Phet Build an Atom Answer Key

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A project at the University of Colorado Boulder called PhET Interactive Simulations is full of simulators. One of them is known as Build an Atom. The topics in this one include atoms, atomic structure, isotope symbols, and atomic nuclei.

After learning about Build an Atom, you will have to take a test. If you have completed everything and are about to take the test, you are suggested to check out the Phet Build an Atom answer key below so that you will be able to get a good result when you take it.


  1. Explore the Build an Atom simulation with your group. As you explore, talk about what you find.

a. List two things your group observed in the simulation.
Responses will vary, but here is what you should see….
· Adding protons changes the identity of the atom ­ adding one moves from right to left on the Periodic Table.
· You can get the atom to be stable/unstable by adjusting the number of neutrons
· Adding electrons will change it from a +ion to neutral atom, to ­ion depending on how many electrons you add
· Adding electrons ­ you can only add 2 to the first orbital and 8 to the second

b. Whatparticle(s)are found in the center of the atom?
Protons and neutrons

  1. Play until you discover which particle(s) determine(s) the name of the element you build. What did you discover?

Protons ­ adding them moves from left to right on the Periodic Table

  1. What is the name of the following atoms?

a. An atom with 3 protons and 4 neutrons: Lithium

b. An atom with 2 protons and 4 neutrons: Heleum

c. An atom with 4 protons and 4 neutrons: Beryllium

  1. Play with the simulation to discover which particles affect the charge of an atom or ion.

a. Fill in the blanks below to show your results:

Neutral atoms have                                           the same number of                                           protons and electrons.

Positive ions have                                           more                                           protons than electrons.

Negative ions have                                            fewer                                               protons than electrons.

b. Develop a relationship (in the form of a single sentence or equation) that can predict the charge based on the number and types of particle.

The charge of an atom is determined by the ratio of protons to electrons

  1. Play with the simulation to discover what affects the mass number of your atom or ion. Click on the green + sign next to Mass to reveal the balance, then continue to play the same way you were before.

a. What is a rule for determining the mass number of an atom or ion?

Mass = the number of protons + the number of neutrons.

  1. Practice applying your understanding by playing 1st and 2nd levels on the Game screen (the tab at the top). If you do well on those first 2 levels, move on the 3rd and 4th level!


  1. Using the Symbol readout box, figure out which particles affect each component of the atomic symbol.

a. In the atomic symbol below, label each letter (a, b, c, and d) with:

· the particle(s) used to determine the letter, and.
· how the value of each letter is determined.

a = element symbol
b = charge
c = atomic number
d = atomic mass

  1. Create a definition (using a complete sentence) for each of these items based on your labels from the atomic symbol above.

a. Element Symbol – a one or two letter abbreviation to represent the full name of the element
b. Charge ­- the difference between electrons and protons (+ has more protons, ­ has more electrons, 0 has same of both)
c. Atomic Number -­ identifies the number of protons
d. Mass Number ­- identifies the number of protons and neutrons

  1. Practice applying your understanding by playing the 3rd and 4th game levels. Play until you can get all the questions correct on the 4th level.
  2. In addition to atomic symbol, we can represent atoms by name and mass number.

a. Complete the table below:

b. Each representation (Symbol and Name) in the table above provides information about the atom. Describe the similarities and differences between the Symbol and Name representations. The symbol is the same as the first letter of the element.


  1. Play with the simulation to determine:

a. Which particles affect the stability of the atom?                                          protons and neutrons_
b. Which particles do not affect the stability of the atom?                                          electrons_

  1. What are the names of the stable forms of oxygen?

a. Oxygen­16

b. Oxygen­_17

c. Oxygen­_18

d. List all of the things that are the same about these atoms (ignore the electrons). the number of protons is the same for all three (8)

e. List all of the things that are different about these atoms (ignore the electrons). the number of neutrons is different for all three atoms (16, 17, 18)

14. The atoms in the previous question are isotopes of each other. Based on this information, list the requirements for two atoms to be isotopes of each other.

Isotopes have the same number of protons, but different number of neutrons

  1. Testyourunderstanding of isotopes by examining the relationships between the pairs of atoms listed below:

Testyourunderstanding of isotopes by examining the relationships between the pairs of atoms listed below


  1. Theperiodictable has a great deal of information about every atom. Using your periodic table, answer the following questions:

a. What is the atomic number of chlorine (Cl)? _17_
b. What is the atomic number of tungsten (W)? _74_
c. How many protons are there in any Cl atom?_17_
d. How many protons are there in any Te atom? _52_
e. Can you tell from the periodic table exactly how many neutrons are in an atom?

No, the mass number is an average mass of all of the isotopes of that element. Since isotopes are based on the number of neutrons, the mass of each isotope of an element will be a little different.

  1. Complete the followingtable:
Name Symbol Atomic




Number of


Number of


hydrogen­2 2H 1 2 1 1 0
hydrogen­3 3H 1 3 2 1 0
sodium­22 22Na+ 11 22 11 10 +1
magnesium­24 24Mg 12 24 12 12 0
magnesium­25 25Mg­1 12 25 13 13 ­1
titanium­46 46Ti­2 22 46 24 24 ­2
gold­107 107Ag 47 107 60 47 0
fluorine­19 19F­1 9 19 10 10 ­1
carbon­12 12C 6 12 6 6 0
carbon­13 13C 6 13 7 6 0
carbon­14 14C 6 14 8 6 0
carbon­12 12C­1 6 12 6 7 ­1
carbon­12 12C+1 6 12 6 5 +1
helium­4 4He 2 4 2 2 0
oxygen­16 16O­2 8 16 8 10 ­2
argon­40 40Ar 18 40 22 18 0
gallium­70 70Ga 31 70 39 31 0
gallium­70 70Ga+3 31 70 39 34 ­3
beryllium­9 9Be+2 4 9 5 2 +2
nitrogen­15 15N­1 7 15 8 8 ­1
  1. Totestyour knowledge of isotopes, draw arrows between all pairs of atoms in the table above that are isotopes of each other.

hydrogen­2 and hydrogen­3 are both isotopes of hydrogen­1 magnesium­25 is an isotope of magnesium­24

carbon­13 and carbon­14 are both isotopes of carbon­12

(carbon­12­1 is an anion of carbon­12, carbon­12+1 is a cation of carbon­12) anions are negative ions with more electrons than protons

cations are positive ions with fewer electrons than protons

(the Gallium examples are also ion

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