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Quantum Model

Need help preparing for the General Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about Quantum Model for Electronic Structure. Watch this video to get all the MCAT study tips you need to do well on this section of the exam! We previously discussed the Bohr model of the atom, and how, while influential, it was ultimately incomplete. One reason why was that in the Bohr model, electrons follow circular orbits around the nucleus, but in reality, electronic orbital patterns are more complex. This is addressed in the quantum model, where electrons reside in more complex, non-circular atomic orbitals. These atomic orbitals can be described in terms of their size and energy, similar to the Bohr model, but are also broken down by the shape of each orbital, and the spin of each electron within the orbital. Principle Quantum Number Similar to the Bohr model, the size of an orbital is determined by its electron shell, or energy level, which is indicated by the principal quantum number (n), with each successively increasing n value corresponding to a new row on the periodic table. In practice, this means that for an atom in the first period (hydrogen and helium), the highest possible n value is 1, meaning that each atom only has one electron shell. Moving down to the second period, our highest n value moves to 2. Atoms in the second period, such as carbon and oxygen, still have electrons in the first electron shell (n = 1), but also have electrons in the second, n = 2 electron shell. Remember from our previous lessons that each successive energy shell is of higher energy and farther from the nucleus, and that the gap between each successive energy shell becomes increasingly smaller. Each electron shell is quantized, meaning each has a specific, discrete energy value, a property also shared by the Bohr model. However, the Bohr model fails to account for the subdivision of electron shells into subshells, viewing instead each electron within a given shell as functionally identical. The subdivision of electron shells into subshells allows for the quantum model to differentiate every electron within an atom. Azimuthal Quantum Number An electron subshell is denoted by the azimuthal quantum number (ℓ), and describes the shape of each atomic orbital. Each successive electron shell has a correspondingly higher number of available subshells, or orbital shapes. This relationship is given by the equation ℓ = 0, 1, 2…n-1, meaning that the azimuthal quantum number can be any integer between 0 and one less than the principle quantum number. Therefore, we would say that the first electron shell, n = 1, could only have one potential subshell, ℓ = 0. The second shell, n = 2, could then have two possible subshells, representing two possible orbital shapes, ℓ = 0 and ℓ = 1. The third shell could then have three possible subshells, with ℓ values of either 0, 1, 2, and so on for each successive electron shell. Just as the principle quantum number corresponded to the periods of the periodic table, the azimuthal quantum number likewise corresponds to specific areas of the periodic table. The first two groups of the periodic table correspond to an azimuthal value of 0, the first possible subshell, known as the s-subshell. For that reason, groups 1 and 2 in the periodic table are referred to as the s-block. The final six groups of the periodic table, groups 13-18, correspond to azimuthal quantum number 1, also known as the p-subshell, and so are collectively called the p-block. The transition metals correspond to the d-subshell, azimuthal quantum number 2, and the lanthanides and actinides correspond to the f-subshell, azimuthal quantum number 3. Note as well that each subshell is increasingly more complex in its shape, from the spherical s-subshell, to the dumbbell shaped p-subshell, and so on. Magnetic Quantum Number Each subshell, or orbital shape, can hold a progressively larger number of orbitals. This is described by the magnetic quantum number (ml), and the range of possible magnetic values includes any integer between ℓ and - ℓ. For the s-subshell, ℓ = 0, that would leave only one possible value for ml, 0. Therefore, we would say that the s subshell only has one possible orbital, of value ml = 0. For the p-subshell, ℓ = 1, there would be three possible orbitals, of values -1, 0 and 1. For the d-subshell, ℓ = 2, the magnetic quantum number ml could be either -2, -1, 0, 1, or 2, giving five possible orbitals within the d-subshell. MEDSCHOOLCOACH To watch more MCAT video tutorials like this and have access to study scheduling, progress tracking, flashcard and question bank, download MCAT Prep by MedSchoolCoach IOS Link: https://play.google.com/store/apps/de... Apple Link: https://apps.apple.com/us/app/mcat-pr... #medschoolcoach #MCATprep #MCATstudytools