MCAT General Chemistry Study Questions

These are questions I came up with when I was studying for the MCAT.


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Atoms and Molecules

  1. What are the components of an atom?  What parts of an atom contribute significantly to the atomic weight?
  2. What would the mass of a mole of neutrons?
  3. What are orbitals?  How many electrons can an orbital hold?
  4. What are the four different quantum numbers?  What is a synonym for the azimuthal quantum number?
  5. What does the term AUFBAU refer to?  What is Hund's rule?  What is the Pauli exclusion principle?
  6. Which will fill up first, 4s or 3d?  Using the periodic table, what is the general pattern for the filling of orbitals?
  7. Where on the periodic table is the s-block?  the p-block? the d-block?  the f-block?  Which block are the transition metals located?
  8. What are the alkali metals?  What are the alkaline earth metals?
  9. What are the halogens?  What are the noble gases?
  10. Which elements are found naturally in a diatomic form?
  11. Why are noble gases so inert?
  12. What two electronic conditions does a nucleus seek?  Which condition is left unsatisfied in radicals?  Which condition is left unsatisfied in ions?
  13. What is the term for two or more atoms connected by a covalent bond?
  14. What on the periodic table is meant by the term FAMILY?  What about GROUP?  What about PERIOD?
  15. What is "energy of ionization"?  What side of the periodic table has elements with low energies of ionization?  What does this mean?
  16. What is meant by "electron affinity"?  How does electron affinity tend to change going left to right and going top to bottom?
  17. Why, along a period, does the atomic radius tend to get smaller?  Among a cation, a noble gas, and an anion with the same electronic configuration, which has the larger radius?  What happens to the radius as one goes from top to bottom on the periodic table?
  18. What is meant by "effective nuclear charge"?  What is shielding?
  19. What is meant by "quantized" energy levels?
  20. What is an electronic transition?  What happens as an electron goes from a lower level to a higher level?  What happens in the reverse?  What is meant by "exited state"?  What is the "ground" state?
  21. What does an s orbital look like (conceptually, at least)?  How many s orbitals are found on a shell?  What do p orbitals look like?  What shell is the first to have p orbitals?  How many are there on that shell?
  22. When drawing orbitals, what do the positive and the negative signs on the orbital lobes indicate?  Better, what DON'T they indicate?
  23. What are valence electrons?  How many electrons does it take to fill hydrogen's valence shell?  How many to fill oxygen's valence shell?
  24. How many electrons are represented in the symbol for a single bond?
  25. In what shell is the first d orbital found?
  26. What is the formula for the energy of a photon?
  27. What is the difference between an ionic bond and a covalent bond?  What is an unevenly balanced covalent bond called?
  28. Besides conjugated organic molecules, what type of atoms/molecules commonly contribute to the colors we see (by absorbing light)?





  1. What is the difference between a solution and a colloid?  Which will scatter a light beam?  What is this phenomenon called?
  2. What is meant when a compound "precipitates"?  What is meant when a solution is "saturated"?  What is the common-ion effect?
  3. What is the difference between molarity and molality?  What are the units of each?  Which is volume dependent?  Which is temperature dependent?  Is volume dependent on temperature?
  4. What is molar solubility?  What is Ksp?  What is the difference between the two? How do you convert between the two?  What is meant by "ion product"?
  5. What do high solubility products indicate?  What do low values indicate? 
  6. Which is the larger number : 3 x 10-6 or 3 x 10-10? (common absent-minded mistake)
  7. What can be said about the solubility of Na+, K+, and NH4+ ionic compounds?  What can be said about the solubility of ionic compounds containing nitrate?
  8. If solubility is a relative concept, then around what value is considered the borderline between "soluble" and "insoluble"?  What is meant by "sparingly soluble"?
  9. Which is more water soluble : NaOH or Mg(OH)2?  What can be done to the solution to make the least soluble compound more soluble?
  10. What is meant by miscible?  Can certain gases be non-miscible?  Can certain liquids be non-miscible?







