Chemical Compounds: Bonding Rules/Types and Molecular Formulas

The Principle… Matter can not be created or destroyed Matter can only change forms So, how does the Oxygen we breathe end up changing into Glucose and Carbon Dioxide? Chemical Bonds Change… Or, as Yoda would say, “Change the Bonds, it does”

Electron Shells & Octet Rule We know that the Atomic Number = Number of Protons The Octet Rule states that atoms tend to gain, lose or share electrons so that they will have a “happy” outer shell The first outer shell holds 2 electrons The second and third outer shell hold 8 Atoms Want To Be Neutral! They WANT the same number of electrons and protons in order to balance their electrical charges

Why is the Octet Rule Important? The octet rule is used to describe the attraction of elements with other elements Whenever possible, every element wants a complete valence shell! That means they are MOST STABLE… A full outer shell with eight (octet) electrons is stable, and many atoms lose or gain electrons to get to “8” electrons That allows them to be “full”, “stable” and happy, happy, happy! Ummmm. What’s a valence electron you ask????

Think about the Octet Rule: C would like to N would like to O would like to Gain 4 electrons Gain 3 electrons Gain 2 electrons

Valence Shell This copper atom has an outside shell with only one electron The outer shell of any atom is called the valence shell This electron is called a valence electron

Valence Electrons In some materials (like copper) the electrons are held so loosely by the protons in the atom, and are so close to the neighboring atoms, that it is difficult to determine which electron belongs to which atom Under these conditions, the valence electrons tend to drift randomly from one atom to its neighboring atoms

Electrons and Electricity Under normal conditions the movement of the electrons is random, meaning they are moving in all different directions at the same time!! However, if some outside force acts upon the material, this flow of electrons can be directed through materials and this flow is called electrical current.

Review… So Far… The valence shell is the outer shell of the atom. Some elements have a “free” electron or two in their valence shell and this electron can easily move from atom to atom. These free electrons are responsible for electrical current. To be stable, (and happy!), elements want a full outer shell They will gain, lose or share electrons to get it!

CHEMICAL BONDING To play the movies and simulations included, view the presentation in Slide Show Mode.

Chemical Bonding How is a molecule held together? Why are atoms distributed at strange angles? Why are molecules not flat? Can we predict the structure? How is structure related to chemical and physical properties? AACCCKKKK! WHY IS THIS SOOOOO CONFUSING!???!

What is Chemical Bonding? A chemical bond is an attraction between atoms or molecules that allows the formation of chemical compounds Chemical compounds contain two or more atoms A chemical bond is the electromagnetic force between opposing charges Positive attracts negative! The strength of bonds varies; there are "strong bonds" such as covalent and ionic bonds and "weak bonds" such as hydrogen bonds

Types of Bonds Why do Atoms Make Bonds? 04/10/99 Types of Bonds There are 3 bond types we will discuss: ionic, covalent and hydrogen Why do Atoms Make Bonds? Most atoms are not stable in their natural state They tend to react (combine) with other atoms to become more stable How do they become stable? The Octet Rule!!!!

Ionic Bonds Positive Ions (called cations) and Negative Ions (called anions) are attracted to one another Remember! Opposites Attract! Cations --> Anions

Ionic Bonding In Ionic bonding one atom has a stronger attraction for electrons than the other, and “steals” an electron from a second atom Remember, an “ion” is an atom or molecule in which the total number of electrons is NOT equal to the total number of protons! causing it to be more positive or negative – so, it is more “attractive” (it exerts a greater “pull” on the other atom’s electrons)

Ionic Bonds = Fatal Attraction!

Ionic bonding Ionic bonding involves 3 steps (3 energies) 04/10/99 Ionic bonding involves 3 steps (3 energies) 1) loss of an electron(s) by one element 2) gain of electron(s) by a second element 3) attraction between positive and negative e– 1) 2) Na Cl 3) Cl– Na+ Cl– Na+

04/10/99 Covalent bonding It two atoms have approximately the same pull on electrons, they will share the electrons (forming a “covalent” bond) Think about the “parts of that word”! “co” (co-operate, co-author) “co” means together; mutually “valent” (valence shell)

Covalent Bonds The number of covalent bonds an atom can form depends on the number of electrons it has to share in its valence shell! “co” valent = “co” operating! (sharing!) Remember – Carbon is an extremely important element… it makes thousands (millions) of different molecules because it has 4 electrons in its outer shell It can gain/lose/share all 4! It is VERY VERSATILE!

Co-Valent (co-valence) Bonding

Now… They are Happy!

Hydrogen Bonding This bond is an “attractive” force that exists between two partial electric charges of opposite polarity Opposite charges attract! The hydrogen bond is weaker than both the ionic bond and the covalent bond As the name "hydrogen bond" implies, one part of the bond involves a hydrogen atom The hydrogen must be attached to oxygen, nitrogen or fluorine for this bond to occur

Hydrogen Bonding in Water

Bonding Review Ionic bonds occur between ions (charged atoms) when one element “steals” an electron because opposite charges attract Covalent bonds occur when two elements “share” valence electrons Think “co-operate”… “co-worker”… Hydrogen bonds occur when Hydrogen is attached to oxygen, nitrogen or fluorine But is also attracted to another heavily charged ion

Molecules A molecule is formed when two or more atoms join together chemically What is a molecule? A group of elements held together by covalent bonds Can be same element OR different elements Molecules “share” valence electrons A molecule of water = H2O A molecule of oxygen = O2

Compounds A combination of two or more different elements held together by Chemical Bonds All compounds are molecules but not all molecules are compounds The characteristics of a compound depend not only on the type and number of atoms in the compound, but also how they are arranged For example, carbon and hydrogen can form thousands of different compounds Natural gas, waxes, plastics, gasoline

Review… Molecular hydrogen (H2), molecular oxygen (O2) and molecular nitrogen (N2) are NOT compounds because each is composed of a single element They are “pure” substances Water (H2O), carbon dioxide (CO2) and methane (CH4) are compounds because each is made from more than one element The smallest bit of each of these substances would be referred to as a molecule

Chemical Formulas Remember, atoms of elements are represented by symbols (found on the periodic table!) Chemical Formulas use Chemical Symbols to represent the atoms of the type of elements (and their ratio) in a chemical compound Glucose = C6H12O6 = 6 carbons, 12 hydrogens, 6 oxygens Aspirin = C9H8O4 = 9 carbons, 8 hydrogens, 4 oxygens

More Numbers! We know the “subscripts” tell you how many of each element are present in a molecule What about THIS? 3H2O2? Where’d that “3” come from???? A Co-efficient! A Number IN FRONT of the chemical formula means you have THAT MANY “molecules” Which means… you multiply each of the element(s) by the coefficient 3 x H2 = 6 hydrogens 3 x O2 = 6 oxygens

Numbers & Bonding The “subscripts” (the little numbers to the right) tell you that there are THAT MANY elements bonded together in a certain formula The “coefficients” (the big number in front of the formula) tell you that there are THAT MANY of the entire molecule bonded together What’s in 2C6H12O6???? # Carbons: __________ # Hydrogens: ___________ # Oxygens: __________