1.4 bonding Flashcards
what is an intramolecular bond?
a bond within the molecule itself
(between atoms that have formed a covalent or ionic bond with each other)
what is an intermolecular bond?
a bond that is formed between molecules
what is an ionic bond?
the electrostatic attraction between oppositely charged ions
(which are formed by the transfer of electrons from one atom to another)
what does ionic bonding occur between?
metals and non-metals
during ionic bonding, what ion does the metal become? non-metal become?
the metal becomes a positive ion -cation
the non-metal becomes a negative ion -anions
(the outermost electrons are lost from the metal and given to the non-metal)
what does ionic bonding form after the ions are formed?
after the ions are formed, they all come together to form a lattice
what is a lattice?
an infinite and repeating arrangement of particles
what are the properties of ionic compounds?
- high melting and boiling points
- conduct electricity when molyen or dissolved
- hard + brittle
why are the melting and boiling points of ionic compounds very high?
- the attraction between oppositely charged ions is very strong
- a lot of kinetic energy is therefore required to overcome them
- so the mp and bp are very high
why don’t ionic compounds conduct electrcity when solid? why do they when dissolved?
- in solid state, the ions are not free to movr since they are held tightly in place by each other. (so they are good insulators)
- in liquid state, the ions are free to move so can conduct electricity
why are ionic compounds hard and brittle?
since ions are held strongly in place by other ions, they cannot move or slip over each other easily
what are the three examples of intramolrcular bonding?
- ionic
- metallic
- simple covalent
what is a covalent bond?
a pair of electrons shared between two atoms
what does simple covalent bonding occur between?
two non metals
what is coordinate bonding?
a pair of electrons shared between two atoms, one which provides both electrons to the bond
(a covalent bond, but one of the atoms provides both electrons in the shared pair) but the shared pair of electrons come from the same atom
what is a coordinate bond represented by?
a short arrow from the electron providing both electrons to the electron providing neither
what are the propertkes of simple covalent molecules?
- low melting points
- poor electrical conductors
- tend to be soft/weak/powdery substances
why do simple covalent bonds have low melting points?
due to the weak intermolecular forces between molecules
why are simple covalent bonds poor electrical conductors?
due to no delocalised electrons or free moving ions
(in liquid and solid state)
where is the attraction and repulsion in covalent bonds? why?
- nuclei in atoms are positive
- electrons are negative
- the electrons in a covalent bond repel each other
- the nuclei between the two atoms in a covalent bond repel each other
- there is an attraction between electrons in the covalent bond and the nuclei in the two atoms involved in the bond
- depending on the strength of attraction/repulsion will dictate the length and strength of the covalent bond
what does coordinate bonding bond between?
an atom with lone pairs with electron deficient species (missing full outer shell)
what is an example of a compound bonded by coordinate bonding
- NH4 +
- H3O +
electronegativity definition
a measure of the ability of an atom to attract electrons in a covalent bond / a bonding pair of electrons
why does electronegativity increase across a period?
as the nuclear charge on the atoms increases but the shielding stays the same, so the electrons are more strongly attractrd to the atom
why does electronegativity decrease down a group?
as the number of shells increases, so shielding increases and the electrons are less strongly attracted to the atom
what are the electronegative elements we need to know? which one is the most electronegative
N, O, F, Cl, Br
Br = 2.8
Cl = 3.0 N = 3.0
O = 3.5
F = 4.0
what is an atoms which has a high electronegativity said to be?
electronegative
what is an atoms which doesn’t have a high electronegativity said to be?
electropositive
are electronegativities relative?
what is the unit for electronegativity?
what is the scale from which it’s measured?
- yes, electronegativity is relative
- it has no units
- is measured on a scale from 0.7 to 4.0
why cannot the nobel gases be ascribed an electronegativity?
since they do not form bonds
(their outermost shells are full, and therefore they do not have a tendency to gain or attract electrons)
why is electronegativity a useful concept?
because it can help predict whether the bonding between two atoms will be ionic, covalent or metallic
what bond must it be if two elements have the same or very similar electronegativities?
COVALENT BOND
- both atoms attract the electrins with similqr power and the electrons will remain midway between the two
- so covalent bond bc the electrins are shared between the two atoms
e.g hydrogen molecule
what bond must it be if two elements have a significant electronegativity difference?
