Structure of the Atom Quiz

Structure of the Atom

• Matter is composed of atoms.
• Atoms have a positively charged nucleus consisting of protons and neutrons (nucleons) and negatively charged electrons revolving in orbital shells.
• Electrons are negatively charged.
• Protons are positively charged.
• Neutrons have no charge.
• The mass of an electron is 1/2000 the mass of a hydrogen atom.
• The mass of a proton is equal to the mass of a hydrogen atom and is taken as 1 unit.
• The mass of a neutron is equal to the mass of a hydrogen atom and is taken as 1 unit.

Nucleus Designation

• A nucleus is designated as ZXA.
• The atomic number (Z) represents the number of protons in the nucleus.
• The number of electrons equals the number of protons.
• The mass number (A) is the sum of the number of protons and neutrons (nucleons) in the nucleus.
• The number of neutrons is the difference between the mass number (A) and the atomic number (Z).

Isotopes, Isobars, Isotones, Metastable State

• Isotopes: Atoms of the same element with the same atomic number but different mass numbers.
• Isobars: Atoms of different elements with different atomic numbers but the same mass number.
• Isotones: Atoms of different elements with different atomic numbers and mass numbers but the same number of neutrons.
• Metastable state: Atoms of the same element with the same atomic and mass numbers but different energy states.

Stable & Unstable Nuclei

• There are approximately 1500 known nuclides.
• Stable nuclei: Approximately 300 nuclides.
• Stable nuclei have an equal number of protons (P) and neutrons (N), with a P/N ratio of approximately 1:1.
• Unstable nuclei (radioactive): Approximately 1200 nuclides.
• 65 natural unstable nuclei exist, and the rest are human-made.
• Unstable nuclei have an imbalance in the number of protons and neutrons, resulting in a neutron excess or deficit.

Radioactive Decay

• A radioactive/unstable nucleus has excess energy.
• To reach a lower energy state, a particle may gain enough energy to escape from the nucleus (alpha, beta, and gamma radiation).
• The ejection of a particle transforms the nucleus into a different form.
• The emission of particles may leave the nucleus in an excited state, leading to the emission of more particles or gamma rays to reach a lower energy state and become stable (or reach the ground state).

Modes of Radioactive Decay

• Alpha decay (α decay):

  • Occurs in high-Z nuclei (heavy atoms) where the Coulomb force of repulsion between protons overcomes the nuclear force binding nucleons.
  • An alpha particle (α) is emitted, consisting of two protons and two neutrons (a helium nucleus).
  • Alpha particles are positively charged, deflected by electromagnetic fields, and stopped by a few sheets of paper or a few centimeters of air.

• Beta decay (β± rays):

  • A) Nuclides with neutron excess (-1β₀ decay):
    • The nucleus is unstable due to a high neutron number.
    • The nucleus loses energy and becomes stable by converting a neutron into a proton and an electron.
    • An electron (negative beta particle) is ejected from the nucleus (not from the atomic orbital shell).
    • Negative beta particles have the same mass and charge as an electron and are deflected by electromagnetic fields, stopped by a few sheets of aluminum.
    • A neutrino is emitted, with no charge and practically no mass, making its interaction highly improbable.
  • B) Nuclides with neutron deficit (+1β₀ decay):
    • The nucleus is unstable due to a low neutron number.
    • The nucleus loses energy and becomes stable by converting a proton into a neutron and a positive electron (positron).
    • A positron is ejected from the nucleus with high energy.
    • A positron combines with a nearby negative electron to form annihilation radiation (two photons).

• C) K Electron Capture (EC):

  • Occurs with a neutron deficit.
  • The nucleus captures an electron from the K-shell, increasing the number of neutrons relative to protons.
  • The daughter nuclide emits characteristic X-rays when the hole created in the K-shell is filled by an electron from an outer shell.
  • K-electron capture is an alternative mechanism to β+ decay.

• Gamma decay (γ):

  • Gamma rays are high-energy photons emitted from the nucleus when it transitions from an excited state to a lower energy state.
  • Gamma rays can be emitted either simultaneously with beta or alpha particles or with a delay after their emission.

