exam 2 chpt 4-6

covalent bond
most common type of chemical bond; 2 atoms share several electrons; results in a molecule; generally form between nonmetal atoms yeilding covalent molecular compounds
ionic bond
transfer of one or more electrons from one atom to another; generally form between metal and nonmetal atoms
bond order

-# of electron pairs shared between atoms (single bond-bond order of 1, double bond-bond order of 2, ect.)

-mult. bonds are shorter and stronger than their corresponding single bond counterparts because there are more shared electrons holding the atoms together

-as the bond order of a bond increases the bond energy increases and bond length decreases

electronegativity

-ability of an atom in a molecule to attract the shared electrons in a covalent bond

-increases from left to right and decreases top to bottom (He has the greatest and Cs has the least)

isoelectronic

when 2 elements and/or ions have the same electronic configurations; 2 chemical species that are isoelectronic tend to have similiar chemical properties.

Element or ion pair Electronic congifuration
Li+, He  1s2 
Be+2, He 1s2 
F-, Ne  1s2 2s2 2p6 
S-2, Ar  1s2 2s2 2s6 3s2 3p6 
Mg+2, Na+   1s2 2s2 2p6
Ca+, K  [Ne]4s1 
P-3, S-2 1s2 2s2 2p6 3s2 3p6

atomic radii

-shrink dramatically in groups 1A & 2A when an electron is removed to form a cation

-expand when converted to anions by the gain of one or more electrons

-increase down a column of the periodic table because succesively larger valence-shell orbitals are occupied

-decrease from left to right across a row of the periodic table because effective nuclear charge increases across the row

lattice energy

-measure of the strength of a solid’s ionic bonds; the amount of energy that must be supplied to break up an ionic solid into its gaseous state, so it has a positive value.

-large when the distance between ions is small and when the ion charges are large; a small distance means that the ions are close together which implies they have a small atomic radii

electron affinity (Eea)

-energy change that occurs when an electron is added to an isolated atom in the gaseous state

-generally negative because energy is usually released when a neutral atom adds an electron

-the more negative electron affinity, the greater the tendency of the atom to accept an electron and the more stable anion results

-7A elements have the most negative electron affinities corresponding to the largest release of energy, but group 2A and 8A elements have near zero or positive electron affinities, corresponding to a small release or even an absorbtion of energy

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VSEPR: Valence-Shell Electron-Pair Repulsion model

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Electrons in bonds and in lone pairs can be thought of as ;charge clouds; that repel one another and stay as far apart as possible, this causing molecules to assume specific shapes.

;2 charged clouds- 180 degrees

3 charged clouds- 120 degrees

4 charged clouds- 109.5 degrees

5 charged clouds- 90, 120 degrees

6 charged clouds- 90 degrees

molecular geometry of atoms with 2,3 and 4 charged clouds
[image]
molecular geometry of atoms with;5 and;6 charged clouds
sigma (;) bonds
a set of head on overlapping orbitals (any convalent bond)
pie (;);bonds
a set of;sideways overlapping orbitals (double bond)
failing octet rule

-often fails for elements towards the right side of the periosdic table (groups 3A-8A) and that are in the 3rd row and lower. atoms of these elements are larger than their second row counterparts, can accommodate more than 4 atoms close around them and therefore more than 4 bonds.

-the elements nitrogen, hydrogen, oxygen, and carbon almost always follow the octet rule

resonance hybrid

when its possible to write more than one valid electron dot structure for a molecule, the actual electron structure is an average of the different possibilities

ex: best describes an;O3 molecule?Each atom in ozone is connected to another atom by 1.5 bonds.

Molecular Orbital
A wave function whose square gives the probability of finding an electron within a given region of space in a molecule.
bonding molecular orbital

-a molecular orbital that is lower in energy than the atomic orbitals it is derived from

-any electrons it contains spends most of their time between 2 nuclei bonding atoms together

antibonding molecular orbital

a molecular orbital that is higher in energy than the atomic orbitals it is derived from

-any electrons in it can’t occupy the central region between the nuclei and can’t contribute to bonding

molecular orbital theory

1. molecular orbitals are to molecules what atomic orbitals are to atoms

2. molecular orbitals are formed by combining atomic orbitals on different atoms. the # of molecular orbitals formed is the same as the # of atomic orbitals combined

3. molecular orbitals that are lower in energy than the starting atomic orbitals are bonding; molecular orbitals that are higher in energy than the starting atomic orbitals are antibonding

4. bond order can be calculated by subtracting the # of electrons in antibonding molecular orbitals from the # in bonding molecular orbitals and dividing the difference by 2.

paramagnetic
substances that contain unpaired electrons are attracted by a magnetic fields; the more unpaired electrons a substance has the stronger the paramagnetic attraction.
formal charge

electron bookeeping device that tells whether an atom is a molecule has gained or lost electrons compared to the isolated atom

formal charge= (# of valence electrons in free atom)-1/2(number of bonding electrons)-(# of nonbonding electrons)

limiting reactant

is the reactant which determines the amount of product formed.

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In the following chemical reaction, 2;mole of;A will react with 1;mole of;B to produce 1 mole of A2B without anything left over:

2A+B=A2B

But what if you’re given 2.8 mol of A and 3.2 mol of B? The amount of product formed is limited by the reactant that runs out first, called the limiting reactant. To identify the limiting reactant, calculate the amount of product formed from each amount of reactant separately:

2.8 mol A x 1mol A2B/2 mol A= 1.4 mol A2B

3.2 mol B x 1mol A2B/ 1mol B= 3.2 mol A2B

 

 

Notice that less product is formed with the given amount of reactant A. Thus, A is the limiting reactant, and a maximum of 1.4 mol of A2Bcan be formed from the given amounts.

Molarity (M):

The number of moles of a substance dissolved in each liter of solution. In practice, a solution of known molarity is prepared by weighing an appropriate amount of solute, placing it in a container called a volumetric flask, and adding enough solvent until an accurately calibrated final volume is reached.

molarity=(moles of a solute)/(liters of solution)

Titration:

A procedure for determining the concentration of a solution by allowing a carefully measured volume to react with a solution of another substance (the standard solution) whose concentration is known.

Once the reaction is complete you can calculate the concentration of the unknown solution.

successful titration does require;The exact reaction between titrant and analyte must be known,The equivalence point must be marked accurately,and The volume of titrant required to reach the stoichiometry point must be known accurately

-successful titration does not require Endpoint could be marked way after the equivalence point.

 

Percent Composition
Expressed by identifying the elements present and giving the mass percent of each.
Empirical Formula
It tells the smallest whole-number ratios of atoms in a compound.