1. GSCI 163 Lecture 4

2. Review • Atoms are made of protons, neutrons and electrons. • Electrons are arranged in shells and orbitals in the atom. • Electrons can move up to an orbital of higher energy by absorbing a photon. • Electrons give off energy in the form of photons when they move down to orbitals of lower energy. • The spectrum of these photons form the emission lines of each element.

3. The Bohr model • How the electrons arrange themselves inside of the atom? The Bohr model of the atom suggested the atom looked like a little solar system. Like the solar system, where most of the mass is due to the sun, the same happens with the nucleus of the atom.

4. Filling up the shells • So we have the shells(closets), the orbitals(shelves) and the energy level (boxes) to fill with electrons. How should we do it? Examples: Hydrogen  1 electron  1s1 Helium  2 electrons  1s2 Lithium  3 electrons  1s2 2s1 Beryllium  4 electrons  ? Boron  5 electrons  ?

5. But the filling gets complicated • Look in the periodic table how the filling proceeds. • Note what happens between element Argon (z = 18) and Scandium (z = 21). • From 3p6 it jumps to 4s1 rather 3d1! Why?

6. The short answer • Instead they have complicated shapes. The way the electrons fill the shells is by staying closer to the nucleus. The long answer? We need to learn quantum physics in detail. • The orbitals are not really spherical as the solar system model suggest.

7. But how do we know how to fill the shells? • Use the following diagram:

8. Bonding of atoms • Very few pure elements are found in our surroundings. We mostly see compounds which are combination of elements by a chemical bond. They form… …Molecules, which is the smallest particle of a compound or a gaseous element that can exist and still retain the characteristic chemical properties of a substance. • As it turns out the electron structure is responsible for the bonding of atoms.

9. Examples • Water: combination of two atoms of Hydrogen and one of Oxygen (H2O) • Sodium Chloride (table salt): combination of one atom of sodium and another of chlorine (NaCl) How do we know how elements associate?

10. The outer shells electrons Outer shells like to be filled with electrons. If two atoms can share the electrons to fill their shells, they will bond. • For most elements the shell to be filled is the second, which has 8 “spaces” (2 spaces for s and 6 for p electrons vacancies) • Electrons in the outer shells are called valence electrons

11. Representing the outer shells To decide whether or not two elements will bond, we must investigate the electron distribution. We can represent that in two ways: • Short shell structure representation • Lewis dot structures

12. Short shell structure representation Since what matters is the outer shell, the structure can be written starting at the lowest filled shell atom. • Take your periodic table and follow the sequence from He (helium) to K (potassium). • Example: Oxygen[He] 2s2 2p4 • Your turn: Boron  Silicon 

13. Lewis dot structure • It consists of drawing dots representing the valence electrons around the chemical symbol of the element • Example: Oxygen O • Your turn: Sodium  Na Chlorine  Cl

14. Bonding elements Atoms can bind together by trading electrons. For example, when sodium meets chlorine, sodium gives up one electron to chlorine and both get a filled outer shell: Na +Cl Na+ + Cl- Giving up an electron requires energy called ionization energy

15. Types of Bonds Bonds can have different characteristics depending on how the electrons are shared. They can be: • Ionic • Covalent • Metallic • van der Waal’s

16. Ionic and covalent • Ionic (strong): happens when one element gladly gives up an electron (or two) and is connected to the other element by the electrostatic force. Example: NaCl • Covalent (not as strong): happens when two elements literally share their electrons so to mutually fill their shells. Example: AlN

17. Metallic & van der Waals • Metallic: happens when the element is so eager to give up electrons that the electrons form a “cloud” that envelops the ions. • van der Waal’s (the weakest): happens when a polarized (with a + and – poles) molecule is attracted to another polarized molecule. Occurs often in biology and polymer science.

18. Next class How reactions occur. • To prepare for the class read: • Handout pages 17 to 20 (day 5) • To prepare for the quiz read: • Handout pages 13 to 16 (day 4) • Power point for this class • Your class notes