Chemical Bonding

Chemical Bonding Chemistry 332

The world is made of atoms • Atoms are not the smallest building block • Protons, neutrons, and electrons make up atoms • Quarks make up protons, neutrons, and electrons • Maybe something makes up quarks • For our laboratory, basic chemistry purposes, though, atoms are the most convenient

Look at the periodic table • How many atoms do you see? • Depending on how old it is a little over 100 atoms • Does that mean that only 100 substances exist? • Hardly • How can new substances be made?

Compounds • Compounds are made when atoms chemically combine with each other to make a new substance

Compare it to words • How many letters of the alphabet are there? • 26 • How many words exist? • Hundreds of thousands • Atoms and compounds are the same way

Different combinations • Combinations can be simple like: • NaCl • Or very complex: • KAl(SO4)2*12 H2O

Each new substance is unique • The compound has its own properties even though it contains these other atoms • Elemental Na is an explosive, highly reactive metal • Elemental Cl is a green, poisonous gas • When combined, NaCl is simply harmless table salt • Na and Cl lose their properties and become a new material

Why do atoms combine? • There must be some reason for atoms to combine with each other • They must “get something” from it or they wouldn’t do it • So where to look? • How about we look at those atoms that DON’T combine with anything? • Do you know of any?

Noble Gases • For some reason, the noble gases never react with anything • They are: • He, Ne, Ar, Kr, Xe, Rn • The are what chemists call “chemically stable” • They never combine with any other element to form a compound

Basic Assumption • All elements that are not noble gases, wish to become stable like the noble gases. • We know that we cannot lose or gain protons or neutrons from the nucleus as that would either change the element or most likely make the nucleus unstable • The only particle left is the electron so let’s focus our model on that.

Helium Two electrons orbiting the nucleus Nucleus with two protons and two neutrons For some reason this is stable!

Neon Ten electrons orbiting the nucleus Nucleus with ten protons and ten neutrons For some reason this is also stable!

Our theory... • Electrons exist in energy levels • Levels can only hold so many electrons • When a level is full, it is considered to be “stable” and will not give up, take, or share electrons with other atoms

Magic Numbers... • Those numbers are 2, 10, 18, 36, 54, and 86 • The higher numbers get far more complicated so let’s just focus on 2 through 18 to illustrate our model

Level capacities • Level 1 can hold 2 electrons • Level 2 can hold 8 electrons • Level 3 can hold 8 more, etc.

The octet rule • Every level past the first can hold 8 electrons • The first level can hold 2 • (This is an extreme oversimplification but for simple covalent bonding it works)

H So how does bonding occur? And compare it to helium Let’s look at the simplest case, hydrogen He 2 electrons 1 electron stable unstable

H H 1 electron 1 electron unstable unstable So how can we bond? But how about 2 hydrogen atoms? What if they SHARE their electrons?

So how can we bond? When they go together…. H H They now each have two electrons Two is a stable number!

Bonds are represented by lines 2 electrons in the bond H H Formula? H2

That was easy! • It gets a little harder, though. • Let’s try Carbon • How many total electrons does an atom of carbon have? • 6 C

H H H H H H Do C & H bond like this? CH6? C

Actually, no... • Carbon and hydrogen only make CH4 • How can this be? • Even though carbon has six electrons, some of them are already stable • We have to look at them in their levels... C

Remember our theory... • Electrons exist in energy levels • Levels can only hold so many electrons • When a level is full, it is considered to be “stable” and will not give up, take, or share electrons with other atoms • Stable levels are 2, 10, & 18

1st level 2nd level stable 2 unstable +4 6 total electrons but only 4 that are unstable and need pairing total 6 The model C Total electrons:

H H H H H H C H H The finished model C CH4

Let’s try with water • Water is H2O • Oxygen has 8 total electrons • 2 go in the first level and become stable • 6 go in the second level • How many more does oxygen need? O

H H O H H2O H Oxygen only needs 2 more electrons O

O If you notice... • The inner levels that are stable are not reacting • They are already filled and do not wish to “play” with others • For bonding purposes, then, let’s just ignore them! O

Valence • All you need to do is count the valence electrons • Valence electrons are any electrons in the OUTERMOST level • For bonding purposes, you can completely ignore inner filled levels.

F F Fluorine • How many TOTAL electrons does Fluorine have? • 9 • 2 in the 1st level, 7 in the second • How many VALENCE electrons does Fluorine have? • 7

Columnn 1 Columnn 4 Columnn 7 He H C Li Be B N O F Ne Anybody noticed a trend? • How many valence electrons does H have? • 1 • How many valence electrons does C have? • 4 • How many valence electrons does F have? • 7

Cl Think Fast... • How many valence electrons does an atom of Cl have? • 7! • It has 17 total electrons • 2 in the first level (stable) • 8 in the second level (stable) • 7 in the third level (unstable) Cl

Columnn 7 He H C Li Be B N O F Ne Si Na Ca Al P S Cl Ar Did you need to do all that? • What column is Cl in? • Column 7 • Because of the stable valence rules we’ve developed, you never have to go through it again! • Just look at what column the element is in and that’s how many valence electrons it has!

So….. • You’ve now learned everything you need to know to draw the molecular structure of certain compounds • These structures are called Lewis Dot structures • They follow the rules we’ve already outlined • Let’s do a quick review to finish

Rules for simple Lewis Dot Structures • Write the formula of the compound • Draw each atom with the correct number of VALENCE electrons • Look at the column the element is in • Atoms that have any unpaired electrons will share or COVALENTLY bond with others • All atoms need 8 electrons in the valence level except H that only needs 2

H H N H N H H H Follow the rules for NH3 Valence electrons: 3 H: 1 each N: 5 NH3

Chemical Bonding