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# Periodic Table of the Elements: Practice Problems

Periodic Table of the Elements: Practice Problems. Atomic Mass, Proton and Neutron Number. Atomic Mass. Protons and Neutrons have almost the same mass. Relatively speaking, electrons are so tiny that they have almost no mass.

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## Periodic Table of the Elements: Practice Problems

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1. Periodic Table of the Elements: Practice Problems Atomic Mass, Proton and Neutron Number

2. Atomic Mass • Protons and Neutrons have almost the same mass. • Relatively speaking, electrons are so tiny that they have almost no mass. • The unit of mass at the atomic level is called an Atomic Mass Unit (AMU). • Since electrons don’t contribute much mass, the mass of an atom is equal to the number of protons (1 AMU each) added to the number of Neutrons (1 AMU each.)

3. Isotopes • The properties of an atom depend on how many protons it has. • The number of protons in an atom’s nucleus determine which element (type of atom) it is. • Atoms of the same element can have different numbers of neutrons without changing the element (atom type.) • Versions of an element that have different numbers of neutrons are known as ISOTOPES. • Each isotope of an element has a different mass from the additional (or missing) neutrons. • Some isotopes are unstable and can be RADIOACTIVE.

4. Atomic Mass • The atomic mass on the PTE is the average of all masses of isotopes of that element. To find the most common mass, round the atomic mass to the nearest whole number. • P+N=AM

5. Isotopic Mass Problem 1 • Find the typical isotopic mass of Carbon (C). • Carbon’s average Atomic Mass (AM) is 12.011. • This rounds down to the typical AM of 12 AMU.

6. Isotopic Mass Problem 2 • Find the typical isotopic mass of Selenium (Se). • The average AM of Se is 78.96 AMU. • This rounds up to the typical AM of 79 AMU.

7. Isotopic Mass Problem 3 • Find the typical isotopic mass of Rubidium (Rb). • The average AM of Rb is 85.468 AMU. • This rounds to a typical AM of 85 AMU.

8. Neutron Number Problem 1 • Find the number of neutrons in a typical isotope of Aluminum (Al). • Al has an atomic number/proton number of 13. • Al has an average AM of 26.982 AMU. This rounds to 27 AMU. • Subtracting the number/mass of protons from the AM gives the neutron mass/number. • 27-13=14 neutrons.

9. Neutron Number Problem 2 • Find the number of neutrons in a typical isotope of Oxygen (O). • O has an atomic number/proton number of 8. • O has an average AM of 15.99 AMU. This rounds to 16 AMU. • Subtracting the number/mass of protons from the AM gives the neutron mass/number. • 16-8=8 neutrons.

10. Neutron Number Problem 3 • Find the number of neutrons in a typical isotope of Lead (Pb). • Pb has an atomic number/proton number of 82. • Pb has an average AM of 207.2 AMU. This rounds to 207 AMU. • Subtracting the number/mass of protons from the AM gives the neutron mass/number. • 207-82=125 neutrons.

11. Proton and Neutron Problem 1 • An element has an Atomic Mass of 35 AMU and a neutron number of 18. What element is it? • P+N=AMU • P=Atomic Number • P+18=35 • P=35-18 • P=17=Chlorine (Cl)

12. Proton and Neutron Problem 2 • An element has an Atomic Mass of 40 AMU and a neutron number of 20. What element is it? • P+N=AMU • P=Atomic Number • P+20=40 • P=40-20 • P=20=Calcium(Ca)

13. Proton and Neutron Problem 3 • An element has an Atomic Mass of 128 AMU and a neutron number of 76. What element is it? • P+N=AMU • P=Atomic Number • P+76=128 • P=128-76 • P=52=Tellurium (Te)

14. Tough Question 1 • An element has a neutral charge with 15 electrons. What is its typical atomic mass? • An atom with a neutral charge has as many protons (+1) as electrons (-1). • If there are 15 electrons, there must be 15 protons. • Proton number is the same as atomic number. • The element with atomic number 15 is Phosphorous (P). Correct.

15. Tough Question 2 • A Noble Gas has 22 neutrons. What element is it? • Move through the Noble Gases (Group 8/18), subtracting 22 from the rounded average atomic mass until you find a number equal to the atomic number. • Helium and neon are too small, since their total mass is less than the mass of 22 neutrons. • Argon’s (Ar) mass of 39.94 rounds to 40 AMU. Subtracting 22 gives 18 protons, which is the same as the atomic number. Correct.

16. Tough Question 3 • A typical isotope of an element has 6 neutrons. What element is it? • It must be an element with low atomic mass, to only have 6 neutrons. To find it, move through the first few periods on the PTE, subtracting the proton/atomic number from the average atomic mass for each element until the total rounds to 6. • Hydrogen and Helium have atomic masses less than 6, so they cannot be the element, since they have to have at least some protons. • Lithium (Li) has an average mass of 6.94. This rounds to a typical mass of 7. Subtract the protons/atomic number of 3 to get 4 neutrons. Incorrect. • Beryllium (Be) has an average mass of 9.01. This rounds to a typical mass of 9. Subtract the atomic number to get 5 neutrons. Incorrect. • Boron (B) has an average mass of 10.81. This rounds to a typical mass of 11. Subtract the atomic number to get 6 neutrons. Correct.

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