Lab: PhET Build an Atom

This lab is a computer-simulation-based activity. The simulator is available for immediate use in any browser at https://phet.colorado.edu/sims/html/build-an-atom/latest/build-an-atom_en.html. Or, perhaps more easily accessed by performing a search for “PhET Build an Atom”.

Background

Atomic structure depends on just three numbers: the number of protons, the number of neutrons, and the number of electrons. Atoms with different numbers of protons are different elements, each with its own atomic symbol, a unique single upper-case letter or upper-case plus one lower-case letter symbol. The number of protons is referred to as the atomic number and we use the letter Z to represent it. The elements are displayed in order by their atomic numbers in the periodic table, where the atomic number is usually given above the atomic symbol.

Every atom has a nucleus, which is a tiny object made entirely of protons and neutrons. Almost all atoms must have neutrons, along with protons, in their nucleus. Protons (p⁺) are sub-atomic particles with a +1 charge and a mass of about 1 atomic mass unit (1 amu). Neutrons (n⁰) are sub-atomic particles with a zero charge (they are neutral) and also have a mass of about 1 amu. Together, the number of protons plus the number of neutrons add up to make the atomic mass number. The atomic mass number is the sum of protons and neutrons in a particular atom of an element. Atomic mass number is represented by the letter A. The isotopes of an element are atoms with the same number of protons but different numbers of neutrons. Each isotope has a unique mass number. The mass number is not the same thing as the mass, which is just the weight of the atom. Mass number is a number and you can tell what it is by counting protons and neutrons, not by weighing the atom. Some isotopes of an element are stable and for most elements these are the only atoms that are found in nature. There are other isotopes which are unstable, that is, radioactive. These can be made in nuclear physics labs, nuclear reactors, and in supermassive stars.

Every atom’s nucleus is orbitted by electrons. Electrons (e^–) are even smaller sub-atomic particles than protons and neutrons with a –1 charge and a tiny mass (0.00054 amu). Electrons do not add anything to the mass number but they do balance out the positive charge of the protons in the nucleus. Each proton has a charge of +1 and each electron has a charge of –1. Electrical forces are very powerful and they create a requirement that (most of the time) the number of electrons in an atom has to be equal to the number of protons. Atoms do lose and gain electrons but when we think of atoms it’s useful to think of them first as neutral, that is with zero charge. In order to be neutral an atom has to have the same number of electrons as protons.

Atoms with the same number of electrons as protons are neutral but atoms with either more or fewer electrons than this are ions. An ion is an atom that has gained or lost one or more electrons. Ions have a net electrical charge calculated by adding up all of the positive charges for the protons with all of the negative charges of the electrons. Atoms with a positive charge have fewer electrons than protons and are called cations. Atoms with a negative charge have more electrons than protons and are called anions.

Exercises

When you open the simulator you will be on the Home tab. Click on the Atom tab at the bottom of the screen. When you get to that tab, click the buttons that will make the “Net Charge” and “Mass Number” displays open up. Check all of the boxes below those displays for Element, Neutral/Ion, and Stable/Unstable. Drag one proton, one neutron, and one electron from the buckets up onto the atomic model so that everything looks exactly as it does in the image above (or at the bottom of the previous page if you have a printed version of this activity.

Fill in the following table by creating all of the stable isotopes of each element with the simulator and recording what you see on your screen. In addition, make sure every atom is neutral. If you are doing this on a piece of paper, please take care to set up a neat table like the one you see on your screen.

Next, click on the Symbol tab at the bottom of the simulation screen. Make everything on your simulation screen look exactly like the image at right. Turn on the “Symbol“ and “Element” displays and check all of the boxes: Element, Neutral/Ion, and Stable/Unstable. Rebuild each atom from the previous exercise and use the simulation to determine the full atomic symbol, which gives all the information needed to know everything about the structure of an atom. Each symbol is now composed of a letter-symbol and three numbers. Fill in the following table with the symbol, drawn exactly as you see it in the simulation. One has been filled in as an example so you know what to do.

1. Which atoms above lost electrons? Write their symbols, showing charge in particular, and write the number of electrons with each one.
2. Which atoms above gained electrons? Write their symbols, showing charge in particular, and write the number of electrons with each one.
3. Do atoms that gain electrons become positive or negative ions? Why?
4. Do atoms that lose electrons become positive or negative ions? Why?
5. Some of atoms in the table above have different numbers of neutrons but the same number of protons. What are such atoms called in relation to one another?
6. Does the isotope of an element matter when it comes to determining the charge? Explain.

Play the Game

The final part of these exercises is to play the Game. Go to the Game tab of the simulation and play each one of the four games until you can get a perfect score. If you are not able to get perfect scores then it may mean you don’t quite understand everything in this lesson yet. Go back over the work you have done and zero in on the mistakes you made in the game. In this way the game can help you to learn. If you are competitive, challenge a classmate by turning on the timer in the game and see who can get the shortest time. If you are not competitive, then this is not required.