CHEMICAL BONDING

THERE ARE TWO KINDS OF PURE SUBSTANCES Elements Compounds

Let’s start the new notes…

Elements  Substance that cannot be separated or broken down into simpler substances by chemical means.  Ex’s- any box on the periodic table  Made of only 1 type of atom  The smallest unit of an element that maintains the properties of that element  HUH?

The smallest unit of an element that maintains the properties of that element???  The element sodium has certain properties  11 protons, 11 electrons, etc and that determines properties like boiling point, etc.

Forming a Compound  Compounds- have properties unlike those of their elements ex. Na-shiny, soft, grey metal Cl- green-yellow gas NaCl- forms table salt Chemical formula- tells what elements it contains & the ratio of the atoms of those elements ex. H 2 O, Na +1 Cl -1 Subscript-indicates #atoms Superscript- indicates charge

Compounds cont…  Substance made of atoms of 2 or more different elements that are CHEMICALLY combined.  Means they are bonded by the valence electrons!  Elements are combined in a definite way and this changes their properties Na - lethal if ingested Cl- lethal if ingested NaCl- table salt

Molecules  Two or more atoms bonded together…  They can be two of the same element or two different elements (compounds)  All compounds are molecules, but not all molecules are compounds.  Ex- O 2, NaCl, etc.

Compounds  Molecules made by atoms of two or more elements bonded together -always in a definite ratio Elements  Made of just one type of atom  Ex. Sodium is only made of Na atoms Na (sodium) NaCl (sodium chloride/salt)

Chemical Bond  Force which holds atoms together.  2 major types: Ionic Covalent

Why Bond??  Chemically stable- if an atom’s outer energy level is completely filled with electrons -most are filled when they contain 8 electrons (OCTET RULE) (exceptions- He, H, Li, Be) ex. Noble gases- inert; chemically stable  So, atoms will try to form compounds by doing chemical reactions and therefore forming bonds.  REMEMBER: ATOMS DON’T LIKE TO BE ALONE!

Why Bond??  Chemical bond- force that holds together the atoms in a substance *Losing, gaining, and sharing of valence electrons are the means that atoms use to become stable & form chemical bonds. ex. LiF- Li loses 1 electron F gains 1 electron

Ionic Bonds (formed in ionic compounds) Formed between metal & nonmetal atoms (elements).  IONS are the basic unit (atom with a charge).  CATION = ion that has a + charge (lost e - ‘s)  ANION = ion that has a – charge (gained e - ‘s)

How are these ions made?  Can you mess with protons?  Would messing with neutrons do anything to the charge?  What must you mess with???  What charge does an electron have?  So what would happen to the atom if they LOSE one? Would they get more positive or more negative overall???

How do ions stick together?  Force of attraction between oppositely charged ions.  (anion & cation)  Ionic bonds are very strong bonds.  Formed between metal & nonmetal atoms (elements).  One atom gives an electron to another atom (remember it’s the losing and gaining of electrons)  Electronegativity difference between the atoms is 1.7 or greater. (see table on last page of notes)  Ex) Na & Cl

Ionic Compounds: pack into a pattern (crystal; lattice) have very high melting points.

Ionic Compound Cont… Dissolve in water (H 2 O), and ions are then free to move (dissociate). Conduct electric current.

Let’s Draw Some:  Do you remember how to draw a bohr model?  Draw one for Sodium and one for Chlorine  What do you think will happen?

Bond Diagrams  Ionic Bond- -lose & gain electrons  Ex. NaCl

Ionic Bonds  Sodium atom, Na, that lost an electron is now a sodium ion, Na 1+  Chlorine atom, Cl, that gained an electron is now a chlorine ion, Cl 1-

Lets make some Lewis Dot diagrams!!!  Draw a Lewis Dot for Na & Cl.  Draw an arrow to show what the electron will do  Finally draw the ions in the bond.

Let’s put it all together…

Lewis Dot Diagrams 

Types of Bonds  Covalent bond – bond that forms b/w atoms when they share electrons -occurs between 2 nonmetals -types- single (shares 1 pair of e) double (shares 2 pairs of e) triple (shares 3 pairs of e)  Usually forms liquids/gases at room temp. *REMEMBER HYDROGEN IS A NONMETAL AND FORMS COVALENT BONDS!!!

Covalent Bonds (formed in molecular/covalent compounds)  Atoms are the basic unit. (atoms bond to form them)  Covalent bonds are firm, but molecules not strongly held together.  Force between atoms that share electrons.

Let’s draw some!  Remember covalent bonds SHARE electrons, so there is no drawing arrows.  Ex- F 2

Lewis Dot Diagrams…

Covalent bonds H 2 -hydrogen gas-sharing of electrons

Steps to help you…  Draw the lewis of each kind of atom in the compound.  Highlight/circle which electrons will be shared  When you draw your molecule-  Draw the center atom in the molecule with all 8 valence electrons (use two “types” like x’s and o’s to show where they came from)  Draw in your other atoms and their valence electrons  Circle each atoms’ “8 electrons”- they will overlap each other where they share

Ex- Cl 2

Kinds of Covalent Bonds  Can share different number of electrons: Single- share ONE pair of electrons Double- share TWO pair of electrons Triple- share THREE pairs of electrons

How do you know which type of bond will form? Use two rules:  Calculation using electronegativity values on a periodic table:  Subtract the values of the 2 elements (larger minus smaller)  If difference is: = nonpolar covalent bond > = polar covalent bond 1.7 & up= ionic bond

Electronegativity Differences summary… If difference is: > Electronegativities are not different enough to steal the electron away, but there will be a “bully” who hugs the electrons closer If difference is 1.7 & up Electronegativities are so great that one atom will steal the electron away difference Electronegativities are the same, so the atoms will share nicely

A second way… “rule of thumb”  2) Periodic table/metal, nonmetal rule (Rule of thumb):  Metal and nonmetal = ionic bond  Nonmetal and nonmetal = covalent

Rule of thumb summary… A metal with a nonmetal Two Nonmetals

Covalent/ Molecular Compounds: Not packed into a pattern. Have low melting points. May not dissolve in water (H 2 O), but some can. Do not conduct electricity; do not dissociate.