  1. How does the equilibrium relate to the rates of the reactions?  Does changing the activation energy of a reaction change the equilibrium?  Why or why not?
  2. What is Le Chatelier's principle?  What does it predict?
  3. What is the general expression for a reaction's equilibrium constant?  Is the same constant used with different temperatures?  Is the same constant used with different reactant/product concentrations?
  4. Adding more product to a reaction at equilibrium will shift the reaction which way?  Adding more product?
  5. What is meant by "Reaction Quotient"?  Under what conditions does the Qc=Kc?  When Qc<Kc, which way will the reaction proceed?
  6. If a reaction is endothermic, will the equilibrium constant be lower or higher at a higher temperature?  What about for exothermic reactions?
  7. What is the difference between the constants Kc and Kp?  How are the amounts of the reactants/products expressed in each of the constants?  For a given reaction of a gases, are the Kp and Kc different values?  Why or why not?
  8. For a reaction of gases in equilibrium, will adding an inert gas to the container shift the equilibrium?  Will this inert gas affect the partial pressures of the gases involved in the reaction?  Will decreasing the volume shift the equilibrium?  Will decreasing the volume affect the partial pressures of the gases involved in the reaction?
  9. What types of reaction participants are excluded in the equilibrium expression?  Are pure gases included?  Are pure liquids?  Are pure solids?  Are impure solids?
  10. Must a reaction's equilibrium expression be determined only by experiment?
  11. In an equilibrium expression, what is on top: reactants or products?
  12. If 2 moles of compound X react with 1 mole of compound Y to form compound  Z, what is the equilibrium expression?
  13. What is meant by a reaction "proceeding to completion"?  How can reactions be encouraged to do so?
  14. What is the molarity of water?  Is this ever included in equilibrium expressions?
  15. What is meant by vapor pressure?  Does a liquid with a high vapor pressure have a high or low boiling point?
  16. What happens to a liquid's vapor pressure as the temperature increases?
  17. What is the temperature referred to at which a liquid's vapor pressure equals atmospheric pressure? 
  18. What happens when a liquid's vapor pressure is higher than the liquid's vapor phase partial pressure but lower than atmospheric pressure?
  19. When water is at room temperature at sea level, is the water's vapor pressure above or below atmospheric pressure?
  20. Does it take longer to boil an egg at higher altitude or a lower altitude?  Why?  Does water at a high altitude boil at a lower or higher temperature than water at sea level?
  21. Suppose a closed container is half full of liquid... what is the pressure of the gas above the liquid equal to?  What would happen to the pressure if the volume above the liquid was halved?  What would happen to the pressure if more pure liquid is added?
  22. What is meant by "dynamic equilibrium"?
  23. Do solids have vapor pressures?




  1. What is a phase?  What is meant by an aqueous phase?  What is meant by an organic phase?
  2. What does the term "fusion" (not the nuclear type) mean?
  3. Is melting endothermic or exothermic?  When is melting spontaneous?  When is melting non-spontaneous?
  4. What is meant by the term "heat capacity"?  What is meant by "specific heat capacity"?  What is the specific heat capacity of water?  How does the heat capacity of water compare to other materials?
  5. How many joules are in one calorie?  How many calories are in one nutritional calorie?
  6. What is the "heat of vaporization"? What is the "heat of fusion"?
  7. At normal atmospheric pressure, what is the temperature of ice water in equilibrium?  What would happen to the temperature if heat were added?  Why?  What would happen if heat (thermal energy) were taken away?
  8. Under normal conditions, why can't you heat boiling water above 100 degrees C?  What happens to the water when more heat is added? 
  9. Where on a P vs. T graph is the triple point?  The critical point?  Where on a P vs. V graph is the critical pressure?
  10. How does the mass of 1 mol of liquid water molecules in liquid form compare to that in ice form?  How does the volume compare?  How does the density compare?
  11. Why is the solid form of water unusual?  What does the slope of the solid-liquid line in a P vs. T graph of water look like?  That of other liquids?
  12. What is meant by the term "specific gravity"?  What is the density of water in SI units?  If something has a specific gravity of 17, what is its density?
  13. What happens to the energy invested in melting a solid?  What happens to the energy used to vaporize a gas?  Why doesn't the temperature increase?



