Middle school students are just at the point developmentally of crossing from concrete understanding to abstract. They can easily get lost in the weeds when it comes to studying the atom- something so small it cannot be seen to be believed. The Bohr model, though not entirely accurate and only useful with the lighter elements, achieves a few key things- First, it gives students a basic, concrete, and reasonable explanation of the atom and its subatomic particles. Second, it sets a solid foundation for understanding ionic and covalent bonds, which leads to a better understanding of chemical compounds and reactions. Ultimately, this is where we want to go with our middle school science students. We want them to understand how simple molecular structures are formed and what occurs during chemical reactions. For this, they need a basic understanding of the atomic structure, one offered by the Bohr model. This is why I take time to teach the Bohr diagrams with my middle school science students. Not a lot of time, but just enough to give them a concrete foundation from which to build the rest of the middle school physical science understanding. Here are a few of the benefits I find from using Bohr diagrams in middle school science…
You can easily identify the valence electrons.
In most cases, the valence electrons (electrons in the last orbital shell of the atom) are responsible for bonding between atoms. When atoms bond, they form molecules and when bonds are broken or created, chemical reactions occur. Understanding chemical reactions and molecular structures is ultimately what we want our middle school science students to achieve, but I believe it’s difficult for them to fully grasp the concept without understanding the structure of the atom and the purpose of the valence electrons first.
I will note that you can achieve the same thing with Lewis Dot Diagrams, but I really like my students to see the structure of the atom and get a sense of how each element is unique based on it’s atomic structure.
They are simple to draw on paper (2-dimensionally).
Because the whole (slightly inaccurate) idea behind the Bohr diagram is that the electrons move around the nucleus in neat, circular orbits, they are easy to draw out on paper in 2-dimension. Additionally, you can gather all the information you need for drawing a Bohr diagram directly from the periodic table.
The “rules” that are used to determine how many electrons can occupy each orbit are accurate for the most part, even in the quantum mechanical model. What is missing are the sub-orbitals, which would create a 3-dimensional structure, and the fact that electrons travel in waves rather than neat little circles. Additionally, the rules don’t work for heavier elements, which is what ultimately lead to further research of the atom and a revised model. Because of this, I typically only use elements 1-36, skipping the transition metals, or elements 1-18. Again, the purpose is to just to set a foundation of basic atomic structure and how atoms can form simple molecules and with this group of elements, you will be able to model many different simple molecules and compound structures.
So what is a Bohr Diagram of an Atom?
The Bohr model represents the atom as having a nucleus in the center which consists of 2 sub-atomic particles, the positive protons and the neutral neutrons, surrounded by rings or orbits where the electrons are found. The number of protons an atom has is unique to each element (Hydrogen has 1 proton, Lithium has 3, and so on). The number of neutrons can vary slightly but they do add to the mass of the atom so it’s possible to have your students determine how many neutrons the atom might typically have (by subtracting the atomic number or number of protons from the atomic mass). When we are modeling atoms, we model them in their neutral state, so the number electrons will be equal to the number protons. Electrons have a negative charge so if the number of protons is equal to the number of electrons, the atom itself has no charge- its neutral. (Atoms rarely exist this way, which is why they form molecules, but that comes later.)
You can determine the number of orbits the atom will have by referring to the periodic table. The rows on a periodic table are called periods. The Period an element is in indicates the number or orbits (or shells) it will have. So, Hydrogen and Helium have 1 orbit while sodium, which is in period 3, will have 3 orbits.
Each orbit represents a different energy level and has a maximum “energy limit”. Because of this, there is a limit to the number of electrons that can occupy each orbit. The first orbit, the one closest to the nucleus, can only hold 2 electrons. Orbits 2 and 3 can each hold 8 electrons, and orbit 4 can hold 18. After period 4 is where the Bohr diagram tends to loose it’s effectiveness as a model, which is why I typically stop with element 36 (Krypton).