Models of the atom

Chapter 21: Atomic physics

Section 2 : Models of the atoms

Open Ended

Describe Thomson's model of the atom.

Early Models of the Atom

• The model of the atom in the days of

Newton was that of a tiny, hard,
indestructible sphere.

• The discovery of the electron in 1897

prompted J. J. Thomson (1856–1940)
to suggest a new model of the atom.

• In Thomson’s model, electrons are

embedded in a spherical volume of
positive charge like seeds in a
watermelon.

Poll

Do you think that Thomson's model of the atom was close to the current model we use nowadays?

• yes

• no

Open Ended

Ernest Rutherford (1871–1937) later proved that Thomson’s model could not be correct.

In his experiment, a beam of positively charged alpha particles was projected against a thin metal foil.

Based on what you see in the picture, what can you conclude about the structure of the atom?

Early Models of the Atom, continued

•Most of the alpha particles passed through the foil. Some were deflected through very large angles.

Alpha particles, also called alpha rays or alpha
radiation, consist of two protons and two neutrons bound
together into a particle identical to a helium-4 nucleus.

Early Models of the Atom, continued

• Rutherford concluded that all of the positive charge in an atom and most of the atom’s mass are found in a region that is small compared to the size of the atom.

• He called this region the nucleus of the atom.

• Any electrons in the atom were assumed to be in the relatively large volume outside the nucleus.

Multiple Choice

Rutherford scattering experiment is based on :

• alpha particle scattering from thin gold foil

• beta particle scattering from thin gold foil

• beta particle scattering from Zn foil

• Gamma rays scattering from thin aluminum foil

Multiple Choice

Rutherford scattering experiment proved

• the existence of the atom

• the presence of the nucleus

• the presence of the electron

• the presence of mass of the nucleus

Poll

Do you think that Rutherford model of the atom is stable?

• yes

• no

Open Ended

An electron revolving around the nucleus continuously should lose energy.

Based on the statement shown above, explain why Rutherford model of the atom couldn't explain the stability of the atom.

• Rutherford proposed that the electrons revolve around the nucleus in fixed paths called orbits. According to Maxwell, accelerated charged particles emit electromagnetic radiations and hence an electron revolving around the nucleus should emit electromagnetic radiation.

• This radiation would carry energy from the motion of the electron which would come at the cost of shrinking of orbits. Ultimately the electrons would collapse in the nucleus.

• Calculations have shown that as per the Rutherford model, an electron would collapse into the nucleus in less than 10^-8 seconds. So the Rutherford model was not in accordance with Maxwell’s theory and could not explain the stability of an atom.

​

Open Ended

What did Ernest Rutherfold's atomic model gets right and wrong?

Rutherford's model

• The Rutherford atomic model was correct in that the atom is mostly empty space.

• Most of the mass is in the nucleus, and the nucleus is positively charged.

• Far from the nucleus are the negatively charged electrons.

• But the Rutherford atomic model used classical physics and not quantum mechanics. T

• his meant that an electron circling the nucleus would give off electromagnetic radiation. The electron would lose energy and fall into the nucleus.

• The atom will collapse.

Early Models of the Atom, continued

• To explain why electrons were not pulled into the nucleus,

Rutherford viewed the electrons as moving in orbits about
the nucleus, much like the planets orbit the sun.

• However, accelerated charges should radiate

electromagnetic waves, losing energy. This would lead
to a rapid collapse of the atom.

• This difficulty led scientists to continue searching for a

new model of the atom.

Open Ended

Describe what you see in the picture?

Open Ended

Look for "Atomic Spectra" and provide your thoughts.

Atomic Spectra

When the light given off by an atomic gas
is passed through a prism, a series of
distinct bright lines is seen. Each line
corresponds to a different wavelength, or
color.

•A diagram or graph that indicates the wavelengths of radiant energy that a substance
emits is called an emission spectrum.

•Every element has a distinct emission and absorption spectrum.

Open Ended

What is the reason for the emission spectrum?

Answers must be based on the knowledge you have about the motion of electron.

Emission spectrum

• When energy is absorbed by electrons of an atom, electrons move from lower energy levels to higher energy levels.

• These excited electrons have to radiate energy to return to ground states from the excited state, which is unstable.

• The emission spectrum is formed by the frequencies of this emitted light.

