Forces Of Atrraction Flashcards
Why are atoms not generally found in their free states
Because they have lower potential energies when in combination with other rather than in isoloation
What is a chemical Bond formed
When the two atom’s attractive forces exceed the repulsive forces
What are intramolecular forces of attraction
attraction that act within particles
comprised of different atoms to hold the atoms tightly together
What is intermolecular forces of attraction
forces of attraction act between
particles to keep them loosely near to each other as solids,
liquids, or gases
What are types of Intramolecular forces
Ionic, Covalent (Simple molecular, Giant molecular), Metallic,
What are types of intermolecular forces
Van der Waals, Hydrogen Bonding, Permanent dipole
What physical properties of matter with ionic bonding (solid, m.p/b.p, electrical conductivity, solubility)
Solid at room temperature
high melting and boiling point
conductors in molten or aqueous state
soluble in polar solvent
What physical properties of matter with Giant covalent bonding (solid, m.p/b.p, electrical conductivity, solubility)
Solid at room temperature
high melting and boiling point
non conductors except graphite
insoluble
What physical properties of matter with simple covalent bonding (solid, m.p/b.p, electrical conductivity, solubility)
Solid liquid or gas at room temperature
low melting and boiling point
non-conductors
usually soluble in polar or non polar solvent
What physical properties of matter with metallic bonding (solid, m.p/b.p, electrical conductivity, solubility)
Solid at room temperature
high boiling and melting points
good electrical conductivity
Dissolves in other metals to form alloys
The strength of metallic bonding increases with
increasing positive charge on the ions
decreasing size of the metal ions
increasing number of delocalised electrons
Explain Metallic bonding
In metallic bonding, metal atoms come together and donate
their valence electrons to form a “sea” of mobile electrons
that is communally shared among the resulting cations.
The cations organise themselves into a lattice structure (aka a
crystal lattice – an ordered 3D arrangement containing
repeating basic units) and the delocalised electrons move
freely throughout the lattice.
*The positive charges are held together by their strong electrostatic
attraction to the delocalised electrons. This strong electrostatic
attraction acts in all directions.
What are the types of structure for metals
Hexagonal close packed (hcp)
= 2 repeating layer arrangements ABABAB
face-centred cubic close packing (ccp or fcc)
= 3 repeating arrangements ABCABC…
Simple cubic (sc)
Body-centered cubic (bcc)
What type of metals have HCP structure
elemental metals including Be, Mg, Ti, Zr,
What type of metals have ccp (fcc) structure
Al, Ni, Cu, Ag
What type of metals have SC structure
Polonium (Po)
What type of metals have bcc structure
Fe, V, Cr, Mo, W
How to measure metallic radius
The distance between the nuclei of two metal atoms in a solid metallic lattice can be measured accurately by X-ray diffraction studies of
metal crystals. Half of this distance is called the metallic radius.
The density (mass per unit volume) of a metal depends on
– the atomic mass
– the size of the atom (i.e. the metallic radius)
– the type of packing
Properties of metals are
high density solids with high melting and boiling points. - It
takes a lot of energy to break the large number of strong forces of
attraction between the ions and the delocalised electrons
Good conductors of heat and of electricity - the delocalised
electrons are free to move when a charge is applied
Shiny- reflecting light of all wavelengths
Malleable and ductile - when stress is applied the cations can slide
over each other and so metals can be beaten into shapes and
drawn into wires
Insoluble in water (although some react with water) - the force of
attraction between the ions and the delocalised electrons is too
strong to be broken up and replaced by bonds to water molecules. Some metals react with water because they lose electrons
Why do metals tend to give up electrons more easily than non-metals
lower ionisation energies
Why do non metals tend to
accept electrons more easily than metals
have more exothermic electron affinities
What affects the rate at which ions become stable
the total of the ionisation energies or electron affinities involved to get them to noble gas configuration
What is ionic bonding
the transfer of electrons from a metal atom to a non-metal atom until the outer shells of the resulting ions have configurations similar to those of a noble gas
Why are ionic compounds said to have 3d crystal structures
each positive ion is surrounded by several negative ions, and each negative ion is surrounded by several positive ions in repeating units
Whats a formula ratio
the ratio in which ions are present in the crystal structure
How do we calculate ionic radius
from interionic distance measurements from the crystal structures of ionic compounds using the O2- ion as a reference.
