Atomic Structure

• What is the charge of the nucleus in an atom?

  Positively charged
• What particles are contained in the nucleus of an atom?

  Neutrons and protons
• What is the relative mass of a proton?

  1
• What is the relative charge of a neutron?

  0
• What is the relative mass of an electron?

  0 (approximately 0.0005)
• What is the typical radius of an atom?

  $1 \times 10^{-10} \text{ metres}$
• How does the radius of the nucleus compare to the radius of an atom?

  The radius of the nucleus is 10,000 times smaller than that of the atom.
• Where is most of the mass of an atom concentrated?

  Most of the mass of the atom is concentrated at the nucleus.
• What is the arrangement of electrons in an atom?

  • Electrons lie at different distances from the nucleus.
  • They occupy different energy levels.
  • Electron arrangements may change with interaction with EM radiation.

  
• What do all atoms of the same element have in common?

  They have the same number of protons.
• What defines a neutral atom?

  A neutral atom has the same number of electrons and protons.
• What are isotopes?


  Isotopes are atoms of the same element with different masses, having the same number of protons but different numbers of neutrons.
• Give an example of isotopes of carbon.

  Carbon-12, Carbon-13, and Carbon-14.
• In the notation $^{A}_{Z}X^{\pm N}$, what does X represent?


  X is the letter of the element.
• In the notation $^{A}_{Z}X^{\pm N}$, what does A represent?


  A is the mass number (number of neutrons and protons).
• In the notation $^{A}_{Z}X^{\pm N}$, what does Z represent?


  Z is the proton number.
• In the notation $^{A}_{Z}X^{\pm N}$, what does N represent?


  N is the charge of the atom.
• What happens when electrons change orbit in an atom?

  Electrons moving to a higher orbit absorb EM radiation, while those falling to a lower orbit emit EM radiation.
• What occurs if an electron gains enough energy?

  It can leave the atom to form a positive ion.
• What did Dalton propose about atomic structure in 1800?


  Dalton said everything was made of tiny spheres (atoms) that could not be divided.
• Who discovered the electron in 1897?

  JJ Thomson
• What was the Plum Pudding Model?

  • Proposed by JJ Thomson.
  • Atoms consist of a positively charged "pudding" with negatively charged electrons dispersed throughout.
• What did Rutherford realize about the atom in 1911?

  Most of the atom was empty space.
• What was the outcome of the Gold Foil Experiment?

  Most alpha particles went straight through, indicating that most of the atom is empty space.
• What did the deflection of some alpha particles in the Gold Foil Experiment suggest?

  The nucleus must be charged, deflecting positive alpha particles.
• What does it mean if alpha particles are deflected by more than 90° in the Gold Foil Experiment?

  The nucleus contains most of the mass of the atom.
• What is the Bohr model of the atom?

  • Proposed in 1913 by Niels Bohr.
  • Electrons exist in fixed 'orbitals' around the nucleus.
  • The model explains electron energy levels.
• Who provided evidence for the existence of neutrons?

  James Chadwick
• What is radioactive decay?

  • A random process where unstable nuclei emit radiation to become more stable.
  • The rate of decay is called activity, measured in Becquerel (Bq).
• What is the difference between activity and count-rate in radioactive decay?

  Activity is the rate at which a source of unstable nuclei decays, while count-rate is the number of decays recorded by a detector per second.
• What are the forms of radioactive decay?

  • Alpha (α): Highly ionising, weakly penetrating (~5cm of air).
  • Beta Minus (β): Medium ionising, medium penetration (~50cm of air, sheet of paper).
  • Gamma (γ): Low ionising, highly penetrating (very far in air, few cm of lead).
• What is a nuclear equation?

  A nuclear equation represents radioactive decay.
• What happens to the mass and charge of the nucleus during alpha decay?

  Alpha decay causes both the mass and charge of the nucleus to decrease.
• What happens to the mass and charge of the nucleus during beta decay?

  Beta decay does not change the mass of the nucleus but increases the charge.
• What happens during gamma decay?

  Gamma decay does not change the mass or charge of the nucleus.
• What is half-life?


  • The time taken for half the nuclei in a sample to decay.
  • Also the time taken for the activity or count rate of a sample to decay by half.
• How can you determine the half-life from a decay process?

  If 80 atoms fall to 20 over 10 minutes, the half-life is 5 minutes.
• What are the characteristics of short and long half-lives?

  • Short half-life:
  • Source presents less long-term risk.
  • Initially very radioactive but quickly decays.
  • Long half-life:
  • Source remains weakly radioactive for a long time.
  • Example: Americium with a half-life of 432 years, used in smoke alarms.

  
• How does Americium function in smoke alarms?

  Americium emits alpha particles into the air around the alarm, which decreases when smoke is present, causing the alarm to sound.
• What is the difference between contamination and irradiation?

  • Contamination:
  • Lasts for a long time.
  • Source of radiation is transferred to an object.
  • Irradiation:
  • Lasts only for a short time.
  • Source emits radiation but does not make the object radioactive.