
are quantized.
Evidence for Energy Levels
• The electric charge temporarily excites an electron
to a higher orbit. When the electron drops back
down, a photon is given off.
“Atomic Fingerprints”
• The emission line spectrum of each element is
unique.
• We can use the line spectrum for the identify of
elements, using their “atomic fingerprint”.
Bohr Model
• Colors from excited gases arise because electrons
move between energy states in the atom.
The Wave/Particle Nature of Light
• In 1900, Max Planck proposed that radiant energy
is not continuous, but is emitted in small bundles.
This is the quantum concept.
• Radiant energy has both a wave nature and a
particle nature.
The Photoelectric Effect
• The photoelectric effect provides evidence for
the particle nature of light -- “quantization”.
• If light shines on the surface of a metal, there is
a point at which electrons are ejected from the
metal.
• The electrons will only be ejected once the
threshold frequency is reached.
• Below the threshold frequency, no electrons are
ejected.
• Above the threshold frequency, the number of
electrons ejected depend on the intensity of the
light.
The Photoelectric Effect
The Quantum Concept
• The quantum concept states that energy is present
in small, discrete bundles.
• For example:
– A tennis ball that rolls down a ramp loses potential
energy continuously.
– A tennis ball that rolls down a staircase loses potential
energy in small bundles. The loss is quantized.
Energy Levels and Sublevels
• It was later shown that electrons occupy energy
sublevels within each level.
• These sublevels are given the designations s, p, d,
and f.
– These designations are in reference to the sharp,
principal, diffuse, and fine lines in emission spectra.
• The number of sublevels in each level is the same
as the number of the main level.
Quantum Mechanical Model
• An orbital is the region of space where there is a
high probability of finding an atom.
• In the quantum mechanical atom, orbitals are
arranged according to their size and shape.
• The higher the energy of an orbital, the larger its
size.
• s-orbitals have
a spherical
shape.
Shapes of p-Orbitals
• There are three different p sublevels.
• p-orbitals have a dumbbell shape.
• Each of the p-orbitals has the same shape, but
each is oriented along a different axis in space.
s-orbitals
p-orbitals
d-orbitals
Orbitals and their Energy
H-Atom
Other atoms
Energy Levels and Sublevels
• The first energy level has 1 sublevel:
– 1s
• The second energy level has 2 sublevels:
– 2s and 2p
• The third energy level has 3 sublevels:
– 3s, 3p, and 3d
Electron Occupancy in Sublevels
• The maximum number of electrons in each of the
energy sublevels depends on the sublevel:
– The s sublevel holds a maximum of 2 electrons.
– The p sublevel holds a maximum of 6 electrons.
– The d sublevel holds a maximum of 10 electrons.
– The f sublevel holds a maximum of 14 electrons
• The maximum electrons per level is obtained by
adding the maximum number of electrons in each
sublevel.
Conclusions
• Light has both the properties of waves and
particles.
• The particles of light are referred to as photons.
• The energy of photons is quantized.
• Electrons exist around the nucleus of atoms in
discrete, quantized energy levels.
• Electrons fill energy sublevels starting with the
lowest energy sublevel and filling each successive
level of higher energy.