the interionic distance is the distance between the centres of the cation and the anion. It is equal to the sum of the ionic radius of the cation (the cationic radius) and the ionic radius of the anion (the
anionic radius)
the radii of any cation can be determined by:
rcation = interionic distance – rO2-
the unknown radius of
another anion bonded to that cation can be found from:
ranion = interionic distance - rcation
What is lattice enthalpy
The lattice enthalpy is defined as the energy absorbed when 1 mole
of an ionic compound is formed from its gaseous ions. It always has
a negative value since this process is exothermic
What is lattice enthalpy used for
Lattice energies can be used as a measure of the relative stabilities
of ionic substances. The more exothermic the lattice energy (i.e. the more heat given off when the ionic bond is formed), the more
stable the resulting ionic compound
Lattice energy becomes more exothermic with:
– increasing ionic charge
– decreasing interionic distances or ionic radii since small distances
means that ions can more tightly pack together in the crystal.
Properties of Ionic Compounds
Strong, rigid, crystalline solids -owing to the regular arrangement of
ions held together by powerful electrostatic forces
High melting points, boiling points, heats of fusion, and heats of
vaporisation - it takes a lot of energy to separate the strongly-attracted ions
Hard and brittle - like charges repel each other when a force is applied
Conduct electricity when molten but NOT when solid because when
molten, ions have enough freedom to move, whereas movement is
restricted in the crystal lattice in the solid state
Soluble in water and other polar solvents. Individual ions need to be
stabilized by polar solvent molecules in order to effectively break apart the crystal lattice in solution
What is covalent bonding in terms of orbital
In terms of orbitals, a covalent bond forms when two atomic orbitals overlap.
The joined orbital is called a molecular orbital
What are sigma bonds
Sigma bonds ( bonds) or sigma molecular orbitals are formed by the overlap of atomic orbitals along a line drawn between the two nuclei
Sigma bonds give the maximum possible electron density
between two nuclei; therefore these are very strong bond
What is the electron density of sigma bond
symmetrical about a line joining the two nuclei
How are Pi bonds formed
Pi bonds ( bonds) or pi molecular orbitals are formed by the sideways overlap of two p atomic orbitals
A pi bond is weaker than a sigma bond; so compounds with pi bonds are reactive
How are electron density of Pi bonds distributed
The electron density of the bond formed is distributed above
and below the plane of the sigma bond
The electron density is zero along the axis of the nuclei
What is a double bond formed by
one sigma bond and one pi bond
What is a triple bond formed by
one sigma bond and two pi bonds
What is bond order
the number of electron pairs being shared by two covalently bonded atoms
What is bond length
the distance between the nuclei
of two atoms that are bonded together
Shorter bond lengths mean that the shared electrons are closer to the nuclei resulting in stronger bonds
What is covalent radius
the distance between the nucleus of a covalently bonded atom and the
outermost shell.
