Atomic Structure and Models

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Questions and Answers

Which statement best describes Thomson's model of the atom?

  • Atoms consist of a central nucleus surrounded by electrons.
  • Atoms are made up entirely of positively charged protons.
  • Atoms are solid spheres with no internal structure.
  • Atoms contain positively charged spheres with electrons embedded within. (correct)

What was a major limitation of Thomson's model of the atom?

  • It proposed that electrons were stationary.
  • It failed to account for the presence of neutrons.
  • It incorrectly described the size of the nucleus.
  • It could not explain the spectral lines of hydrogen. (correct)

What significant discovery did Rutherford's alpha-ray scattering experiment lead to?

  • The concept of electron orbits around atoms.
  • The discovery of electrons being negatively charged.
  • The construction of atomic models using protons and neutrons.
  • The presence of the atomic nucleus. (correct)

How does the size of the atomic nucleus compare to the size of the atom?

<p>The nucleus is much smaller than the atom by a factor of $10^5$. (C)</p> Signup and view all the answers

What is the total charge of an alpha particle?

<p>2 positive charges. (C)</p> Signup and view all the answers

At the distance of closest approach, what happens to the kinetic energy of an alpha particle?

<p>It reduces to zero. (A)</p> Signup and view all the answers

Which of the following equations represents the relationship between kinetic energy and electric potential energy at the distance of closest approach?

<p>$KE = PE$ implies $ rac{1}{2} mv^2 = rac{1}{4 rac{ ext{pi}}{ ext{epsilon}_0}} rac{(ze)(2e)}{r_0}$. (B)</p> Signup and view all the answers

In Rutherford's model, what provides the necessary centripetal force for electrons to revolve around the nucleus?

<p>Electrostatic force of attraction. (C)</p> Signup and view all the answers

What is the equation that relates the energy difference between two states to the frequency of emitted radiation?

<p>$E_2 - E_1 = hv$ (D)</p> Signup and view all the answers

In which spectral series do the transitions occur from any higher energy state to the ground state in the hydrogen atom?

<p>Lyman Series (C)</p> Signup and view all the answers

Which of the following spectral series contains lines in the visible region?

<p>Balmer Series (A)</p> Signup and view all the answers

What is the Rydberg formula used for?

<p>Calculating the wavelength of emitted light (A)</p> Signup and view all the answers

In the hydrogen atom, what is the value of Z?

<p>1 (A)</p> Signup and view all the answers

What does the wave number represent in the context of the spectral lines?

<p>Number of complete waves in unit length (C)</p> Signup and view all the answers

Which series involves transitions to the third orbit in hydrogen?

<p>Paschen Series (B)</p> Signup and view all the answers

What is represented by an energy level diagram?

<p>The energies of electrons in different stationary orbits (B)</p> Signup and view all the answers

What does the impact parameter measure in relation to an alpha particle?

<p>The perpendicular distance of the particle's velocity vector from the nucleus (B)</p> Signup and view all the answers

According to the limitations of Rutherford's atom model, what is a consequence of electrons emitting energy?

<p>There should be a continuous emission of electromagnetic radiation (A)</p> Signup and view all the answers

What is the expression for the radius of the stationary orbit of an electron in a hydrogen atom?

<p>r = n^2 h^2 / (4 imes rac{ heta^2}{2} mk e^2) (D)</p> Signup and view all the answers

In Bohr's model of the hydrogen atom, what is the significance of the quantization of angular momentum?

<p>It defines specific allowed radii for electron orbits in the atom (D)</p> Signup and view all the answers

What determines the energy difference when an electron transitions between stationary orbits in Bohr's model?

<p>The difference in total energy of the electron in different orbits (A)</p> Signup and view all the answers

What can be concluded about the total energy of an electron in a stationary orbit?

<p>Total energy is negative, indicating the electron is bound to the nucleus. (B)</p> Signup and view all the answers

How does the orbital velocity of an electron in outer orbits compare to that in inner orbits?

<p>Orbital velocity is smaller in outer orbits compared to inner orbits. (B)</p> Signup and view all the answers

How does Bohr's model describe the forces acting on the electron in its orbit?

