unit 7 - bonding Flashcards
chemical bond
an attraction between atoms for electrons
(electronegativity)
a chemical bond involves what
attraction between nuclei for electrons
when forming a bond, electrons may be…
lost, gained, or shared
The # of electrons in a chemical bond is such that each atom fills its valence shell
- 8 electrons fill the valence shell (Most elements).
- Some exceptions: Ex. H and He need only 2
He and H only need how many valence electrons for full valence shell
2 electrons
when a bond is broken, what happens to heat
heat energy is absorbed
(endothermic process)
when a bond is formed, heat energy is
released
(exothermic process)
if sign on table I is “-“ , reaction is overall
exothermic
Heats of Reaction:
Ex. H2 + I2 +53 KJ => 2 HI
- Endothermic (Energy is on Left/Absorbed)
- More energy used to break bonds
Heats of Reactions
Ex. 2 H2 + O2 => 2 H2O + 483.6 KJ
- Exothermic (Energy is on Right/Released)
- More energy released in forming bonds
potential energy
Amount of Energy stored within the bonds (distance between the atoms involved in the bond)
When more energy is used to break bonds Reaction is (endo) –> PE
increases
When more energy is used to form bonds Reaction is (exo) –> PE
decreases
Ionic Bonds
Form when metals transfer valence electrons to non-metals.
Ionic Bonds are formed from the
attraction between positive and negative ions
Ionic Substances are called
Ionic Compounds NOT molecules
Ex. All Salts
why aren’t ionic substances called molecules
molecules have neutral particles
Ionic bond characteristics
- ionic bonds are very strong
- ions are held in a fixed position in a “crystal lattice structure”
Ionic compounds are what at room temp?
Solids at room temp
- high melting points (ex: NaCl = 801C, KCl = 771C)
- high boiling points (ex: NaCl = 1465C, KCl =1420C
Ionic compounds are:
- poor conductors as solids
- good conductors when melted (molten)
- soluble in polar solvents (ex. Water)
why are ionic compounds good conductors when dissolved
good conductors when dissolved in water because ions are free to carry a current
ionic bonds
form when elements with large differences in electronegativity combine
if the difference in electronegativity is 1.7 or greater
ionic bond
the greater the difference in E.N.
the stronger the Ionic Bond
covalent bonds
involve the sharing of valence electrons between 2 non-metals
- no ions are involved; only molecules = covalent
4 types of Covalent Bonds:
- non-polar covalent
- polar covalent
- co-ordinate covalent
- network covalent
properties of covalent bonds
- aka molecular substances
- most are poor conductors in all three phases of matter (inlcluding water)
- have low melting/boiling points
- weak forces hold molecules together
non polar covalent bonds
- no difference in electronegativity
- involve the EQUAL sharing of electrons
- electrons spend same amount of time in the valence shell of both atoms
- form between 2 identical non-metal atoms
ex: H2, O2, N2, etc.
polarity
- refers to charge; so Non-Polar refers to NO DIFFERENCE in charge
the electronegativity difference for nonpolar covalent bonds is
zero
allotropes
- different forms of the same element
** different physical properties (color, shape, hardness, melting point) due to *different arrangement of atoms with different structure - ex: diamonds & graphite (all carbon)
- ex: oxygen (O2) and ozone (O3)
- **all have non polar covalent bonds
polar covalent bonds
- a bond in which atoms of 2 different non metals share electrons unequally
electronegativity difference of polar covalent bonds
is between 0 & 1.7
compounds that are composed of both IONIC AND COVALENT BONDS contain
POLYATOMIC IONS
co-ordinate covalent bonds
- AKA: free loader bond
- one atom supplies both electrons (lone pair) to be shared
ex: ammonia + proton -> ammonium ion
water + proton -> hydronium ion
network bonds
- consists of special covalently bonded atoms in a network
- no separate particles, considered one giant “macro-molecule”
- strong bonds; ultra high melting points
ex: diamonds - poor conductors, very hard substances (generally)
- ex: diamond (carbon), graphite (carbon), SiO2 (quartz), and SiC (silicon carbide)
metallic bonds
- bonds that hold metal atoms together
- very strong bonds
- ”+” nuclei in a “sea of mobile electrons”
- free moving electrons give metals luster and conductivity
- metals are good conductors in all three states of matter
bond types
metallic, ionic, covalent (network, polar, nonpolar, co-ordinate)
molecule type
polar molecule, nonpolar molecule
nonpolar molecules have
EQUAL charge distribution and are SYMMETRICAL molecules
** bond type does not always match molecule type
nonpolar molecule examples
LINEAR:
- CO2, H2, O2, N2, Cl2, Br2, F2, I2
TETRAHEDRAL:
- CH4, SiBr4, CCl4, SH4
polar molecules
- have unequal/uneven charge distribution
- are NOT symmetrical
polar molecule examples (linear)
HF, HCl, HI, HBr
polar molecule examples (tetrahedral)
CH3Cl, CH2Cl2, SiH3Cl, SiH2Cl2
polar molecule examples (bent)
H2O, H2S, H2Se, H2Te
polar molecule examples (pyramidal)
NH3, PH3, AsH3
SNAP
- SYMMETRICAL - NONPOLAR molecule
- ASYMMETRICAL - POLAR molecule
intramolecular
A bond WITHIN a molecule
Intermolecular
A bond between molecules
Intermolecular Force: Dipole-Dipole
Bond between 2 identical polar molecules
Hydrogen Bonding
- (special dipole-dipole)
- a bond BETWEEN a hydrogen atom of one molecule (polar molecule) and a highly electronegative atom Ex: (F, O, N) of another molecule
Ex. Between molecules of HF, H2O, and NH3
Hydrogen Bonding characteristics
- Strong bonds (strong intermolecular force)
- Reason why water has a high melting point/high boiling point for its size
- the stronger the intermolecular
force, the higher the boiling point
Van der Waals Forces
- attraction between non polar molecules (symmetrical)
- weak force of attraction so low melting and boiling point
- reason why non polar molecules exist as liquids at low temperature and high pressure
- ex: if a solid and gas at same temperature, gas has weaker intermolecular force
When is van der waals forces most effective
Most effective when molecules are close such as between molecules with more electrons and a larger molecular mass
The STRONGER the Van der Waals force
the HIGHER the melting/boiling point of the substance
MOLECULE/ION ATTRACTION
- attraction between a + or – ion (ionic compound) and the opposite charged ends of a solvent molecule (a liquid in which a substance dissolves)
Ex: H2O and NaCl
Ionic substances (salt) dissolve in
Polar substances
