bonding Flashcards
what do bonds always involve
the bonds between atoms always involve their outer electrons
why do chemical bonds form
noble gases have full outer main levels of electrons and are very unreactive
When atoms bond together they share or transfer electrons to achieve a more stable electron arrangement, often a full outer main level of electrons, like the noble gases
what are the three types of strong chemical bonds
there are three types of strong chemical bonds:
- ionic
- covalent
- metallic
what is the easiest way for metals to gain an electron structure of a noble gas
metals have 1, 2, 3 electrons in their outer main levels, so the easiest way for them to attain the electron structure of noble gases to lose their outer electron
what is the easiest way for non-metals to gain an electron structure of a noble gas
non-metals have spaces in their outer main levels so that the easiest way for them to attain the electron structure of a noble gas is to gain their electrons on their outer shell
when does ionic bonding occur
ionic bonding occurs between metals and non - metals
Electrons are transferred from metal atoms to non - metals
This causes the formation of positive and negative ion
The two ions are attracted to each other and to other oppositely charged ions in the compound by electrostatic forces of attraction
what is an example of ionic bonding
sodium chloride has ionic bonding
The sodium, Na, has 11 electrons (and 11 protons)
The electron arrangement is 1s2, 2s2, 2p6, 3s1
chlorine has 17 electrons (and 17 protons). The electron arrangement is 1s2, 2s2,2p6,3p5
An electron is transferred. The single outer electron of the sodium atom moves into the outer main level of the chlorine atom ( the 3s1 electron from the sodium atom to the 3p orbital on a chlorine atom)
Now both sodium and chlorine have a noble gas electron arrangement - sodium has the neon noble gas arrangement whereas chlorine has the argon noble gas arrangement
what are ions
the two charged particles that result from the transfer of an electron are called ions
e.g. the sodium ion is positively charged because it has lost a negative electron
the chloride ion is negatively charged because it has gained a negative electron
the two ions are attracted to the oppositely charged ion in the sodium chloride compound by electrostatic forces of attraction
This attraction extends throughout the compound
what is ionic bonding the result of
ionic bonding is the result of electrostatic forces of attraction between oppositely charged ions
This attraction extends throughout the compound
what do ionic compounds always exist as
a lattice
what are the properties of ionically bonded compounds
- ionic compounds are always solids at room temperature
- They have giant structures and therefore high melting temperatures. This is because in order to melt an ionic compound energy must be supplied to break up the lattice of ions (break electrostatic forces of attraction) - ionic compounds conduct electricity when molten or dissolved in water (aqueous) but not when solid.
- This is because the ions that carry the current are free to move in the liquid state but are not free in the solid-state - ionic compounds are brittle and shatter easily when given a sharp blow. This is because they form a lattice of alternating positive and negative ions
when do covalent bonds occur
non - metal atoms need to receive electrons to fill the spaces in their outer shells
a covalent bond forms between a pair of non - metal atoms
The atom share some of their outer electrons so that each atom has a stable gas arrangement
what is a covalent bond
a covalent bond is a shared pair of electrons
what is a molecule
a small group of covalently bonded atoms is called a molecule
they are three dimensional and they come in many different shapes
e.g. chlorine exit as a gas that is made up of molecules Cl2
two chlorine atoms make a chlorine molecule
how does a chlorine molecule form
the two chlorine atoms share one pair of electrons
each atom now has a stable noble gas arrangement
The formula is Cl2
why are molecules neutral
molecules are neutral because no electrons have been transferred from one atom to another
how do you represent one pair of shared electrons
you can represent one pair of shared electrons in a covalent bond by a line e.g. Cl - Cl
how do sharing electrons hold atoms together
atoms with covalent bonds are shared together by the electrostatic attraction between the nuclei and the shared electrons
This takes place within the molecule.
The simplest example is hydrogen. The hydrogen molecule consists of two protons held together by a pair of electrons
The forces within this molecule
( repulsion and attraction) are balanced when the nuclei are a particular distance apart
what are double covalent bonds
in a double bond, four electrons are shared
e.g. the two atoms in an oxygen molecule share two pairs of electrons so that the oxygen atoms have a double bond between them
You can represent the two pairs of shared electrons in a covalent bond by a double line O=O
what are the properties of substances that are covalently bonded together (have a molecular structure)
- substances composed of molecules are gases, liquids or solids with low melting temperatures.
- This is because the strong covalent bonds are only between the atoms - There is only weak attraction between the molecules so the molecules do not need energy to move apart from each other ( low melting point)
- They are poor conductors of electricity because the molecules are neutral overall.
This means that there are no charged particles to carry the current - If they dissolve in water, and remain as molecules, the solutions do not conduct electricity.
Again, this is because there are no charged particles
what is co - ordinate bonding
single covalent bonds consist of a pair of electron shared between two atoms
In most covalent bonds each atom provided one of the electrons
But, in some bonds, one atom provides both the electrons. This is called co - ordinated bonding
It is also called active covalent bonding
what is involved in a dative bond
In a co - ordinate or dative covalent bond:
- the atom that accepts the electron pair is an atom that does not have a filled outer main level of electrons -the atom is electron deficient
- the atom that is donating the electrons has a pair of electrons that is not used in a bond, called a lone pair
what are dative bonds represented by
by an arrow
The arrow points towards the atom that is accepting the electron pair
what is the length and strength of coordinate bonds
coordinate bonds have exactly the same strength and length as ordinary covalent bonds between the same pair of atoms
what is the strength of covalent bonds
double and triple bonds are stronger than single bonds but they are shorter than single bonds
more bonds =higher bond strengths
fewer bonds = smaller bond strength
what are metals
metals are shiny elements made up of atoms that can easily lose up to three outer electrons, leaving positive metal ions
what is metallic bonding
the atoms in a metal element cannot transfer electrons ( as happens in ionic bonding) unless there is a non -metal atom present to receive them
In a metal element, the outlet main levels of atoms merge
- The outer electrons are no longer associated with any particular atom.