  1. What is an ideal gas?  What is a real gas?  How do they differ?  Under what conditions does a real gas approximate an ideal gas? (temperature? pressure?)
  2. What is the ideal gas formula? 
  3. What does Boyle's law say?  What does Charles law say?
  4. What is Avogadro's law say?
  5. What is meant by partial pressure?
  6. What many torr is atmospheric pressure?  bar?  Pa?  atm?  mmHg?  What is the relationship between bars and pascals?  What is the relationship between mmHg and torr?  Which is the SI unit?
  7. What is the difference between absolute pressure and gauge pressure?  Which one can be negative?  How?
  8. What effect does temperature generally have on the solubility of a gas in a liquid?  How is this different than a solid in a liquid?
  9. How many liters will one mole of gas molecules take up at STP? <memorize this number>
  10. What are the main components of air?
  11. What is the weight of the air in the atmosphere over an area of one square meter?




Colligative Properties

  1. What are the four main colligative properties?  Are the properties of these solutions changed by the amount of substance present?  Are they changed by the nature of the substance present?
  2. What does dissolving salt in water do to its melting point?  to its boiling point?  Will a fresh water freeze before salt water?  Will fresh water boil before salt water?
  3. What constant is used to calculate the change in the freezing point?  What are its units? 
  4. What is the approximate mass of water in a liter of aqueous solution?
  5. What does hyperosmolar refer to?  What is meant by hypoosmolar?  What type of membrane must be present to observe osmosis?
  6. Suppose a membrane permeable to most ions separates two compartments of water.  When salt is added to one compartment, what will happen?  Is this osmosis?
  7. Suppose a membrane that is permeable to only water separates two compartments of water.  When salt is added to one compartment, what is observed?  Is this osmosis?
  8. What effect does dissolved non-volatile solutes have on the vapor pressure of a liquid?
  9. What is the procedure for using freezing-point depression to determine the molecular weight of a substance?
  10. What is meant by van't Hoff factor?  Ideally, what would that be for sodium chloride?  How about that for calcium chloride?



  1. What distinctions must be made between the equilibrium of a reaction and the rate at which the reaction takes place?  How are these two concepts related?
  2. What is the activation of energy of a reaction?  What two values are subtracted?  How is this different from the free energy change of a reaction?  Which two values are subtracted for this?
  3. How does a catalyst change the rate of a reaction?  Does it change the rate of the reverse reaction?  How does a catalyst affect the equilibrium?
  4. Must a real reaction's rate constant expression be determined only by experiment?
  5. What is meant by ELEMENTARY reaction?  How is this treated differently than a multi-step reaction?  How is the rate expression determined for an elementary reaction?
  6. For a multi-step reaction in which all the individual reactions are known, which step will decide the expression for the rate constant?
  7. When a slow elementary step is preceded by a fast reaction, how are the reactant concentrations for the slow step approximated?
  8. What is being measured by the value "reaction rate"?  Is the rate of a reaction affected by the concentrations of the products or reactants?
  9. The reaction rate is equal to the rate expression multiplied by what?
  10. What happens when the rate of the forward reaction equals the rate of the reverse reaction?  At this point, what is the overall rate of the reaction?
  11. What is meant by "reaction order"?  For a real life reaction, how must this reaction order be determined?
  12. What is a zero-order reaction?  Does the rate of the reaction change as the reaction proceeds?  What is a first order reaction?  Does the rate of the reaction change as the reaction proceeds?
  13. What is the order of a reaction that decays with a constant half life?  How many half lives will it take for a reactant to fall to below 13% of its original amount?
  14. Why do kinetics experiments often measure only the first part of a reaction?
  15. Assuming none of the reactants or products decay with increased temperature, how do higher temperatures affect the rate of a reaction?  Why?  How does temperature affect equilibrium constants?
  16. When reacting compounds X and Y to form Z, what is suggested when the rate of Z formation doubles when the concentration of X is doubled?  If the rate increases 9 fold when the concentration of Y is tripled, what is the order of Y?  What is the overall reaction order?
  17. Why are high temperatures often used to determine what the thermodynamic properties of a reaction are?
  18. What is the difference between the reaction constants k and K?




