Science Unit 4.1-4.3 Cheat Sheet
how to remember the periodic table groups
1
alkali metals
1 valence electron; very reactive; usually lose 1 electron.
17
halogens
7 valence electrons in the usual main-group pattern; need 1 more for a full shell.
18
noble gases
full outer shells; generally unreactive.
2. ions and ionic compounds
``term | what you need to know |
|---|---|
ion | an atom or group of atoms with an electric charge. |
positive ion | forms when a neutral atom loses electrons. |
negative ion | forms when a neutral atom gains electrons. |
polyatomic ion | an ion made of more than one atom. the group has an overall charge. |
ionic bond | the attraction between oppositely charged ions, usually after electrons transfer from a metal to a nonmetal. |
ionic compound | a neutral compound made of positive and negative ions whose charges balance. |
properties of ionic compounds | usually hard, brittle crystals with high melting points; conduct electricity when molten or dissolved in water. |
the most important trick in chemistry:
lose negative electrons → become positive
gain negative electrons → become negative
positive and negative charges attract
ionic compounds have a total charge of zero
3. covalent bonds and molecules
term | what you need to know |
|---|---|
chemical bond | the force of attraction that holds atoms together. |
covalent bond | a bond formed when nonmetal atoms share electrons. |
single bond | 1 shared pair of electrons = 2 electrons. |
double bond | 2 shared pairs of electrons = 4 electrons. |
triple bond | 3 shared pairs of electrons = 6 electrons. |
polar covalent bond | electrons are shared unequally, producing slight positive and negative charges. |
nonpolar covalent bond | electrons are shared equally between identical nonmetal atoms, as in your gimkit definition. |
molecule | a neutral group of atoms joined by covalent bonds. |
properties of molecular compounds | generally lower melting and boiling points than ionic compounds and poor electrical conductors. |
bond comparison
1 pair
single bond
2 shared electrons
2 pairs
double bond
4 shared electrons
3 pairs
triple bond
6 shared electrons
4. formulas and forces
term | what you need to know |
|---|---|
subscript | the small number in a chemical formula that tells you how many atoms of an element are present. |
van der waals forces | weak attractions between neighboring molecules or particles due to partial charges. |
example: \(H_2O\)
H₂ = 2 hydrogen atoms
O = 1 oxygen atom because there is no written subscript
the formula contains 3 atoms total
⭐ the five comparisons you MUST know
don't mix these up! | difference |
|---|---|
positive ion vs. negative ion | loses electrons vs. gains electrons |
ionic vs. covalent | electrons transfer vs. electrons are shared |
polar vs. nonpolar | unequal sharing vs. equal sharing |
ionic vs. molecular compounds | usually high melting points vs. generally lower melting points |
chemical bond vs. van der waals forces | holds atoms together vs. weak attraction between neighboring particles |