ppt 2 Flashcards
Most of the mass is in the ______
nucleus
containing protons and neutrons
Surrounded by an electron cloud containing electrons in a large volume of space
nucleus
protons + neutrons
Most of the volume is in the _______
electron cloud
variation of an element that possesses the same atomic number but a different mass number
any element will always have the same number of protons and electrons
They will differ in the number of neutrons held by their respective nuclei.
isotopes
the exact location of the electron cannot be known; only the electron density can be known
Quantum Mechanical Model
the probability that the electron will be in a certain region of space (orbital) at a given instant
Electron density
Electrons do not circle around the nucleus in fixed orbits but rather in _______
Each _____ has a characteristic shape and size (energy)
orbitals
used to describe orbitals
QUANTUM NUMBERS
Principal
- n
- Size (energy level, shell)
- positive integers (1,2,3,…)
Azimuthal/ Angular Momentum
- l
- Shape (subshell)
- 0 to (n-1) / s, p, d, f
Magnetic
- ml
- Orientation (orbital)
- (-l to +l)
Electron Spin
- ms
- Direction
- +1/2,-1/2
each box represents ________
one orbital
each cluster of boxes represents _________
one subshell
each row represents ________
one shell
describes the orbitals occupied by the atom’s electrons when they are all in the available orbitals with the lowest energy
Ground-state electron configuration
states that an electron occupies orbitals in order from lowest energy to highest (increasing order)
Aufbau Principle
Every orbital in a sublevel is singly occupied before any orbital is doubly occupied.
All of the electrons in singly occupied orbitals have the same spin (to maximize total spin).
Hund’s Rule
no two electrons can have the same four electronic quantum numbers.
As an orbital can contain a maximum of only two electrons, the two electrons must have opposing spins.
This means if one electron is assigned as a spin up (+1/2) electron, the other electron must be spin-down (-1/2) electron.
Pauli’ Exclusion Principle
states that we cannot know both the position and speed of a particle, such as a photon or electron, with perfect accuracy;
the more we nail down the particle’s position, the less we know about its speed and vice versa
Heisenberg’s Uncertainty Principle
electrons below the outermost shell
Core electrons
electrons in the outermost shell
Valence electrons
an attractive force between two ions or between two atoms
CHEMICAL BOND
Why do chemical bonds form?
Because the compound that results is more stable and lower in energy than the separate atoms
How do chemical bonds form?
Octet rule
atom is most stable if its outer shell is either filled or contains eight electrons, and it has no electrons of higher energy
OCTET RULE
An atom will give up, accept, or share electrons in order to achieve a filled outer shell containing eight electrons EXCEPT _______ and ________
hydrogen and helium
Chemistry of main-group elements is governed by their tendency to take on the electron configuration of the nearest
noble gas
attractive forces between opposite charges
Electrostatic attraction
- bond formed as a result of the electrostatic attraction between ions of opposite charge
- bond formed from the transfer of electrons
- usually formed from the reaction of metals with nonmetals
Ionic bond
compounds formed by ionic bonds
Ionic compound
bond formed as a result of sharing electrons between two nuclei
COVALENT BOND
compounds formed by covalent bonds
Molecular compounds
measure of the ability of an atom to pull the bonding electrons toward itself
ELECTRONEGATIVITY
a covalent bond between atoms with the same electronegativity
nonpolar covalent bond
covalent bond between atoms with different electronegativities
polar covalent bond
bond where no electrons are shared; opposite charges attract each other
ionic bond
bond where electrons are shared equally
nonpolar covalent bond
Polar molecules have _______
dipoles
a pair of equal and oppositely charged poles separated by a distance
Dipole
- measure of dipole
- magnitude of the charge on either atom x distance between the two charges
Dipole Moment
electron-dot structures
Lewis Structures
line-bond structures
Kekulé Structures
DRAWING CHEMICAL STRUCTURES
- Chemical symbols
- Covalent bond
- Nonbonding/lone-pair electrons
- Formal charges
- not an actual charge
- used for bookkeeping of electrons
- the charge the atom would have if each bonding electron pair in the molecule were shared equally between atoms
- the difference between the number of valence electrons an atom has when it is not bonded to any other atoms and the number it “owns” when it is bonded
Formal Charge
formal charge equation
formal charge = no of valence electrons in free atom - no of valence electrons in bonded atoms
omitting of the covalent bonds and listing
atoms bonded to a particular carbon (or nitrogen or oxygen) next to it (with a subscript if there is more than one of a particular atom)
Condensed Structures
- Carbon atoms aren’t usually shown.
- A carbon atom is assumed to be at each intersection of two lines (bonds) and at the end of each line.
- Hydrogen atoms bonded to carbon aren’t shown.