  1. Is HCl a molecule or an ionic compound?  What about NaOH?  Is it possible to have a covalent compound in solid form that ionizes upon addition to water?
  2. What is an acidity constant?  How is this abbreviated?  What is a pKa?  What type of acid is indicated by a low acidity constant?  a low pKa?
  3. What is the equilibrium expression for Ka?  What is the equilibrium expression for Kb?
  4. What is Kw?  What is the value for pKw?  Does this change with temperature?  Does this equilibrium always hold?
  5. What is meant by amphoteric?  Does an amphoteric acid have a conjugate base?  Does a non-amphoteric have a conjugate base?
  6. Water is amphoteric.  If you know the pKa for water,   14 - pKa   represents the pKb for what molecule?  What would you guess is a common mistake that students make in regards to what molecule the pKb for an amphoteric acid/base refers to?
  7. What is the conjugate acid of water?  What is the conjugate base of water?  What is the pKa of water? (not as easy as you may think)
  8. How do you convert from pKa to Ka?  How do you convert from Ka to pKa?  8.0 x 10-3 would be within what pKa range (by quick estimate)?
  9. How do you calculate the Kb of the conjugate base from the Ka?  What about the pKb of the conjugate base from the pKa?
  10. What does pH measure?  How is this concentration value converted into pH?  How is pH converted into this concentration value?
  11. What is pOH?  How is is calculated from pH? 
  12. What is the difference between pH and pKa?  What about pOH and pKb?
  13. How does one derive the Henderson-Hasselbach equation from pH and pKa? (derive it yourself)
  14. Which definition of acids and bases is the most generic?  Which is the most specific for aqueous solutions?
  15. What is the pH when 10-12 mols of HCl are added to 1 liter of water?  Why isn't this value 12?  What must not be ignored when adding amounts of acid this small that can be ignored when adding a large amount of strong acid? (common trap)
  16. Suppose you have a liter of water... what is the [H+] after about 500 mL of 0.1 M HCl has been added?  Why isn't it 0.05?  What must not be forgotten when mixing two aqueous solutions together? (common trap, esp. in titrations)
  17. What is the end point of titration?  What is the equivalence point?  What is the difference?
  18. What does the slope of the line at the equivalence point in a titration graph look like?  What about the slope of the line at the pKa?
  19. What is the concentration of an acid compared to its conjugate base when pH=pKa?
  20. What are buffers?  What is the relationship between the two components in a buffer?  What are their relative concentrations when the buffer is at its maximum capacity?  Why don't strong acids or strong bases work as buffers?  What does the titration graph look like as it progresses through a buffered region?
  21. In aqueous solution, what makes an acid strong?  Is H20 a stronger acid than HCl?  Is H30+ a stronger acid than HCl?  Is H20 a stronger acid than acetic acid?  Is H30+ a stronger acid than acetic acid?
  22. In aqueous solution, what makes an base strong?  Is H20 a stronger base than NH3?  Is OH- a stronger base than NH3?    Is H30+ a stronger acid than NH4+?
  23. At what point does water "level" the strength of an acid?  Is HCl in aqueous solution stronger than other strong acids?  What type of solvent would allow fewer acids to be considered "strong"?
  24. Which of the following is the strongest acid : HF, HCl, HBr, or HI?  Which is a weak acid in aqueous solution?  Which has the strongest conjugate base?
  25. When a strong base is titrated with a strong acid, what is the pH of the equivalence point?  Which would have an equivalence point with pH < 7 : a strong acid + weak base OR a weak acid + strong base?
  26. When titrating with a strong acid, does a weak base require more acid than a strong base?  (common trap)
  27. How does ammonia increase the concentration of hydroxide ions when added to water?
  28. Is ammonia commonly considered an acid or base?  How about ammonium?  Which has a lone pair of electrons?
  29. What approximations can sometimes be made when calculating the pH of a solution?
  30. What are indicators?  Where in a titration graph would the best indicator's range be located?



