Molecular Structure

Question 1

pure substance that is composed of two or more elements in a fixed proportion
can be broken down chemically to produce constituent elements
may be either ionic or covalent

Answer 1

compounds

Question 2

combination of two or more atoms held together by covalent bonds
smallest unit of a compound that displays the properties of that compound
may contain 2 atoms of the same element or may be two or more different atoms
ionic compounds not considered to be made up of these b/c they are vast lattices of ions

Answer 2

molecules

Question 3

sum of the weights of the atoms that make up the molecule
found by multiplying the number of atoms found in that molecule times its atomic weight
sum atomic weights for all elements in the molecule

Answer 3

molecular weight

Question 4

mass of 1 mole of a substance

Answer 4

molar mass

Question 5

Equation: Number of moles for a molecule

Answer 5

of moles = mass of molecule (g) / molar mass of the molecule (g/mol)

Question 6

simplest whole number ratio of elements in a compound
gives ratio between the atoms in a molecule
describes a formula unit

Answer 6

empirical formula

Question 7

gives the exact number of atoms of each elements in a molecule of the compound
multiple of the empirical formula

Answer 7

molecular formula

Question 8

if given the empirical formula, can determine the correct molecular formula if we also know the ________

Answer 8

molecular weight

Question 9

Strategy: Find molecular formula from empirical formula

Answer 9

1. Find the formula weight using the empirical formula and atomic weights of elements
2. Determine the correct multiple for the empirical formula by the ratio of molecular weight to weight of empirical formula
3. Multiply empirical formula by that factor

Question 10

percentage by mass contributed by each element in a compound

may be determined using either the empirical or molecular formula

Answer 10

percent composition by mass

Question 11

Equation: Percent Composition

Answer 11

% comp = (mass of X in formula / formula weight of compound) * 100%

Question 12

Strategy: Determine empirical formula from percent comp

Answer 12

1. Assume 100 g sample
2. Translate percent comp for each element to the same number of grams
3. Convert grams to moles by dividing the weight of each element by its molar atomic mass
4. Find the simplest whole number ratio of the elements by dividing the number of moles by the smallest number obtained in the previous step
5. Convert number obtained into whole numbers

Question 13

What atoms can violate the octet rule?

Answer 13

Hydrogen, Boron, Beryllium, and elements from period 3 and below

Question 14

What element forms the central position in a lewis dot structure?

Answer 14

the least electronegative element

Question 15

an atom will form bonds in order to have 8 valence electrons (noble gas config)

Answer 15

octet rule

Question 16

______ and ______ usually occupy the end positions in lewis dot structures, as these elements can only form one bond

Answer 16

hydrogen and halogens

Question 17

used as an aid in determining alternative Lewis structures

structure with the smallest ____ is preferred

Answer 17

formal charge

Question 18

Equation: Formal Charge

Answer 18

Formal Charge = (# valence electrons) - (# nonbonding electrons + # of bonds)

Question 19

structures with the same lewis dot structure that just differ in the way that electrons are distributed
arrangement of actual atoms does not change

Answer 19

resonance structures

Question 20

resonance hybridization diffused the charge of a polyatomic ion, stabilizing the structure
leads to the stabilization of a molecule or an ion

Answer 20

delocalization

Question 21

theory that states that the 3D arrangement of atoms surrounding a central atom is determined by the repulsion between the bonding and nonbonding electron pairs in the valence shell of the central atom
these electron pairs arrange themselves as far apart as possible, minimizing repulsion

Answer 21

VSEPR theory (valence shell electron pair repulsion theory)

Question 22

geometry of a molecule with:
2 electron pairs (Ex: BeCl2)
3 atoms (x —– A——x)
no lone pairs

Answer 22

shape: linear

angle b/w electron pairs: 180

Question 23

geometry of a molecule with:
3 electron pairs
4 atoms (Ex: BH3)
no lone pairs

Answer 23

shape: trigonal planar

angle b/w electron pairs: 120

Question 24

geometry of a molecule with:
4 electron pairs
4 atoms (Eh: NH3)
1 lone pair

Answer 24

shape: trigonal pyramidal
angle: 107

Question 25

geometry of a molecule with:
4 electron pairs
5 atoms (Ex: CH4)
no lone pairs

Answer 25

shape: tetrahedral
angle: 109.5

Question 26

geometry of a molecule with:
5 electron pairs (Ex: PCl5)
6 atoms
no lone pairs

Answer 26

shape: trigonal bipyramidal
angles: 90, 120, 180

Question 27

geometry of a molecule with:
6 electron pairs (Ex: SF6)
7 atoms
no lone pairs

Answer 27

shape: octahedral
angles: 90, 180