The Bohr model offers a simplistic, two-dimensional drawing of an atom’s structure, which is widely used for instructional purposes. Developed in 1913 by physicist Niels Bohr, this representation shows electrons orbiting a central nucleus in fixed, circular paths, much like planets around a sun. Each circular path represents a specific, quantized energy level that electrons can occupy, helping to visualize the discrete nature of electron energy states. While modern quantum mechanics presents a more complex picture of electron distribution, the Bohr model remains an effective tool for introducing the concept of electron shells and energy levels to a general audience.
Gathering the Essential Atomic Information
To create a Bohr model, first determine the number of subatomic particles for the specific element you wish to model. This information is available on the periodic table. The atomic number, typically found above the element’s symbol, is a whole number displayed in every element’s box. This number establishes the element’s identity and equals the number of protons in the nucleus.
For a neutral atom (the type most often represented), the number of protons must equal the number of electrons. Therefore, the atomic number provides the count for both protons and electrons.
To find the number of neutrons, locate the atomic mass (usually a decimal number). Round this atomic mass to the nearest whole number to get the mass number (total count of protons and neutrons). The number of neutrons is then calculated by subtracting the atomic number (the number of protons) from this rounded mass number.
For example, the element Carbon has an atomic number of 6, meaning it has 6 protons and 6 electrons. Its atomic mass is approximately 12.011, which rounds to a mass number of 12. Subtracting the atomic number (6) from the mass number (12) gives you 6, which is the number of neutrons.
Constructing the Atomic Nucleus
After calculating the subatomic particle counts, the first step in drawing the Bohr model is creating the nucleus. The nucleus is the atom’s dense, central core containing protons and neutrons, and holds the vast majority of the atom’s mass. Draw a single circle in the center of your diagram to represent the nucleus.
Inside this central circle, clearly label the quantity of protons and neutrons you calculated from the periodic table data. A common method is to write “P = [number of protons]” and “N = [number of neutrons]” within the circle. For a Carbon atom, this would mean labeling the nucleus with P=6 and N=6.
The positive charge of the atom is concentrated entirely within this nucleus due to the presence of the protons. The neutral neutrons contribute to the mass but do not affect the overall charge of the core. Having the nucleus clearly defined allows you to move outward to the shells, which will contain the equal number of negative charges needed to balance the atom.
Mapping the Electron Shells
The next step involves mapping the electron shells, which are the fixed, circular orbits surrounding the nucleus. These shells represent distinct energy levels; those closer to the nucleus correspond to lower energy states. Electrons fill these shells from the inside out, occupying the lowest energy levels first.
Each shell has a specific maximum capacity that must be satisfied before electrons can be placed in the next shell outward. The innermost shell (the first energy level) holds a maximum of two electrons. Once full, the second shell can hold a maximum of eight electrons.
The third energy level can technically hold up to 18 electrons, but for elements in the first three rows, a simplified 2-8-8 rule is often applied. This rule dictates that the second and third shells will each hold a maximum of eight electrons for basic modeling purposes. This pattern is important because atoms are generally most stable when their outermost shell, known as the valence shell, contains eight electrons, following the octet rule.
You draw these shells as concentric circles around the nucleus, adding a new circle for each energy level required to accommodate the total number of electrons calculated earlier. If you are modeling Neon (10 electrons), you would need two shells: the first holding two electrons and the second holding the remaining eight. If the total number of electrons exceeds the capacity of the first two shells, a third concentric circle must be drawn to hold the overflow.
Placing Electrons and Checking Your Work
With the shells mapped out, begin placing the total number of electrons onto the concentric circles according to the capacity rules. The electrons should be represented as small dots or symbols placed directly on the circular paths. Start with the innermost shell, filling it completely with two electrons before moving remaining electrons to the next outer shell.
Continue distributing the electrons, filling subsequent shells to their maximum capacity of eight until all electrons have been placed. The outermost shell that contains electrons is called the valence shell, and the electrons within it are the valence electrons. The number of valence electrons is particularly significant because it largely determines the element’s chemical behavior and its tendency to form bonds.
Once all electrons have been positioned, a final review ensures the model accurately represents the atom. First, count every electron placed on the shells to verify that the total number matches the proton count in the nucleus, confirming the atom is electrically neutral. Second, check that all inner shells are completely filled before any electrons were placed in the outer shells, adhering to the fundamental energy level rules. Finally, confirm that the nucleus is clearly labeled with the proton and neutron counts, resulting in a complete and accurate Bohr model diagram for that specific element.