• Isomeric Transition (IT):

  • Occurs when gamma rays are not emitted immediately but after a significant delay after beta particle emission.
  • An isomeric transition is a type of radioactive decay involving a rearrangement of a nucleus from a metastable (excited) state to a lower energy state.
  • This state is often referred to as a nuclear isomer, and it can exist for an observable time before decaying.

• Internal Conversion (IC):

  • The energy liberated during isomeric transition can be transferred to an electron, causing it to be ejected from the atom.
  • This ejected electron is called a conversion electron.
  • The atom's electrons then rearrange to fill the vacancy left by the internal-conversion electron, resulting in the emission of characteristic X-rays and/or Auger electrons.

Spontaneous Fission

• A very rare type of radioactive decay in which the nucleus of an atom splits into two or more lighter nuclei, along with the emission of neutrons, gamma rays, and other particles.

Summary of Decay Processes

• General Equation: XA YA + W+Q Z Z
• Alpha decay: XA YA + α +Q (α decay) Z Z-2
• Beta minus decay: XA YA + B-+V+ Q (B- decay) Z Z+1
• Beta plus decay: XA YA + B++ γ (γ decay) Z Z-1
• Isomeric transition: XA (M) XA+ γ (IT) Z Z
• K-electron capture: XA + e- YA +V+ Q (EC) Z Z-1

Modes of Decay: α β γ

• Beta Minus (β-) Decay:
  • Excess of neutrons
  • A neutron transforms into a proton and an electron.
  • The electron (beta particle) is ejected from the nucleus.
• Beta Plus (β+) Decay:
  • Excess of protons
  • A proton transforms into a neutron and a positron.
  • The positron is ejected from the nucleus.
• Alpha (α) Decay:
  • Excess of nucleons (heavy nuclei)
  • The nucleus ejects an alpha particle (helium nucleus, consisting of 2 protons and 2 neutrons).

Decay Series

• The emission of multiple particles or gamma rays occurs when one particle emission leaves the nucleus in an excited state and it needs to reach a lower energy state (ground state).

Exponential Law of Decay

• The rate of decay of a radioactive isotope is proportional to the number of radioactive atoms present.
• Equation: N=N0e-λt
• N: The number of radioactive atoms at time t.
• N0: The number of radioactive atoms at time t0 (initial quantity).
• λ: Decay, disintegration, or transformation constant.
• The value of the transformation constant is inversely related to half-life time.

Physical Half-Life (T1/2)

• The time interval required for a given nucleus type to decay to half its original value.

Biological Half-Life

• The time required for the body to eliminate half of the dose of a substance through metabolic processes and excretion.

Effective Half-Life

• The combined effect of physical and biological decay on the amount of radioactivity in a body, organ, or tissue.
• Equation: 1/ effective t1/2=1/ biological t1/2 + 1/physical t1/2

Units of Radioactivity

• Curie (Ci): An older unit of radioactivity, representing the activity of one gram of radium 226.
• Becquerel (Bq): The SI unit of radioactivity, representing one disintegration per second.
• Conversion: Ci=37GBq

Decay Equilibrium

• Decay equilibrium: A condition established in a parent/daughter mixture when both parent and daughter are radioactive; the daughter's half-life is shorter than that of the parent.

• Secular equilibrium:

  • Occurs when the half-life of the daughter radionuclide is much shorter than the half-life of the parent (parent half-life >> daughter half-life).
  • Achieved after 6 half-lives of the daughter.
  • The decay rate of the parent and the production rate of the daughter are approximately equal.
  • The daughter seems to decay with the half-life of the parent.

• Transient equilibrium:

  • Occurs when the half-life of the daughter is shorter than the half-life of the parent (parent half-life > daughter half-life).
  • Achieved after four half-lives of the daughter.
  • Example: 99Mo (67 h) decaying to 99mTc (6 h).

Test your knowledge on the structure of atoms, including the characteristics of protons, neutrons, and electrons. This quiz covers concepts such as atomic designation, isotopes, and nuclear composition. Challenge yourself with questions that delve into the fundamental building blocks of matter!