2.01 Elements and The Periodic Table

Try It

Atomic Mass Calculations

For the following Try It, you may need to use The Periodic Table of Elements.

In this review game, you'll practice using the periodic table and these equations:

mass number = number of protons + number of neutrons

mass number - atomic number = number of neutrons

1. If you know the atomic symbol of an element, where can you find more information about it, including its atomic number?
   
   1. The periodic table
   2. The internet
   
   
   Answer: The periodic table



2. How can you find the atomic number of an element?
   
   1. Look on the periodic table.
   2. Use the calculation "mass number - # of neutrons = # of protons
   
   
   Answer: Look on the periodic table.



3. The atomic number is equal to ______.
   
   1. The number of neutrons in an element
   2. The number of electrons in an element
   3. The atomic mass of an element
   4. The number of protons in an element
   
   
   Answer: The number of protons in an element



4. The element nitrogen (N) has an atomic number of 7. Knowing the atomic number and the atomic symbol, what is the easiest way to find out the number of protons of this element?
   
   1. Atomic number = # of protons. So, nitrogen has 7 protons.
   2. Use the equation: atomic mass - # of neutrons = # of protons
   
   
   Answer: Atomic number = # of protons. Knowing that the atomic number is equal to the number of protons in an element is the fastest way, in this scenario, to determine the number of protons in an element. Nitrogen has an atomic number of 7, meaning it has 7 protons. No math or calculations needed!



5. We have determined that nitrogen has an atomic number of 7 and has 7 protons. Next, we want to find the atomic mass of nitrogen. How can we find this information?
   
   1. Use the equation: atomic mass - # of neutrons = # of protons
   2. Look on the periodic table, which lists the atomic mass
   
   
   Answer: Look on the periodic table, which lists the atomic mass of each element.



6. We know that nitrogen's atomic number is 7, meaning it has 7 protons, and that the periodic table lists nitrogen's atomic mass as 14. Using this information, how many neutrons does nitrogen have?
   
   1. 10 neutrons
   2. 6 neutrons
   3. 8 neutrons
   4. 7 neutrons
   
   
   Answer: 7 neutrons. Using the equation "atomic mass number - number of protons = number of neutrons" allows us to calculate the number of neutrons. Atomic mass of 14 - 7 protons = 7 neutrons.



7. In a stable, neutrally charged atom, the number of protons is ________ the number of electrons?
   
   1. greater than
   2. equal to
   3. greater than by one
   4. less than
   
   
   Answer: equal to. In a stable, neutrally charged atom, the number of protons is equal to the number of electrons.



8. In this review quiz, you have determined the number of protons, neutrons, electrons, and the atomic mass of an element using your knowledge of the periodic table, atomic structure, and the following equations:
   "mass number = number of neutrons + number of protons"
   "mass number - atomic number = number of neutrons"
   Use all of this information to answer the remaining questions in this review.
   First, what is the atomic symbol for the element with the atomic number of 11? How many protons does this element have?
   
   1. Atomic symbol Na and 11 protons
   2. Atomic symbol Na and 12 protons
   3. Atomic symbol Mg and 12 protons
   4. Atomic symbol Ne and 10 protons
   
   
   Answer: Atomic symbol Na and 11 protons. Using the periodic table, you can determine that atomic number 11 is Na, or sodium. The atomic number of an element is equal to the number of protons, so Na has 11 protons.



9. You have determined that the element Na has 11 protons. What is the atomic mass of Na? How many neutrons does it have? How many electrons does it have?
   
   1. Atomic mass of Na is 22, 12 neutrons, and 11 electrons
   2. Atomic mass of Na is 23, 12 neutrons, and 11 electrons
   3. Atomic mass of Na is 25, 13 neutrons, and 12 electrons
   4. Atomic mass of Na is 20, 10 neutrons, and 11 electrons
   
   
   Answer: Atomic mass of Na is 23, 12 neutrons, and 11 electrons. We know that the number of protons is equal to the number of electrons, so there are 11 electrons. According to the periodic table, sodium has an atomic mass of 23. Atomic mass (23) - number of protons (11) = 12 neutrons.



10. You know the following about an elements: It's atomic symbol is Mo, it has 54 neutrons, and has the atomic mass of 96. Determine the following for this element: atomic number, number of protons, and number of electrons.
    
    1. Atomic number 25, 25 protons, and 25 electrons
    2. Atomic number 42, 42 protons, and 42 electrons
    3. Atomic number 43, 43 protons, and 43 electrons
    4. Atomic number 41, 41 protons, and 41 electrons
    
    
    Answer: Atomic number 42, 42 protons, and 42 electrons. The element Mo (molybdenum) has an atomic number of 42 and has 42 electrons and 42 protons. Knowing the element's atomic symbol, you looked on the periodic table to find its atomic number, 42. Since the atomic number is equal to the number of protons (plus the number of protons and electrons in a stable atom are equal), you only needed the periodic table to find the rest of the information about this element!



11. You know the following about an element: It has 103 neutrons and 70 electrons. Determine the following for this element: the atomic symbol, atomic number, number of protons, and atomic mass.
    
    1. Atomic symbol Yb, atomic number is 70, 70 protons, and atomic mass of 173
    2. Atomic symbol Er, atomic number is 68, 68 protons, and atomic mass of 167
    3. Atomic symbol Y, atomic number is 30, 30 protons, and atomic mass of 89
    4. Atomic symbol K, atomic number is 19, 19 protons, and atomic mass of 39
    
    
    Answer: Atomic symbol Yb, atomic number is 70, 70 protons, and atomic mass of 173. To determine this information, you know that the number of electrons is equal to the number of protons in a stable atom. Therefore, 70 electrons indicates 70 protons. Knowing that "number of protons + number of neutrons = mass number", you can use this equation to determine the atomic mass. 70 protons + 103 neutrons = 173 (atomic mass). The number of protons is equal to the atomic number, meaning the atomic number is 70. Knowing the atomic number, you can look on the periodic table to determine the atomic symbol (and the name) of the element. The element in question is Yb (ytterbium).



12. You know the following about an element: It has 55 protons and an atomic mass of 133. Determine the following for this elements: the atomic symbol, atomic number, number of neutrons, and number of electrons.
    
    1. Atomic symbol Ba, atomic number 56, 81 neutrons, and 56 electrons
    2. Atomic symbol Pt, atomic number 78, 117 neutrons, and 78 electrons
    3. Atomic symbol Co, atomic number 27, 32 neutrons, and 27 electrons
    4. Atomic symbol Cs, atomic number 55, 78 neutrons, and 55 electrons
    
    
    Answer: Atomic symbol Cs, atomic number 55, 78 neutrons, and 55 electrons. To find this information, you know that the atomic mass minus the number of protons is equal to the number of neutrons. 133 (atomic mass) - 55 protons = 78 neutrons. The number of protons is equal to the number of electrons, so there are 55 electrons. The number of protons is also equal to the atomic number, so the atomic number is 55. Looking on the periodic table tells us the element with the atomic number 55 is Cs (cesium).

You've completed this review of atomic mass math. If you missed 2 or more questions, review the lesson again before continuing.