POLAR COVALENT
- one attracts the electrons more strongly and the electrons are on average closer to one atom than the other
- electrons still shared, but one atom has a sight deficit of electrons and thus a slight positive charged and the other a slight surplus of electrons and therefore a slight negative charge
e.g a water molecule
what is a polar bond?
a bond that contains an unequal sharing of electron density between two atoms in a covalent bond
(a dipole is therefore created (separation of charge) within the molecule. due to the differences in electronegativity between different atoms in a bond)
what bond must it be if two elements have extreme differences in electronegativity?
IONIC BONDING
- if difference so large, the sharing of the electrons is so uneven
- electrons have essentially been transferred from one atom to the other
e.g sodium chloride
what bond must it be if two elements are both electropositive?
METALLIC BONDING
- neither has the ability to attract electrons
- electrons don’t remain localized in the bond at all
- they are free to move
- both atoms gain a positive charge
is SH2 a dipole?
is NH3 a dipole?
SH2 = not a dipole
NH3 = yes is a dipole
what does it mean that a molecule has a no net dipole?
if all atoms in a molecule are pulling electron density in opposite directions, then the net result is that there is no permanent dipole
e.g O=C=O
<— —>
no net dipole (0)
what are molecules that have a net dipole of zero said to be? do they have low or high boiling points?
- said to be non-polar
- and hence have lower boiling points
are intermolecular forces strong or weak?
very weak
(much weaker than their intramolecular counterparts)
an increase in amount of intermolecular forces will _____ a substances boiling point
increase
what are the 3 main types of intermolecular forces?
- hydrogen bonds
- Van der Waals / temporary dipole-dipole
- permanent dipole-dipole
what are Van der Waals forces? (VdW)
- these charges produce very weak electrostatic attractions between all atoms and molecules
- electrons in atoms are always moving so it’s likely that at any given time the distribution of electrons will not be exactly symmetrical
- the molecules get a slight negative and slight positive charge and so VdW are made between the neighbouring molecules/atoms
- (on an adjacent molecule, the electrons are now repelled by the negative part of the dipole and attracted to the positive part)
- makes instantaneous dipoles
- form + break over and over again
- weakest intermolecular force
- if bigger (Mr), more VdW, higher boiling point)
which is the weakest intermolecular force?
Van der Waal’s forces
where do VdW forces occur?
- they exist everywhere
what does the strength of the Van der Waal’s forces in between molecules depend on?
- the number of electrons in the molecule
- the surface area of the molecules
explain why chlorine is a gas but iodine is a solid at room temperature and pressure. [3]
- VdW forces
- I2 larger (surface area)
- more VdW so more energy needed, so higher temp
do temporary dipoles exist in all molecules?
do permanent dipoles exist in all molecules?
- yes, temporary dipoles exist in all molecules
- no, in some molecules there is also a permanent dipoles
does the presence of polar bonds mean a permanent dipole on the molecule?
no not necessarily
as there are other polar bonds (dipoles) in the same molecules which have the effect of cancelling each other out
e.g O=C=O is a dipole resulting from polar bonds but the vector sum of these dipoles is 0. the molecule has no overall dipole and is said to be non-polar
what are polar molecules?
those in which there are polar bonds and in which the dipoles resulting from the polar bonds do not cancel out
what is permanent dipole / dipole-dipole bonding between?
between a permanent dipole on one molecule and a permanent dipole on another
what is a permanent dipole?
a molecule that is polar
(the dipoles resulting from polar bonds whose vector sum is not 0)
what is the middle strength bond of intermolecular forces?
permanent dipole / dipole-dipole bonding
does dipole-dipole bonding increase the boiling points compared to temperary dipole bonding?
yes
what do you need for hydrogen bonding to occur?
- an electronegative element (N, O, F, Cl, Br) DIRECTLY bonded to hydrogen
what is the strongest intermolecular bond?
hydrogen bonding
(relative)
where does the hydrogen bond form?
beween the H (sight positive charge) or one molecule with the O (slight negativr charge) of another molecule
- the more dipole-dipole forces and hydrogen bonds, the _____ the boiling point
- the higher the difference in electronegativity between two atoms, the _____ the dipole on the atoms and thus the ____ the dipole-dipole or hydrogen bond is
- higher
- larger, stronger
the more hydrogen bonds a molecule can make with water, the ___ soluble in water that substance is
more soluble in water
what do polar solvents e.g water dissolve?
what do nonpolar substances e.g octane dissolve?