<p>The centripetal force required is provided by electrostatic forces (A)</p> Signup and view all the answers

What is the ionization energy for a hydrogen atom?

<p>13.6 eV (D)</p> Signup and view all the answers

What defines a 'stationary orbit' as per Bohr's postulates?

<p>An orbit in which the electron does not lose energy or radiation (D)</p> Signup and view all the answers

The frequency of an electron in a stationary orbit is determined by which of the following equations?

<p>f = 2 heta kze^2 / nh (D)</p> Signup and view all the answers

What role does Planck's constant play in Bohr's model?

<p>It defines the relationship between energy differences and frequency of radiation (D)</p> Signup and view all the answers

Which of the following represents the relationship between kinetic energy and potential energy in a stationary orbit?

<p>KE = -PE (B)</p> Signup and view all the answers

What is the mathematical expression for the centripetal force acting on an electron in a hydrogen atom according to Bohr's model?

<p>$F = rac{mv^2}{r}$ (A), $F = kze^2 / r^2$ (B)</p> Signup and view all the answers

What does the Rydberg formula describe within the context of atomic energy levels?

<p>The energy transitions between various atomic orbits. (D)</p> Signup and view all the answers

Which statement is true regarding the energy of an electron in higher orbits?

<p>Energy values are progressively smaller (less negative) in higher orbits. (A)</p> Signup and view all the answers

What is the energy of the electron in the second orbit of a hydrogen atom?

<p>-3.4 ev (A)</p> Signup and view all the answers

At what energy level does an electron in a hydrogen atom achieve ionization?

<p>0 ev (C)</p> Signup and view all the answers

According to Bohr's second postulate, what is quantized in the model of the hydrogen atom?

<p>Angular momentum (B)</p> Signup and view all the answers

Which of the following correctly represents the relationship for permissible orbits in the hydrogen atom?

<p>$2\pi r_n = n\lambda$ (B)</p> Signup and view all the answers

What happens when an electron absorbs energy and jumps to a higher energy level?

<p>It becomes excited (C)</p> Signup and view all the answers

How does the energy of an electron change as it moves to higher orbits in a hydrogen atom?

<p>It increases without bound (A)</p> Signup and view all the answers

Which limitation is NOT associated with Bohr's theory of the hydrogen atom?

<p>It describes elliptical orbits accurately (D)</p> Signup and view all the answers

What is the de Broglie wavelength associated with an electron in the nth orbit?

<p>$\lambda = h/mv_n$ (D)</p> Signup and view all the answers

Flashcards

What is Thomson's model of the atom?

Atoms consist of a uniformly distributed positive charge within a sphere with negatively charged electrons embedded within it.

What is an alpha particle?

Helium nucleus containing two protons and two neutrons.

What is the atomic nucleus?

The tiny, dense core of an atom containing protons and neutrons, where most of the atom's mass is concentrated.

What is the distance of closest approach?

The distance an alpha particle comes close to a nucleus before being repelled by its positive charge.

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What is Dalton's atomic theory?

Dalton proposed that all matter is made up of tiny, indivisible particles called atoms.

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What was Rutherford's alpha-ray scattering experiment?

Rutherford's experiment involved firing alpha particles at a thin gold foil. Most particles passed through, but some were deflected at large angles.

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What are the limitations of the Thomson model?

The Thomson model couldn't explain the spectral lines of hydrogen and other atoms and didn't account for the scattering of alpha particles at large angles.

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What is Rutherford's atomic model?

Rutherford's model proposed that atoms have a small positively charged nucleus surrounded by negatively charged electrons orbiting it.

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Impact Parameter

The perpendicular distance between the initial path of an alpha particle and the center of the nucleus it's approaching. It essentially determines how close the alpha particle gets to the nucleus.

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Quantized Orbits

The idea that electrons can only exist in specific orbits around the nucleus, not anywhere in between.

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Electron Jumps and Energy

Bohr's idea that an electron only emits or absorbs energy when it jumps from one allowed orbit to another.

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Electrostatic Force of Attraction

The force that keeps an electron in its orbit around the nucleus, provided by the attraction between the positive nucleus and negative electron.