How do we calculate covalent radius
half the distance between the nuclei of two identical atoms bonded covalently via a single bond
What is Electronegativity
Electronegativity is a measure of the ability of an atom to attract
an electron
It is derived by combining data on ionisation energies and electron
affinities for the atom
How does electronegativity increase across the periodic table
Electronegativity increases from left to right across a period and
decreases down a group in the periodic table (i.e. it generally
follows the same trend as ionisation energy
Fluorine is the most electronegative element
When is a bond said to be non-Polar
If two atoms forming a covalent bond have electronegativity values
that are the same in magnitude then the shared electrons will be
symmetrically distributed
When is a bond said to be polar
If the two atoms forming the covalent bond have different
electronegativities, then the atom with the greater electronegativity
will pull the shared electrons closer to itself, causing the electrons to
be unsymmetrically distributed between the atoms
Where is the partial negative and positive charge in a polar bond
the bond has a partial negative charge (–) at the more electronegative end, and a partial positive charge (+) at the less electronegative end
Where does the dipole point to and from
from less electronegative atom to more electronegative atom
What is bond enthalpy (bond dissociation enthalpy)
the energy required to disssociate 1 mole of molecules into atoms
Factors affecting bond energy/bond strength include:
– Bond length – shorter bonds are stronger
– Bond order – multiple bonds are more difficult to break
– Types of bonds – sigma bonds are stronger than pi bonds
– Bond polarity – higher polarity leads to stronger bonds
molecules that are unable to complete the octet of electrons
when they form covalent bonds are called
electron deficient
Whats a coordinate bond or dative covalent bond
one atom provides both the electrons for the covalent bond
For coordinate bonding to occur, we need:
– one atom with a lone pair of electrons
– a second atom with an unfilled orbital
In the displayed formula, a coordinate bond can be indicated by
an arrow pointing away from the atom donating the electrons
Molecules are said to have an expanded octet of electrons when…
number of electrons in their outer shell ismore than 8
example sulphur hexafluoride SF6
What does VSEPR Theory stand for and what does it do
Valence Shell Electron Pair Repulsion
predicts the shapes and bond angles of simple covalent
molecules
What does this theory suggest
the electron pairs around an atom
repel each other and arrange themselves in space so as to
minimize the repulsive forces
What determines the shape of molecules
The shapes of molecules are determined primarily by the
numbers of bonding pairs and lone pairs of electrons around a particular atom called the central atom
Interactions between electrons in two different bonds are called
bonding pair-bonding pair repulsions
Interactions between lone pairs and bonding pairs are called
lone pair-bonding pair repulsions
Interactions between two different lone pairs are
lone pair-lone pair repulsions
The repulsion between electron pairs is inversely proportional to the ____ between them
distance
the shorter the distance the greater the repulsion
the strength of repulsions increases in the order
Bond pair-bond pair < lone pair-bond pair < lone pair-lone pair
the strength of the repulsion depends on the proximity of
each pair to the central atom. Lone pairs are closer to the central atom
than bonding pairs since there is no other nearby positive nucleus pulling the electrons away
2 groups attached without lone pairs
Two groups
Equal repulsion
Bond angles both 180
Shape is linear
2 groups attached with lone pairs
Two groups
Two Lone pairs
Greatest repulsion between lone pairs
Less repulsion between bond pairs
Bond angles 104.5
Shape is non linear, V shaped, bent
3 groups attached without lone pairs
Three bond pairs
Equal Repulsion
All angles 120
Shape is trigonal planar
3 groups attached with lone pairs
On lone pair
Three bond pairs
Greater repulsion between lone and bonding pairs
Bond angles 107
Shape pyramidal
4 groups attached without lone pairs
Four bond pairs
Equal Replusion
all bond angles 109.5
Shape is tretrahedral
5 groups attached without lone pairs
Equal Repulsion
Five bond pairs
Three angles form the trigonal plane with bond angles for 120
Two remaining bond angles are 90
Shape: trigonal bipyramidal
6 groups attached without lone pairs
Six bond pairs
Equal repulsion
Bond angles all 90
Shape octahedral
What are the two questions that VSEPR theory does not address
How are the valence (or outer shell) electrons used to form
covalent bonds if the electrons are paired in various subshells?
How do we account for the shapes of the resulting molecules
using molecular orbital theory?
What is Hybridization
Hybridization is the mixing of different atomic orbitals to form
new orbitals of equal energy
What happens in sp3 hybridization
one s orbital and three p orbitals combine to form four equal in energy sp3 hybrid orbitals (sppp = sp3)
After hybridization, the four resulting sp3 hybrid orbitals are
oriented in a tetrahedral arrangement. When these four
orbitals form bonds to other atoms, the resulting bonds take
on a tetrahedral shape in the molecule. (109.5)
What happens in sp2 hybridization
In sp2 hybridization, one s orbital and two p orbitals combine
to form three equal in energy sp2 hybrid orbitals (spp = sp2).
One p orbital remains unhybridised
After hybridization, the three resulting sp2 hybrid orbitals are
oriented in a trigonal planar arrangement. When these three
orbitalsform bonds to other atoms, the resulting bonds take
on a trigonal planar shape in the molecule. (120)
The remaining unhybridised p orbital is perpendicular to the
plane containing the sp2 hybrid orbitals and is used for
bonding to create a double bond
What happens in sp hybridization
In sp hybridization, one s orbital and one p orbitals combine
to form three equal in energy sp hybrid orbitals. Two p orbitalsremain unchanged.
