REMEMBER…. THERE ARE TWO KINDS OF PURE SUBSTANCES Elements Compounds
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, reactivity, etc.
Compounds cont… Substance made of atoms of 2 or more different elements that are CHEMICALLY combined. 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- this would be a compound All compounds are molecules, but not all molecules are compounds. Ex- O 2, NaCl, etc.
Compounds Molecules made by two or more elements bonded together - always in a definite ratio Elements Molecules made of just one element Na (sodium) NaCl (sodium chloride/salt)
Compound/Molecule Chemical combination of 2 or more elements. Properties of the compound are unlike those of the elements that make it. Represented by a formula that gives the ratio of atoms of the elements. Ex- The formula NaCl tells you that salt is a compound made of one Na and one Cl. The ratio is then 1:1
Chemical Bond Force which holds atoms together. 3 Types: Metallic Ionic Covalent Polar Nonpolar
Metallic Bonds Formed between 2 or more metal atoms (elements). Valence electrons overlap. The electrons are free to move about all the atoms; therefore, they are good conductors of electricity. Ex) Mg, Cu, Fe, etc. form these types of bonds
Ionic Bonds (formed in ionic compounds) 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). 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.
Covalent Bonds (formed in molecular/covalent compounds) Atoms are the basic unit. Covalent bonds are firm, but molecules not strongly held together. Force between atoms that share electrons.
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 Can be polar covalent or nonpolar covalent bonds.
Nonpolar Covalent bonds: Electrons are shared equally in the bond. Does not have (+) and (-) end to the bond. Formed between 2 or more of the same nonmetal. Electronegativity difference between the atoms (elements) is 0. (see table on last page of notes) Ex) Cl & Cl
Polar Covalent bonds: Electrons are NOT shared equally in the bond. Has a (+) and (-) end to the bond. Formed between 2 different nonmetals. Electronegativity difference between the atoms (elements) is 0.1- 1.6 (see table on last page of notes) Ex) H & O C & O
How do you know which type of bond will form? Use two rules: Calculation using electronegativity values on a periodic table: (Note: if they are both/all metals, automatically, it is a metallic bond!) Subtract the values of the 2 elements (larger minus smaller) If difference is: 0= nonpolar covalent bond 0.1-1.6= polar covalent bond 1.7 & up= ionic bond
Electronegativity Differences summary… If difference is: 0.1-1.6 Electronegativities are not different enough to steal the electron away, but there will be a “bully” who hugs the electrons closer If differenc is 1.7 & up Electronegativities are so great that one atom will steal the eletron away 0 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 Metal = metallic bond Metal and nonmetal = ionic bond Nonmetal and nonmetal = covalent All same nonmetal = nonpolar covalent bond Different nonmetals = polar covalent bond
Rule of thumb summary… Two different nonmetals A metal with a nonmetal Two Metals Both the same metal
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.