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Electron Speed Equation

The equation that relates the electron's speed, mass, charge, and the distance from the nucleus, used to calculate the electron's speed in a particular orbit.

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Quantized Angular Momentum

A key concept in Bohr's model, it states that the angular momentum of an electron in a stable orbit is a whole number multiple of h/2Ï€ (Planck's constant).

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Radii of Bohr's Orbits

The radii of the allowed orbits for an electron around a hydrogen nucleus, calculated using Bohr's postulates.

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Bohr's Model of Hydrogen Atom

The model of the hydrogen atom proposed by Niels Bohr, incorporating both classical physics and quantum concepts to explain the atom's behavior.

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Radius of Stationary Orbits

The radius of a stationary orbit in the Bohr model is quantized, meaning it can only take on discrete values. This is determined by the principal quantum number 'n'. Larger 'n' corresponds to larger orbit radii.

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Electron Velocity in Bohr Orbits

The velocity of an electron in a Bohr orbit is inversely proportional to the principal quantum number 'n'. This means that electrons in higher energy levels move slower.

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Electron Frequency in Bohr Orbits

The frequency of revolution of an electron in a Bohr orbit is the number of times it completes a circular path around the nucleus per second. It is related to the electron's velocity and the orbit's radius.

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Total Energy of Electron in Bohr Orbit

The total energy of an electron in a Bohr orbit is negative, indicating that it is bound to the nucleus. It consists of both kinetic and potential energies, with the potential energy being negative and greater in magnitude than the kinetic energy.

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Ionization Energy

The minimum energy required to remove an electron from its ground state and completely free it from the atom. For hydrogen, this is 13.6 eV.

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Spectral Lines

The discrete wavelengths of light emitted or absorbed by an atom when electrons transition between energy levels. This is explained by the Bohr model and the Rydberg formula.

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Rydberg Formula

Describes the wavelengths of spectral lines emitted by hydrogen, and it is based on the quantized nature of electron energy levels and transitions between them.

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Ground State

The lowest energy state of an atom, corresponding to the electron occupying the innermost orbit (n=1).

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E2 - E1 = hv

The energy difference between two energy levels in an atom, where 'E2' is the higher energy level and 'E1' is the lower energy level, is equal to the energy of a photon emitted or absorbed during the transition between these levels. It is a fundamental equation used in Atomic Physics.

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What is the Rydberg Formula for Hydrogen?

The Rydberg formula relates the wavelengths of spectral lines emitted by a hydrogen atom to the energy levels involved in the transitions. It predicts the wavelengths of the different spectral series observed in the hydrogen spectrum.

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What is the Rydberg Constant?

The Rydberg Constant is a physical constant used in the Rydberg Formula to calculate the wavelengths of spectral lines emitted by a hydrogen atom. It is a fundamental constant in atomic physics.

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Lyman Series

A series of spectral lines observed in the ultraviolet region of the hydrogen spectrum, emitted when electrons transition from higher energy levels to the ground state (n1=1) of the atom.

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Balmer Series

A series of spectral lines observed in the visible region of the hydrogen spectrum, occurs when electrons transition from higher energy levels to the second energy level of the atom (n1=2).

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Paschen Series

A series of spectral lines observed in the infrared region of the hydrogen spectrum, created by electronic transitions from higher energy levels to the third energy level of the atom (n1=3).

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Brackett Series

A series of spectral lines in the infrared region of the hydrogen spectrum, produced by electronic transitions from higher energy levels to the fourth energy level of the atom (n1=4).

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What is an energy level diagram?

A graphic representation of the permitted energy levels of electrons in an atom (the stationary states, as described by Bohr's model). The different energy levels correspond to different electron orbits around the nucleus. The energy levels are shown on a vertical axis, with higher energies at the top. Electrons can jump between these energy levels, absorbing or emitting photons of light. This process gives rise to the characteristic spectral lines observed in atomic emission spectra.

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Energy of an electron in a hydrogen atom

The total energy of an electron in the nth orbit of a hydrogen atom. It's expressed in electron volts (eV).

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Hydrogen atom energy levels

The energy level diagram of a hydrogen atom shows different energy levels available for electrons. Each level is labeled by its principal quantum number, 'n'.

