High School Chemistry : Unit #3 - The Atom : Lesson #2

Atoms: The Big Picture, The Little Picture

Objective: Students will be able to name the parts of the atom and their location within the atom. They will also be able to calculate the number of atoms in a sample of an element.
Standards: HS-PS1-1 SP2 XC-SPQ-HS-2
Subject(s): Science
60 minutes
1 Introduction - 0 minutes

In this lesson students learn the parts of the atom. Some students may already know this from middle school, but many do not. They then use atomic numbers and atomic masses from the periodic table and a plastic model of the atom to show how elements differ. As an extension some students also find the mass of different element samples and calculate the number of atoms that are in the sample using Avogadro’s number.

This lesson relates to the NGSS Crosscutting concept of Scale, proportion, and quantity by helping students to relate relatively small amounts of elements to the incredibly large amounts of atoms contained in the sample. It also relates to the NGSS Practice of Developing and using models by giving students the chance to use a model to show differences between atoms. It corresponds to the NGSS Disciplinary Core Idea of Structure and Properties of Matter by working with the concept "Each atom has a charged substructure consisting of a nucleus, which is made of protons and neutrons, surrounded by electrons."

Materials list:

  • a modeling system that enables students to place protons and neutrons in a nucleus and electrons in a cloud (I use this one)
  • a periodic table (the most up to date one can be found here)
  • Samples of elements
  • An electronic balance


2 Do Now/Activator - 10 minutes

Do Now: To get students starting to think about the atom I ask them to read a page in their text about the atom. (if you do not have one this link can give some of the same information). I ask them to record in their notes the three parts of the atom and explain where these parts are located, what their charge is, and what their relative size is. I choose this activity because it should be review and easily accessible material, which allows me to take attendance and make sure class is settled.

Activator: I invite students to participate in the aluminum foil challenge, which is this: take a piece of foil, cut it in half, then cut one of the halves in half. Take one of those halves and cut it in half. Keep repeating the procedure until you have the smallest piece possible. The person with the smallest piece wins. After students have done this for about 2 minutes, I call time.

I then ask the participants to parade around the room showing off their piece, and the people who did not participate have to judge to see which piece is the smallest. After everyone has paraded, and a winner has been announced, I explain that there is one more contestant who did not parade. I then make a show of parading around my finger which appears to have nothing on it.

I explain with braggadocio that while my opponents paraded around with millions or billions of atoms, only I am parading with just 1 atom on my finger. Amidst the groans and moans I explain that while I am being silly, the truth of the matter is that you can only divided up an element so many times, and that the least amount of an element you can have is one atom, and that is a very small amount.  

This is a somewhat silly exercise that gives students a chance to be kinesthetic learners. It gives students a chance to blow off some steam. However, it also exemplifies Democritus's thought experiment from 2,000 years ago, in which he rightly assumed that eventually you would have to get to the smallest amount of an element.

3 Mini-lesson and Guided Practice - 15 minutes

Mini-lesson: I link the activator to the lesson by noting that today we are going to look at the atom. I use cold call to ask the class what the three parts of the atom are; students know that the parts are proton, neutron, and electron.

Part 1. I explain that the number of protons determines which element you have. I note that it is rather incredible that just 3 particles—protons, neutrons, and electrons—make up all matter. From their notes students also note that protons and neutrons are found in the nucleus and electrons are found in a cloud outside of the nucleus, and that protons and neutrons are about 1800 times more massive than electrons, even as it is the electron cloud that is 10,000 times more voluminous than the nucleus. I emphasize that most of matter is actually empty space, but that due to the scale we do not perceive it in this way.

I then teach that different atoms have different amounts of protons, neutrons and electrons. I teach students that to know how many protons or electrons they can just look a the atomic number on the periodic table. For neutrons, they need to round the atomic mass and subtract from it the atomic number. 

The structure of the atom is central to the understanding of chemistry, and so choosing this idea to start students on their path toward understanding bonding and chemical reactions feels like the right foundation.

Guided Practice: I model the first example and then release students to work on this by using plastic models and the Atomic Structure Practice probems

Part 2. I teach students how to find the number of atoms in a sample. Here is a student explaining it almost as well as I can in this atoms in a sample video. In watching this video I recognize that I will need to go back and name Avogadro's number and further clarify the relationship between Avogadro's number  and atomic mass, but overall the student knows how to calculate the number of atoms in a sample, so I will leave the teaching to her! I chose this activity because I want students to appreciate how small the atom is, and the fact that a small amount of matter contains vast quantities of atoms.

4 Application - 25 minutes

Student Activity:

Part 1: In response to the first part of the lesson students work on modeling the atom. They use the periodic table to video and the atom modeling kits to figure out how atoms for each of the first twenty elements are comprised. They record their work. Here is a student modeling the atom video that shows how I support a student as he is first trying to understand how all of the parts of the atom fit together.  I am thankful to have these models--I believe that when students have the opportunity to work with a model they then have a powerful bridge between an abstract concept and a conceptual understanding.

Part 2: after the second part of the lesson students move on to calculate how many atoms are in a sample by working on Scaffolded mass to atoms practice problems.

Students who finished with this part of the lesson weigh out some samples of elements and use their understanding of the math to calculate how many atoms they had in their sample. In this way, the whole class is reminded that what we are talking about is real, and not just academic.


Sometimes you gotta be wrong before you can be right

Students are sometimes not engaged in science investigations because they are afraid to be wrong. My favorite part of this video is when the student makes a small mistake while modeling the atom--for the first time. His response is that this is why he does not like to be filmed. What he meant was that he did not like to make a mistake, as if mistakes when doing something for the first time are bad. Something we are working on in our school culture is the idea of mindset, as developed and popularized by Carol Dweck. I hope in this video I was supportive, but what the video made me realize in retrospect is that I need to remind students that our scientific knowledge and discoveries are all based on a million false starts. I used to have a sign in my classroom that read "If you are afraid to be wrong you can't do science." Maybe it is time to reemphasize this important point. A child learns to walk by falling repeatedly, but she never gives up, and that is how we should view all learning.

5 Debrief - 10 minutes

We start debrief with some students showing a small sample of an element and how many atoms were in their samples, as depicted in this video about how to find the number of atoms in a sample. I emphasize that atoms are very small, and that the numbers that the students show are unimaginably large, beyond the scale of how we are used to thinking about quantity.

I then ask students to turn and talk about the following questions:

What are the parts of the atom?

Where are they found?

How do you know how many protons, neutrons, and electrons are found in an atom for any element?

I then cold call a few students to report out answers:

Protons and neutrons are found in the nucleus, electrons are found in an electron cloud outside the nucleus. The atomic number indicates the number of protons and electrons, while the rounded atomic mass minus the atomic number indicates the number of neutrons.

Students can check their work using this atom composition answer key, which I project.

I close class reminding students that they have to take their element quiz next class. I have created a simple game called Element Stand-off so they can check their readiness. One student starts by saying hydrogen. The second person has to say the next element, and then the first person has to say the next, and this continues back and forth until all twenty elements have been recited. The person who says the last element wins, and if the pair gets all the way to calcium they both win. Here is a video showing the Element Stand-off. This game turned out to be a fun way for students to test their knowledge and to show off what they know while giving the audience a new way to think about the first 20 elements.