Chapter 5 Models of the Atom

Models of the Atom Rutherford’s atomic model could not explain the chemical properties of elements. Explaining what leads to the chemical properties of elements requires a model that better describes the behavior of electrons within atoms. Niels Bohr (1885-1962) Danish physicist and student of Rutherford believed the previous model needed improvement. Bohr proposed that an electron is found only in specific circular paths or orbits around the nucleus. Each possible electron in Bohr’s model has a fixed energy.

Models of the Atom Energy levels- the fixed energies an electron can have. Ex. Think of the rungs of a ladder. The lower the rungs are lower energy. As you increase energy you go up rungs on the ladder. As you move up the ladder the rungs get closer together. Pg.129 Quantum of energy- amount of energy required to move an electron from one energy level to another energy level.

Models of the Atom 1926-Erwin Schrodinger- devised and solved a mathematical equation describing the behavior of the electron in a hydrogen atom.   The quantum mechanical model determines the allowed energies an electron can have and how likely it is to find the electron in various locations around the nucleus. The quantum mechanical model description of how an electron moves around the nucleus is similar to the motion of a rotating propeller. The quantum mechanical model increases the probability of finding the electron. The area where the electron can be found is in a fuzzy cloud around the nucleus.

Models of the Atom Each energy sublevel corresponds to an orbital of a different shape, which describes where the electron is likely to be found. The energy levels of electrons in the quantum mechanical model are labeled by principal quantum numbers (n). These are assigned values n=1,2,3,4 and so on. Atomic orbital- region of space where there is a high probability of finding an electron Each orbital my contain 2 electrons

Models of the Atom Different atomic orbitals are denoted by letters. S= spherical shape P= dumbbell shape (3 orbitals) D= clover leaf shape (5 orbitals) F= more complicated d orbitals (7 orbitals)

Models of the Atom The numbers and kinds of atomic orbitals depend on energy level. The lowest principal energy level (n=1) has one sub-level called 1s The second principal energy level (n=2) has two sublevels 2s and 2p. The 2p is higher energy than 2s The third principal energy level (n=3) has three sublevels 3s and 3p and 3d. The fourth energy level (n=4) has four sublevels. 4s and 4p and 4d and 4f. 2n2 -where n is the principal quantum number. This formula tells you the number of electrons allowed in each of the first four energy levels.

Arrangements in Atoms Electron configuration- The ways in which the electrons are arranged in various orbitals around the nucleus of an atom. Table 5.7 pg.133

Arrangements in Atoms Three rules tell us how to find the electron configuration of the atoms.   Aufbau Principal- electrons occupy the orbitals of lowest energy first. Pauli exclusion principal- An atomic orbital may hold only two electrons and they must have different direction of spin. Hund’s rule- states that electrons occupy orbitals of the same energy in a way that makes the number of electrons with the same spin direction as large as possible. Example table 5.3 pg.134