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Energy level diagram of hydrogen

The energy level diagram of a hydrogen atom shows different energy levels available for electrons. Each energy level is labeled by its principal quantum number, 'n'.

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Excitation potential

The minimum potential required to excite an electron in an atom from its ground state to a higher energy level.

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Ionization potential

The potential required to completely remove an electron from an atom, leaving it positively charged.

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de Broglie's explanation of Bohr's postulate

de Broglie proposed that particles exhibit wave-like properties. He explained Bohr's quantization postulate by suggesting that electron orbits contain an integral number of electron wavelengths.

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Limitations of Bohr's theory

Bohr's model, while successful for hydrogen, had limitations. It couldn't explain the fine structure of spectral lines, the intensities of spectral lines, and it only worked for hydrogen.

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Study Notes

Atomic Structure

  • Dalton postulated that matter is made of atoms
  • Thomson proposed a model of the atom: positively charged spheres with electrons embedded within
  • Rutherford's experiment (using alpha particles and gold foil) discovered the atomic nucleus
  • The nucleus is small, dense, and positively charged, containing almost all the atom's mass
  • Protons and neutrons (in the nucleus) have approximately the same mass; protons have a positive charge and neutrons no charge.
  • Electrons are negatively charged and orbit the nucleus (e-).
  • The number of protons defines the atomic number of the element

Thomson's Atomic Model

  • Positively charged sphere containing negatively charged electrons
  • Electrons are embedded throughout like plums in a pudding.
  • The atom is electrically neutral (equal positive and negative charges)

Rutherford's Atomic Model

  • Experiment involved firing alpha particles at a gold foil
  • Most alpha particles passed straight through the foil
  • Some particles were deflected significantly, indicating a small but dense positive nucleus at the center of the atom
  • Nucleus contains most of the atom's mass

Limitations of Thomson's Model

  • Could not explain the scattering of alpha particles
  • Could not explain the line spectrum of hydrogen

Rutherford's Experiment - Observations

  • Most alpha particles pass straight through
  • Some alpha particles are deflected at large angles
  • A few alpha particles are reflected back

Rutherford's Experiment - Conclusions

  • Atom has a tiny, dense, positive nucleus at the center
  • Electrons orbit the nucleus

Rutherford's Atomic Model - Limitations

  • Could not explain the stability of the atom (electrons losing energy and spiralling into the nucleus)
  • Could not explain the line spectra of atoms

Bohr's Model of the Atom

  • Electrons orbit the nucleus in specific, quantized energy levels
  • Electrons can only exist in these specific orbits and do not continuously emit energy
  • When an electron moves from one energy level to another, it absorbs or emits energy in the form of a photon
  • Electrons do not spiral into the nucleus due to the quantized nature of their energy levels
  • Angular momentum is quantized, which stabilizes the atom

Bohr's Postulates

  • Electrons revolve in stable orbits without radiating energy
  • Electrons can only exist in certain specific energy levels
  • Emission and absorption of energy occur when the electron jumps between these energy levels
  • Quantized angular momentum of electrons restricts possible orbits

Bohr's Explanation of Spectral Lines in Hydrogen

  • Electrons can jump between energy levels, emitting or absorbing photons
  • The frequency of the emitted or absorbed light corresponds to the energy difference between the energy levels
  • Different spectral lines correspond to different electron transitions.

Limitations of Bohr's Model

  • Applicable only for the simplest atoms (e.g., hydrogen)
  • Does not account for the fine structure of spectral lines
  • Does not explain the behaviour of multi-electron atoms

Atomic Spectra

  • Line absorption spectra: dark lines on a bright background, indicating specific wavelengths of light being absorbed
  • Line emission spectra: bright lines on a dark background, indicating specific wavelengths of light being emitted

Energy Level Diagram

  • Diagram showing the energy levels of an atom's electrons
  • The energy levels are quantized
  • Transitions between energy levels correspond to specific frequencies of emitted or absorbed light.
  • Ionization energy: energy required to remove an electron from the outermost orbit

de Broglie's Explanation

  • Electrons have wave-like properties
  • The electron orbit must have an integral number of de Broglie waves in it.

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