1. Electrons

2. Models of the Atom

4. Electrons Electrons do not orbit the nucleus like the planets orbit the sun. Electrons are located in the electron cloud around the nucleus. The fixed energies an electron can have are called energy levels. The closer an electron is to the nucleus, the less energy it has.

5. Energy Levels Electrons either gain or lose energy to move between energy levels. A quantum of energy is the amount of energy required to move an electron from one energy level to another. Electrons cannot exist between energy levels. Energy levels are not evenly spaced in atoms.

6. Atomic Orbitals An atomic orbital is a region of space where there is a high probability of finding an electron. Orbitals hold at most two electrons. Each type of orbital has a specific shape There are four types of atomic orbitals: s, p, d, and f

7. Atomic Orbitals

8. The principal quantum number (n) indicates the energy level of an electron. The number of electrons allowed in each of the first four principle energy levels are shown here.

9. Atomic Orbitals The numbers and kinds of atomic orbitals depend on the energy sublevel.

10. Electron Configurations The ways in which electrons are arranged in various orbitals around the nuclei of atoms are called electron configurations. Sodium (Na): 1s 2 2s 2 2p 6 3s 1 There are three rules used for writing electron configurations. The Aufbau Principle The Pauli Exclusion Principle Hund’s Rule

11. The Aufbau Principle The Aufbau principle states that electrons occupy the orbitals of lowest energy first. 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p

12. The Pauli Exclusion Principle The Pauli exclusion principle states that atomic orbitals may at most describe two electrons. Those two electrons must have opposite spins. correct incorrect

13. Hund’s Rule Hund’s rule states that electrons spread out before they pair up in orbitals of the same energy. Electrons that are by themselves in an orbital are called unpaired electrons.

14. Electron Configurations The most stable electron configurations are those that are completely filled: He: 1s2 Ne: 1s 2 2s 2 2p 6 Ar: 1s 2 2s 2 2p 6 3s 2 3p 6 Kr: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 Xe: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 6 Rn: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 6 6s 2 4f 14 5d 10 6p 6 Each of the above elements are noble gases. The noble gases are stable because they all have full principle energy levels.

15. Electron Configurations Half-filled sublevels are not as stable as filled sublevels, but they are more stable than other configurations. These are called pseudo noble gas configurations. Some elements deviate from their predicted electron configuration to achieve pseudo noble gas configuration. Predicted: Chromium (Cr): 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 4 Actual: Chromium (Cr): 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 5 Predicted: Copper (Cu): 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 9 Actual: Copper (Cu): 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 10

16. Ground State vs. Excited State The lowest potential energy arrangement of electrons in an atom is called the ground state. The ground state of carbon is: 1s 2 2s 2 2p 2 An excited state electron configuration is any electron configuration for an atom that contains the correct total number of electrons but has a higher total electron potential energy than the ground state electron configuration. One possible excited state of carbon is: 1s 2 2s 1 2p 3