Multiple Choice

Which source is associated with a line emission spectrum ?

• Electric fire

• neon street sign

• red traffic light

• sun

Multiple Choice

Atomic spectrum is also produced by the heating of elements in

• gaseous state

• solid state

• liquid state

• plasma

Open Ended

Based on the picture you see, try to define absorption spectrum?

Atomic Spectra, continued

• An element can also absorb light at specific wavelengths.

• The spectral lines corresponding to this process form what is known as

an absorption spectrum.

• An absorption spectrum can be seen by passing light containing all

wavelengths through a vapor of the element being analyzed.

• The absorption spectrum consists of a series of dark lines placed over

the otherwise continuous spectrum.

• Each line in the absorption spectrum of a given element coincides with

a line in the emission spectrum of that element.

Emission and Absorption Spectra of Hydrogen

The various absorption lines seen in the solar spectrum
have been used to identify elements in the solar
atmosphere.
With careful observation and analysis, astronomers have
determined the proportions of various elements present
in individual stars.

Open Ended

Rutherford's model couldn't explain the fact of the fact that each element has a unique series of spectral lines.

Search for the model that followed Rutherford's model of the atom.

The Bohr Model of the Hydrogen Atom

• In 1913, the Danish physicist Niels Bohr (1885– 1962) proposed a new model

of the hydrogen atom that explained atomic spectra.

• In Bohr’s model, only certain orbits are allowed. The electron is never found

between these orbits; instead, it is said to “jump” instantly from one orbit to
another.

• In Bohr’s model, transitions between stable orbits with different energy levels

account for the discrete spectral lines.

Formula used to find the frequency of the emitted radiation

The Bohr Model, continued

• When light of a continuous spectrum shines on the atom, only

the photons whose energy (hf ) matches the energy separation
between two levels can be absorbed by the atom.

•When this occurs, an electron jumps from a lower
energy state to a higher energy state, which
corresponds to an orbit farther from the nucleus.

•This is called an excited state. The absorbed photons
account for the dark lines in the absorption spectrum.

Photon: a particle representing a quantum of light or other
electromagnetic radiation. A photon carries energy
proportional to the radiation frequency but has zero rest mass.

The Bohr Model, continued

• Once an electron is in an excited state, there is a certain

probability that it will jump back to a lower energy level by
emitting a photon.

• This process is called spontaneous emission.

•The emitted photons are responsible for the bright lines
in the emission spectrum.

•In both cases, there is a correlation between the “size”
of an electron’s jump and the energy of the photon.

Fill in the Blank

In the Bohr model, which used quantum theory, the electrons exist only in --------------------- orbits and can move between these orbits.

Multiple Choice

When electrons move from a lower energy level to a higher energy level, energy is

• absorbed

• emitted

• both (a) and (b)

• none of the above

Multiple Choice

The energy of each orbit is

• same

• fixed

• changes with time

• none of the above

Multiple Choice

When electrons move from the higher energy level to a lower energy level, energy is

• absorbed

• emitted

• both (a) and (b)

• none of the above

Multiple Choice

The radius of Bohr’s orbit is……..

• directly proportional to principal quantum number

• directly proportional to square of principal quantum number

• inversely proportional to principal quantum number

• inversely proportional to square of principal quantum number

Multiple Choice

If frequency required for transition of electron from 2^nd orbit to 5^th orbit is 4×10¹⁵ Hz, the energy absorbed during the transition is…..

• 16.5 eV

• 26.5 eV

• 16.5 J

• 26.5J

Chapter 21

Quantum Energy

• Einstein later applied the concept of quantized

energy to light. The units of light energy called
quanta (now called photons) are absorbed or
given off as a result of electrons “jumping” from
one quantum state to another.

E = hf

energy of a quantum = Planck’s constant  frequency

Planck’s constant (h) ≈ 6.63  10–34J•s

• The energy of a light quantum, which

corresponds to the energy difference between two
adjacent levels, is given by the following equation:

Chapter 21

Quantum Energy

• If Planck’s constant is expressed in units of J•s, the equation E = hf gives the energy in joules.
• However, in atomic physics, energy is often expressed in units of the electron volt, eV.
• An electron volt is defined as the energy that an electron or proton gains when it is

accelerated through a potential difference of 1 V.

• The relation between the electron volt and the joule is as follows:

1 eV = 1.60  10–19J