Thermal Energy

  1. What is thermodynamics?  Does thermodynamics predict whether a reaction will occur spontaneously?  Does it predict how fast that reaction will reach equilibrium?  Does it predict what that equilibrium will be?
  2. What is the first law of thermodynamics?  A machine that violated this would be able to do what?
  3. What is the second law of thermodynamics?  A machine that violated this would be able to do what?
  4. What is meant by "spontaneous"?  What value measures the spontaneity of a reaction?  (do not say enthalpy)  Is this value positive or negative for a spontaneous reaction?
  5. What does entropy measure?  What type of entropy change (positive or negative) contributes to a reaction being spontaneous?
  6. What does enthalpy measure?  What type of enthalpy change contributes to a reaction being spontaneous?
  7. What is "enthalpy of formation"?  How can this be used to determine the heats of reaction?  What is the enthalpy of formation of oxygen gas?
  8. What is the formula for Gibb's Free Energy?  What are the two main components for determining  ΔG?  Which should each be to contribute to a more negative value for ΔG?
  9. What effect will a catalyst have on ΔG?  Why?
  10. Assuming ΔH and ΔS are constant over a range of temperatures, which of these components has its effect on ΔG modified by temperature?
  11. Reactions with negative ΔH  values are called what?  What about positive ΔH  values?
  12. Increasing the temperature of a reaction with a negative change in entropy will ideally do what to the spontaneity of the reaction?  What about a reaction with a positive change in entropy?
  13. What type of change in entropy can be expected for reactions in which a solid becomes a liquid?  For reactions in which a liquid becomes a solid?  For a reaction in which two moles of gas react to form three moles of gas?
  14. What can be said about  the spontaneity of reaction with a negative change in enthalpy and a positive change in entropy?  Can a reaction be endothermic and still be spontaneous?  Is so, how?  If not, why not?
  15. For what conditions is ΔH defined?  What conditions must exist for ΔG to be calculated?  What about for ΔG to be meaningful?
  16. What is the ΔG of a reaction at equilibrium?
  17. What is the kinetic theory of gases?  What does temperature represent in a gas? 
  18. What term is used to denote the velocity of gas molecules which would correspond to the average kinetic energy of the gas molecules?  How might this be different from simply the average velocity?
  19. Doubling the velocity of gas molecules will do what to the temperature of the gas?
  20. What are the conditions in STP?  How does this differ from the conditions at which most thermodynamic values are cited? 
  21. Without doing any calculations, what is the volume (in liters) of 1 mol of an ideal gas at STP? (memorize)
  22. What is a heat engine?  What must exist in order for a heat engine to do work?
  23. How is the efficiency of a heat engine calculated?  What does an efficiency of 1 mean?  What law of thermodynamics would this violate?
  24. How is the change in entropy calculated?  What is the  ΔS for 40 J of energy leaving a heat reservoir at 400 K?
  25. What is the difference between ΔG and ΔG?  Suppose ΔG is cited for 25 C, under what conditions would  ΔG= ΔG?
  26. What is the formula for determining ΔG of reaction from the known ΔG at that temperature and the concentrations of the reactants and products?
  27. What is meant by internal energy?  Is potential energy part of internal energy?  Is organized kinetic energy part of internal energy?  Is disorganized kinetic energy part of internal energy?  Are molecular vibrations part of internal energy?
  28. What does adiabatic mean?  What is the difference between the terms ADIABATIC and ISOTHERMAL?  For an adiabatic process, an increase in internal energy must be attributed to what form of energy input?
  29. What is an isothermal process? In this type of process, what happens to the energy provided by work on a gas?
  30. What is PV work?  Can a process at constant pressure perform PV work?  Can a process at constant volume perform PV work?
  31. In a constant-V process, what happens to the energy transferred to a system as heat?  In a constant-P process, what happens to the energy transferred to the system as heat?  Assuming the same amount of heat was transferred, which system will have a higher internal energy?  What value represents the change in internal energy plus the work performed by the system?
  32. If the number of moles and the temperature of an ideal gas were held the same, what would the P versus V graph look like?  How would the graph look if plotted for a different constant temperature?  What are isotherms?  On a P-V graph, what shape are isotherms of an ideal gas?
  33. What is occurs when the state of a gas is taken from a point on a P-V graph to one further to the right?  What about from the right to the left?  For an isothermal process, what can be assumed about the heat input as the state moves to a point further on the right?  For an adiabatic process, what is known about the heat input for the same change in volume?  Which will have the lower pressure (y-axis): the result of the isothermal process or of the adiabatic process?
  34. In a P-V graph, what is represented by the area encompassed by the lines in a cyclic process?
  35. What is a state function?  Is entropy a state function?  Is internal energy a state function?  Is heat a state function?  Is work a state function?  Is enthalpy a state function?  Why or why not?
  36. The combustion of a physical chemistry textbook is spontaneous... why is it still on my bookshelf?  Why don't some spontaneous reactions proceed?
  37. If there is no working air conditioner on a hot day, would keeping the door to the refrigerator open be a good way in the long term to cool down the room? 