- Atoms other than carbon and hydrogen are shown
Skeletal Structures
3D Structures
Perspective Drawing
Ball-and-Stick Model
Space-Filling Model
shows 3D shape
Perspective Drawing
shows bond angles accurately
Ball-and-Stick Model
shows atoms in scale
Space-Filling Model
bond lies in the plane of the paper
normal bond
bond extends backwards, away from the viewer; “into” the paper
bond behind a page
dashed bond
bond protrudes forwards, towards the viewer; bond out of page; “out of” the paper
wedged bond
- Geometry based on arrangement of atoms in a molecule
- Defined by bond angles
MOLECULAR GEOMETRY
- Geometry based on valence electron pairs (bonding and non
bonding) around a central atom- Bonding electron pairs
- Nonbonding/lone electron pairs
- Defined by bond angles
ELECTRON PAIR GEOMETRY
A model for the prediction of molecular geometry based on the minimization of electron repulsion between regions of electron density around an atom
VALENCE-SHELL ELECTRON-PAIR
REPULSION (VSEPR) MODEL
The best arrangement of a given number of electron pairs (bonding and nonbonding) is the one that
minimizes the repulsions among them.
Without lone pairs:
electron pair geometry __ molecular geometry
=
equal
assumes that the electrons in a molecule occupy overlapping atomic orbitals of the individual atoms
Valence Bond theory
assumes the formation of molecular orbitals from the atomic orbitals
Molecular Orbital Theory
formed when atomic orbitals on neighboring atoms overlap one another
Covalent bonds
head-on overlap of atomic orbitals; stronger
σ
sideway overlap of atomic orbitals; weaker
π
atomic orbitals obtained when two or more nonequivalent orbitals of the same atom combine for covalent bond formation
Hybrid orbitals
Produces the molecular geometry of the molecule.
HYBRIDIZATION
one 2s and three 2p orbitals into four sp3 orbitals
four sp3 orbitals are 109.5° away from each other → tetrahedral geometry
sp3 Hybrid Orbitals: CH4 (Methane)
one 2s and two 2p orbitals into three sp2 orbitals + one unhybridized p orbital
three sp2 orbitals are 120° away from each other: trigonal planar geometry + unhybridized p orbital perpendicular to it
sp2 Hybrid Orbitals: C2H4 (Ethylene)
one 2s and one 2p orbitals into two sp orbitals + two unhybridized p orbital
two sp orbitals are 180° from each other → linear geometry + two unhybridized p orbitals mutually perpendicular to it
sp Hybrid Orbitals: C2H2 (Acetylene)
bond strength _____ as bond length ______
increases, decreases
As s character increases, bond angle ________
increases
three sp3 orbitals w/ bonding pairs + one sp3 orbital w/ lone pair (trigonal pyramidal)
N
two sp3 orbitals w/ bonding pairs + two sp3 orbitals w/ lone pairs (bent)
O
Single bonds (no π bonds): sp3 w/ lone pairs in the hybrid orbitals
N & O
one sp3 orbital with bonding pair + three sp3 orbitals w/ lone pairs
X
describes a region of space in a molecule
where electrons are most likely to be found
molecular orbital
2 ways to combine atomic orbitals into molecular orbitals
additive and subtractive
lower energy, bonding molecular orbital
Additive
higher energy, antibonding molecular orbital
Subtractive
Occurs when an electron is shared by more than 2 atoms with π bonds (p orbitals)
π ELECTRON DELOCALIZATION
Structures are represented by resonance contributors
π ELECTRON DELOCALIZATION
Some substances can’t be represented by a single line-bond structure and must be considered as a _______ of two or more structures.
resonance hybrid
The two individual line-bond structures are called ________, and their special resonance relationship is indicated by the double headed arrow between them.
resonance forms
It has a single unchanging structure that is a resonance hybrid of the two individual forms and has characteristics of both.
resonance
Individual resonance forms are ______
imaginary, not real
Resonance forms obey normal rules of _______
valency
The _________ is more stable than any individual resonance form
resonance hybrid
resonance leads to
stability
The larger the number of resonance forms, the ____ stable a substance is
more
vacant p orbital, single electron, or lone pair of electrons on atom Z (resonance forms)
asterisk *
from electron-rich species (negative charge, lone pair, double bond)
tail
to electron-deficient species (positive charge, electronegative atom)
head
between resonance forms
double headed arrow
movement of electrons
movement of two electrons
movement of one electron
curly arrow
curly arrow (full)
curly arrow (half)
Double bonds separated by single bonds
Refers to a system that has a p orbital adjacent to a π bond allowing delocalization of π electrons
conjugation
Compounds with conjugated double bonds are thermodynamically ____ stable (lower energy) than compounds with isolated double bonds (higher energy).
more
Special kind of conjugation
Cyclic unsaturated molecules with unusual stability
Originally used to classify benzene and its derivatives because many of them have distinctive odors
AROMATICITY
Six p orbitals align and overlap to form six molecular orbitals
benzene
TWO CRITERIA FOR AROMATICITY
- It must have an uninterrupted cyclic cloud of p electrons (called a ____) above and below the plane of the molecule.
p cloud
huckel’s rule for aromaticity
Hückel’s Rule: 4n + 2
Not all _______ (carbon ring systems with alternating single and double
bonds) are aromatic.
annulenes