- polar solvents dissolve polar substances
- non-polar solvents dissolve non polar substances
is any molecule that can hydrogen bond able to form hydrogen bonds with water molecules?
yes theoretically
(the more hydrogen bonds a molecule can make with water, the more soluble in water that substance is)
what is another word for insoluble?
immiscible
the more intermolecular bonds that can form between molecules, the ___ solublr the substance
the more soluble
what happens to e.g hexane in water? (has no dipoles/polar bonds)
- no dipoles/polar bonds so no attraction between molecules
- so you would see two layers of substances if you added these two chemicals together as no intermolecular bonds can form between them
- they are IMMISCIBLE
hydrophilic definition
water loving
hydrophobic definition
water hating
what is electron pair repulsion theory?
the shape of a molecule depends upon the number of electron pairs surrounding the central atom
(electron pairs repel one another and move as far away as possible)
how can molecular shapes be predicted?
using the valence shell electron pair repulsion theory (VSEPR)
how does the VSEPR theory work?
- draw a dot and cross diagram for the molecule
- cound the number of bonded electron pairs around the central atom
- note the arrangement that this numver of electrons will adopt
- write the symbol of the central atom and arrange the other atoms around it in the expected shape
- the overall shape can be distorted by lone pairs, like it is in ammonia and water
what is the shape and bond angle of a molecule with 2 bond pairs?
linear
bond angle = 180.
what is the shape and bond angle of a molecule with 3 bond pairs?
trigonal planar
bond angle = 120.
what is the shape and bond angle of a molecule with 4 bond pairs?
tetrahedral
bond angle = 109.5
(is 3D)
what is the shape and bond angle of a molecule with 5 bond pairs?
trigonal bipyramid
bond angle = 90. and 120.
(3D)
what is the shape and bond angle of a molecule with 6 bond pairs?
octahedral
bond angle = 90.
what is the order of increasing repulsion of lone pair-lone pair, lone pair-bond pair and bond pair-bond pair?
^ lone - lone
I bond - lone
I bond - bond
greater repulsion ^
what are the physical properties of molecules effected by?
intermolecular forces
what are the chemical properties of molecules effected by?
the bonds (as bonds are broken during chemical reactions)
(e.g ionic/covalent)
what slight charge does the atom with the higher electronegativity have?
slight negative charge
(because it has more electrons)
what is the bond angle for water?
104.5.
what shape does H2O make? (has lone-lone bonding/ 2 lone pairs)
non-linear
use VSEPR theory to state and explain the shape of the molecule BCl3 [3]
- has 3 bonding electron pairs
- has 0 lone pairs
- the bond pairs adopt the position of least repulsion creating a trigonal planar shape
use VSEPR theory to state and explain the shape of the molecule NCl3 [3]
- has 3 bonding electron pairs
- has 1 lone pair
- the shape is distorted from the expected
- making it a pyramidal shape due to the greater repulsion between lone and bond pairs
- resulting in a bond angle of 107.
why do both electrons in the covalent bond have opposing spins?
to minimise this repulsion
the greater the electronegativity value, the ____ the attracting power of the element for the bonding pair of electrons
stronger
what is a dipole?
a difference in charge between two atoms in a bond
what shape and bond angle would three bonding pairs and one lone pair make? e.g ammonia
trigonal pyramidal
bond angle = 107.
what shape and bond angle would two bonding pairs and two lone pair make? e.g water
bent / v-shaped
bond angle = 104.5.
(bond angle less than in tetrahedral molecules bc lone-lone repulsion forces the bonding pairs even closer and reduces the angle between them)
why does electronegativity increase along a period?
as atomic radius decreases
why does electronegativity decrease down a group?
as shielding increases
are intermolecular forces weaker or stronger than ionic/covalent bonds?
weaker
what is meant by a polar covalent bond?
a covalent bond where the electrons are not shared equally between the atoms
give a reason why some covalent bonds are polar
bonds are polar if the elements/atoms involved have a difference in electronegativity
are coordinate bonds always polar?
yes
sincr the atom giving both electrons to the bond cannot completely lose its rights over one electron
the melting temperature of sodium chloride is 1074K but sodium iodide has a melting temperature of 934K.
suggest why the melting temperature of sodium iodide is lower than that of sodium chloride [1]
- iodine ion larger than chloride ion so less attraction to sodium ion
- less difference in electronegativity so NaI not as ionic as NaCl
while chlorine has a boiling temperature of 238K, the boiling temperature of iodine monochloride , ICl, is 371K.
suggest why the boiling temperature of iodine monochloride is higher than that to chlorine?
- ICl larger molecule than Cl2 so more electrons
- more VdW forces between molecules
OR
- permanent dipoles between molecules in ICl but Cl2 has only temporary dipoles/vdw forces between molecules
- permanent dipoles stronger than temporary dipoles/vdw forces
do you say e.g 2 lone pairs or 2 lone bonds?
2 lone pairs