  1. What is reduction?  What is oxidation?
  2. What unit measures a compounds desire to undergo either a reduction or oxidation?
  3. What happens to the oxidizing agents in a reaction?
  4. What are reduction potentials?  How are they measured?  What are they relative to?  What does a positive value indicate?
  5. How does one obtain an oxidation potential given the value of the reduction potential?
  6. How does one calculate the standard potential for a reaction?  Are reduction potentials affected by stoichiometry?
  7. How is the E of a redox reaction calculated from its E?  What are the concentrations of the products and reactants during a standard reaction?   How is ΔG calculated from the E?
  8. In the equation ΔG = -nFE, what does F represent?  What value represents "moles of electrons"?  What is the sign of the change in free energy when E is positive?
  9. What is an electrochemical cell?  What are the two main types?
  10. What electrode is defined as the one where oxidation occurs?
  11. What electrode is defined as the one where reduction occurs?
  12. What electrode is positive in a voltaic cell?  Which is positive in an electrolytic cell?
  13. Which type of cell requires an input of energy?  Which type of cell produces electricity?
  14. What type of half reaction occurs when a metal is converted into a cation? 
  15. What occurs when iron rusts?  What happens to the oxidation number on the iron atom?  What are three common forms (oxidation states) of the iron atom?
  16. What type of half reaction is occurring when atoms in solution deposit on the electrode to form a solid?
  17. When performing electrophoresis, do anions migrate towards the anode or the cathode?  What type of cell can this be considered?  Is an outside source of electricity required?
  18. How can a reaction that has the same type of atoms in equal amounts on both sides still be unbalanced?
  19. What is the purpose of a salt bridge?  What would happen in a voltaic cell if a salt bridge (or equivalent) were not present?  Which half cell would become more positive?  Which half cell would become more negative?
  20. Can a cell with the standard potential of zero still produce a current?  What is a concentration cell?
  21. What is a "sacrificial anode"?  How does this protect certain metal?
  22. Why does elemental sodium react violently with water?  What gas is produced in this reaction?  What happens to the pH of the water?
  23. What is the photoelectric effect?  What is the work function?  What when the energy of a photon is below that of the work function?  What happens to the extra energy when photon energy is above the work function?
  24. Does increasing the intensity of incident light cause electrons of higher energy to be ejected?  Why or why not?



Nuclear Chemistry

  1. What is alpha radiation?  Beta radiation?  Gamma radiation?  Which type is electromagnetic in nature?  Which types have particles with mass?  Which types have particles with significant mass?  Which types have charged particles? 
  2. Alpha radiation particles can be thought of as nuclei of what atom?
  3. Which type of radiation can penetrate the deepest? 
  4. What is the difference between atomic number and atomic mass?  Which number is the superscript and which is the subscript?  Which one determines the element?  For a given element, which number will determine the isotope?
  5. For carbon-12, what does the number 12 represent?
  6. What is the mass number of an alpha radiation particle?  What is the atomic number of an alpha radiation particle?
  7. What type of elements tend to emit alpha radiation?  Where on the periodic table are they?
  8. When an atom loses an alpha particle, what happens to the mass number?  What happens to the atomic number?  Is this atom a different element?
  9. What is emitted when a proton converts to a neutron?  What does this conversion do to the atomic number of the atom?  What does it do to the mass number?
  10. What is emitted when a neutron converts to a proton?  What does this conversion do to the atomic number of the atom?  What does it do to the mass number?
  11. What is a positron?  What is it analogous to? 
  12. What is nuclear fission?  What type of elements tend to undergo fission (as opposed to fusion)?  What is nuclear fusion?  What types of elements undergo fusion?
  13. What does "critical mass" refer to?  How was an element of critical mass produced in the first atomic bomb dropped on Japan?
  14. How might gas molecules containing isotopes of an element be purified to increase the percentage of one of the isotopes?
  15. What is the name for a hydrogen atom with a neutron?  With two neutrons?
  16. What does iron represent in terms of nuclear stability?
  17. Where in the electromagnetic spectrum are gamma rays found?
  18. What is the reaction order for nuclear decay?
  19. What is meant by the term fissile?  What about fissionable? 
  20. What does the equation E=mc2 mean?  What does each symbol in the equation represent?  What units?
  21. What are the four fundamental forces?  What keeps the nucleus together?  What force acts against the nucleus?
  22. Why might the mass of a nucleus differ from the sum of the masses of the individual components?  What is binding energy?  How does this relate to nuclear fusion?