Structure of the Atom (DP IB Physics)

The proton number of an atom is the number of protons in its nucleus. This number defines the chemical element.

The mass number of an atom is the total number of protons and neutrons in its nucleus.

False.

The periodic table is ordered by proton number, not mass number.

The relative charge of an electron is -1e, where e is the elementary charge.

False.

Most of the mass of an atom is concentrated in the nucleus. Electrons have a negligible mass compared to the mass of protons and neutrons.

In nuclear notation:

• X represents the chemical symbol

• A represents the mass number (number of protons and neutrons)

• Z represents the atomic number (number of protons)

Therefore, Z = number of protons = number of electrons, and A - Z = number of neutrons.

The mass number (top number) of carbon is 12.

The proton number (bottom number) of carbon is 6.

The mass number (top number) of beryllium is 9.

The proton number (bottom number) of beryllium is 4.

Therefore, an atom of beryllium contains 4 protons, 5 neutrons and 4 electrons.

An emission spectrum is a set of discrete wavelengths represented by bright lines on a black background.

These lines represent the photons emitted when electrons in excited atoms transition from higher to lower energy levels.

An absorption spectrum is a set of discrete wavelengths represented by dark lines across a continuous spectrum.

These lines represent the photons absorbed when white light passes through a low-pressure gas.

False.

Emission and absorption spectra for the same element have lines at the same wavelengths.

Atomic spectra provide evidence that electrons in atoms can only transition between discrete energy levels.

The main difference between emission and absorption spectra is that emission spectra appear as bright lines on a dark background, whilst absorption spectra appear as dark lines on a bright, continuous background.

True.

Each element produces a unique pattern of spectral lines, which can be used to identify the element.

A photon is a quantum of electromagnetic energy.

They are particles with no mass and carry energy in discrete quantities.

The equation for photon energy is:

Where:

• = Planck's constant (6.63 × 10^-34 J s)

• = speed of light (3.0 × 10⁸ m s^-1)

• = frequency of photon (Hz)

• = wavelength of photon (m)

False.

The energy of a photon is inversely proportional to its wavelength.

Ionisation energy is the minimum energy required to remove an electron from the ground state of an atom.

When an atom absorbs a photon, an electron moves to a higher energy level.

The atom is said to be in an excited state.

False.

The ground state is the lowest possible energy state of an atom.

De-excitation is when an electron transitions from a higher energy level to a lower energy level and a photon is emitted.

The equation for the difference between the two energy levels is:

Where:

• = the energy of the lower level (J)

• = the energy of the higher level (J)

• = Planck's constant (6.63 × 10^-34 J s)

• = the frequency of the emitted or absorbed photon (Hz)

The Fermi radius is the radius of a hydrogen nucleus.

It is equal to the constant of proportionality = 1.20 × 10^-15 m

The equation for determining the nuclear radius of an atom is:

Where:

• = Fermi radius

• = mass number

False.

Nuclear density is the same for all nuclei and is independent of nuclear radius.

The expression for nuclear density is derived using the equations:

• density:

• volume of a sphere:

• nuclear radius:

• mass of a nucleus:

The point of closest approach is the point where the alpha particle's kinetic energy is fully converted to electric potential energy, momentarily stopping the particle before it is repelled.

The deviations from Rutherford's predictions in the alpha scattering experiment provide evidence for the existence of the strong nuclear force.

False.

Rutherford's original alpha scattering model only accounted for electrostatic repulsion and did not include the strong nuclear force.

At very high energies, the number of back-scattered alpha particles in Rutherford's alpha scattering experiment sharply decreases to zero, deviating from Rutherford's predictions.

At high energies, the alpha particles become close enough to the nucleus that the strong nuclear force has a greater influence than electric repulsion, which creates deviations in the electrostatic repulsion pattern.

The atomic number of the target nucleus affects deviations from Rutherford's predictions of his alpha scattering experiment because nuclei with lower atomic numbers show greater deviations at lower energies compared to nuclei with higher atomic numbers.

The Bohr model states that electrons can only exist in certain stable orbits defined by their angular momentum.

Electrons can only move to a larger or smaller orbit by absorbing or emitting a photon of light.

The equation for the discrete energy levels in a hydrogen atom is

Where:

• 13.6 eV = energy of the ground state of hydrogen

• = principal quantum number

True.

The Bohr model predicts electrons possess fixed orbital radii, quantised energies and quantised angular momenta.

The Bohr condition for the quantisation of angular momentum is

Where:

• = mass of an electron (kg)

• = velocity of electron (m s^–1)

• = radius of orbit (m)

• = integer number of energy level

• = Planck's constant (J s)

False.

The Bohr model accurately describes the energy levels of hydrogen atoms but fails for more complex atoms.

False.

The Bohr model is not compatible with the predictions of classical physics.

According to classical physics, atomic electrons should not have fixed orbital radii. Instead, they should radiate energy and gradually spiral into the nucleus due to the centripetal force.