ELECTRONS IN ATOMS

Electrons in Atoms The chemical properties of atoms, ions, and molecules are related to the arrangement of the ELECTRONS within them. Over time, the atomic model has evolved regarding where electrons are located.

Atomic Theory / Electrons Democritus and Dalton Atom is indivisible—no electrons Thompson’s Model (Plum Pudding) Described the atom as a ball of positive charge containing randomly placed electrons. Rutherford’s Model Electrons are still randomly placed, but now there’s a positive nucleus.

Atomic Theory The Next Step Max Planck’s Quantum Energy Theory Energy is always absorbed or emitted in “packets” which he called quanta. Photon= quantum of light 10th grader = High school student Bohr’s Model (Planetary) Bohr used the quantum energy theory and proposed that electrons move in particular paths with fixed energy in “orbits” around the nucleus.

Atomic Theory / Electrons In Bohr’s Model… Electrons cannot exist between energy levels, but they can move from one level to another. Think of a ladder, and the electrons can only jump from rung to rung. To move from one energy level to another one, an electron must gain or lose just the right amount of energy. This amount of energy is called a quantum of energy.

Atomic Theory / Electrons Bohr’s Model was only partly successful. Two other contributions led to a new atomic theory. Louis de Broglie illustrated that electrons have a dual nature with properties of both particles and waves. Heisenberg’s Principle of Uncertainty stated that it was impossible to know both the momentum and the position of an electron.

Modern Atomic Theory Schrodinger’s Quantum Mechanical Model is the atomic model in use today. It was able to explain many previously unexplainable phenomena. Restricts energy of electrons to certain values Same as Bohr Does NOT define an exact path around the nucleus Unlike Bohr Estimates the probability of finding an electron in a certain position (Heisenberg).

Quantum Numbers: Where is the electron?? In Schrodinger’s model, there are four “quantum” numbers that tell us where an electron is likely to be located. Principal (n), 1-7, gives the energy level Subshell (l), s-p-d-f, gives the shape of region Orbital (m), gives the orientation in space of the shapes Spin (s), clockwise or counterclockwise No two electrons have the same 4 quantum numbers.

Principal Quantum Number (n) Designates the energy level Tells how far away from the nucleus an electron is likely to be. Assigned values in order of increasing energy: n = 1, 2, 3, 4, 5, 6, or 7 Corresponds to the rows/period on the periodic table.

Principal Quantum Number (n) Lower value of n means the electron is closer to nucleus and has less energy. + n = 1 n = 2 n = 3

Subshell (l), s-p-d-f (Shapes) Within each principal energy level there are a certain number of sublevels Energy level 1 has 1 sublevel; energy level 2 has 2 sublevels; energy level 3 has 3 sublevels, etc. These sublevels are called s, p, d, or f. Each sublevel has a unique shape These shapes describe the region of space where the electron is likely to be located s p d

Principal Energy Level Subshell, (l), s-p-d-f Principal Energy Level Number of Subshells Type of Subshell n = 1 1 1s n = 2 2 2s, 2p n = 3 3 3s, 3p, 3d How many sublevels will principal energy level 4 have, and what shapes will they be?

Orbitals (m) Each orbital can only hold 2 electrons. (The orbital names s, p, d, and f stand for names given to groups of lines in the spectra of the alkali metals. These line groups are called sharp, principal, diffuse, and fundamental—This is not important.) Within each sublevel there are a certain number of orbitals. s sublevel—1 orbital p sublevel—3 orbitals d sublevel—5 orbitals f sublevel—7 orbitals Orbitals within each sublevel have similar shapes, but are oriented differently in space. Each orbital can only hold 2 electrons.

s - orbitals Spherical shape Hold a maximum of 2 electrons First found in the 1st energy level

p-orbitals Pear or dumbbell shaped 3 different orientations Each orientation holds a max of 2 e- The p sublevel as a whole holds a maximum of 6 e- First found in the 2nd energy level p-orbitals

d - orbital 5 different orientations Each holds 2 e-’s The sublevel as a whole holds a maximum of 10 e- First found in the 3rd energy level

"f" Atomic Orbitals Do not appear until the 4th shell (energy level) and higher.

Maximum eֿs Allowed The maximum # of electrons that can occupy a principal energy level is given by the formula 2n². How many eֿs are allowed in energy level 3? 18

Let’s Review How many orbitals are in the following sublevels? 3p 2s 3 3d 3 1 7 3 5

Electron Configuration: How are the electrons in any given element arranged? In an atom, electrons are positioned around the nucleus in the most stable arrangement possible. This is the arrangement that has the lowest energy. There are 3 important rules to help us find the configuration of electrons in atoms.

Aufbau’s Principle Electrons will occupy orbitals having the lowest energy FIRST and then in order of increasing energy. The "Ground State" of an atom is when every electron is in its lowest energy orbital.

Pauli’s Exclusion Principle An orbital can be empty, have one electron, or AT MOST have two electrons. Electrons occupying the same orbital must have opposite spins! One clockwise, and one counter clockwise

Hund’s Rule When electrons occupy orbitals of equal energy, one electron enters each orbital until all the orbitals contain one electron with parallel spins.

Chemistry Riddle Q: What happens when electrons lose their energy? A: They get Bohr’ed!