After hybridization, the two resulting sp hybrid orbitals are
oriented in a linear arrangement. When these two orbitals
form bonds to other atoms, the resulting bonds take on a
linear shape in the molecule (180)
The remaining unhybridised p orbitals are perpendicular to
the plane containing the sp hybrid orbitals and are used for
bonding to create a triple bond
What a conjugated bond system
a system of alternating single and double bonds
Example A special type of sp2 hybridization occurs in the ring structure of benzene (C6H6)
Covalently bonded substances can form the following crystal lattices:
– simple molecular lattices with weak intermolecular forces
between the separate molecules
– giant molecular structures with strong covalent bonds throughout
the entire lattice made up of different atoms
– giant atomic structures with individual atoms of one type
covalently bonded in a giant structure (e.g. diamond, graphite)
Whats the differences between Simple Molecular Lattices and Giant Molecular Structures
In simple molecular lattices, weak intermolecular forces operate
between the separate molecules
In giant molecular structures, strong covalent bonds occur throughout the entire lattice
What are the properties of Simple Molecular Substances
Since simple molecular solids have weak van der Waals forces
between the molecules; small amounts of heat will provide
sufficient energy to separate the molecules. So, molecular
substances have low melting and boiling points and are often
found as liquids or gases at room temperature.
Molecular solids have no free electrons and thus are poor
conductors of electricity
Molecular solids tend to be soft and brittle
Most molecular solids are only slightly soluble in water
What are the properties of Giant Covalent Substances
Giant covalent structures have very high melting and boiling points
since it requires a lot of energy to break the strong covalent bonds
throughout
Giant molecular and atomic solids with no free electrons are poor
conductors of electricity (e.g. SiO2, diamond). Exception: Graphite is
a good conductor.
Because of the strong covalent bonds throughout, giant covalent
structures other than graphite are hard solids
Giant covalent structures are insoluble in water
Why is graphite a conductor of electricty
Graphite contains unbonded or delocalised electrons that can move throughout the p orbitals above and below the plane.
What are Intermolecular forces
the forces of attraction that operate between separate molecules holding them close to one another
Which are stronger intermolecular or intramolecular
Intermolecular forces are weak when compared with the
intramolecular forces responsible for ionic, covalent, and metallic
bonding
What are the three intermolecular forces
permanent dipole-dipole attractions, van der Waals forces, and hydrogen bonding
The comparative strength of intermolecular forces varies as
follows:
van der Waals < permanent dipole – dipole < hydrogen bonding
What is a non polar molecule
In a non-polar molecule, the electrons are on average evenly
distributed throughout the molecule
Whats a polar molecule
In a polar molecule, the electrons on average spend more time at one
end of the molecule than the other. One end of the molecule is slightly
negatively charged (-) while the other end is slightly positively
charged (+)
What are Permanent dipole – dipole forces
exist between polar molecules.
They are weak attractive forces between the + of the dipole of one
molecule and the - of the dipole of a neighbouring molecule
What is an instantaneous dipole
An instantaneous dipole is a temporary dipole that occurs when a molecule’s electrons are distributed unevenly around the nucleus, creating a dipole of uneven charges. This happens due to the constant motion of the electrons.
van der Waals forces determine
the distances between atoms in
liquids and solids
The strength of the van der Waals Forces increases with
the # of electrons and molar mass of the atom or molecule
the shape of the molecule:
elongated molecules are more easily polarised than compact,
symmetrical molecules and therefore have stronger van der Waals forces
the size of the molecule:
small molecules have weak van der Waals forces between them
Larger molecules have more points of contact between them and thus
more opportunities for induced dipoles to form leading to stronger vander Waals forces
the degree of branching:
linear molecules have stronger van der Waals forces between them
than branched molecules. Branching reduces the number of points of contact between the two molecules leading to weaker van der Waals forces overall
What are van der Waals forces
temporary instantaneous induced
dipole – dipole forces that exist in all atoms and molecules
including noble gases
What is a hydrogen bond
The force of attraction between an electron-deficient hydrogen
bonded to fluorine, oxygen or nitrogen and a lone pair on a
neighbouring fluorine, oxygen or nitrogen is called a hydrogen bond