A simple positive picture of metallic bonding is that metals consist of a lattice of positive ions existing in a “sea” of outer electrons
what are delocalised electrons
delocalised electrons are the electrons that a free in the lattice of positive metal ions
what are the forces within metallic bonding
the positive ions tend to repel one another and this is balanced by the electrostatic forces of these positive ions and the negatively charged “sea”of delocalised electrons
what do the number of delocalised electrons depend on
the number of delocalised electrons depends on how many electrons have been lost by each metal atom
why do metals have giant structures
the metallic bonding spreads throughout so metals have giant structures
what are the properties of metals
- metals are good conductors of electricity
- metals are also good conductors of heat
- metals tend to be strong
- metals are malleable and ductile
- metals have high melting points
what does the strength of metals depend on
- the charge on the ion
- the greater the charge on the ion, the greater the number of delocalised electrons and the stronger the electrostatic attraction between the positive ions and the electrons - the size of ion
- the smaller the ion, the closer the electrons are to the positive nucleus and the stronger the bond
why are metals a good conductor of electricity
- delocalised electrons that can move throughout the structure explains why metals are good conductors of electricity
- an electron from the negative terminal of the supply joins the electron sea at one end of a metal wire while at the same time a different electron leaves the wire at the positive terminal
why are metals good conductors of heat
- metals are also good conductors of heat
- they have a high thermal conductivities. The sea of electrons is partly responsible for this property, with energy also spread by increasingly vigorous vibrations of the closely packed ions
why are metals strong
- metals tend to be strong
- the delocalised electrons also explain this. These extend throughout the solid so there are no individual bonds to break
why are metals malleable and ductile
- metals are malleable and ductile
metals are malleable ( they can be beaten into shape) and ductile (they can be pulled into thin wires)
After a small distortion, each metal ion is still in exactly the same environment as before so thee new shape is retained - different with the brittleness of ionic compounds
why do metals have a high melting and boiling points
- metals have high melting points
metals generally have high melting and boiling points because they have giant structures.
There is a strong attraction between metal ions and the delocalised sea of electrons.
This makes them difficult to separate
what is the electron pair repulsion
electrons in molecules exists in pairs in volumes of spaces called orbitals
You can predict the shape of a simple covalent molecule
e.g. one consisting of a central atom surrounding by a number of atoms, by using the ideas that:
- each pair of electrons around an atom will repel all other electron pairs
- the pairs of electrons will therefore take up positions as far apart to minimise repulsion
This is called the electron repulsion theory
what are the two types of electron pairs
electron pair may be a shared pair or a lone pair
what does the shape of a sim0ple molecule depend on
the shape of a simple molecule depends on the number of pairs of electrons that surround the central atom
To work out the shape of any molecule you first need to draw a dot and cross diagram to find the number of pairs of electrons
what is the shape of a molecule when there is two pairs of electrons
if there are two pairs of electrons around the atom, the molecule will be linear
The furthest away from each other the two pairs can get is 180° part
e.g. beryllium chloride for example
Cl- Be-Cl
what is the shape of a molecule when there are three pairs of electrons
if there are three pairs of electrons around the central atom, they will be 120° apart.
The molecule is planar (flat) and is called trigonal planar
what is the shape of a molecule when there are four pairs of electrons
If there are four pairs of electrons, they are furthest apart when they are arranged so that they point to the four corners of a tetrahedron ]
This shape, with one atom position at the centre is called tetrahedral
The angles that the bonds are a is 109.5°.
It is a 3-D shape not planar so thee sim of the angles can be more than 360°
This is better than a square planar because bonds are further apart
what is the shape of a molecule when there are five pairs of electrons
If there are five pairs of electrons, the shape usually adopted is that of a trigonal bipyramid
1 angle is 90° and the rest are 120°
what is the shape of a molecule when there are six pairs of electrons
If there are six pairs of electrons, the shape adopted is octahedral, with bond angle at 90°
Remember that an octahedral has eight faces but six points
do the overall charge affect the shape of a molecule
no
what do the wedges and dashes represent
in three - dimensional representaions of molecules a wedge is used to represent a bond coming our of the paper
and a dashed line represents a one going into a paper away form the reader
what is a lone pair
lone pairs is a pair of electrons that are unshared in a molecule
Lone pairs can affect the shape of molecules
what are the rules for working out the shape of a molecule
- central atom
- group (this tells us the number of outer electrons
- charge - if it has lost or gained an electron
- bonds - number of bonds on the molecule
- add up the total of step 2, 3, 4
- divide by to to work out the number of electron pairs
- draw out the structure
- look at steps 6 and 4 to work out the lone pairs ) bonds + lone pairs = total electron pairs)
- the shape ( the shape does not include the lone pairs)
- work out the bond angles
what does the forces that hold the atoms together depend on
the attraction between positive charges and negative charges
e.g.
In ionic bonding there is complete transfer of electrons from one atom to another
in covalent bonds, the electrons shared by atoms will not be evenly spread if one of the atoms is better at attracting electrons than the other
what is electonegativity
electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons in a covalent bond
what does the term electron density mean
when chemists consider the electrons as charge clouds the term electron density is often used to describe the way the negative charge is distributed in a molecule
what are the four most electronegative atoms
nitrogen =3.0
oxygen = 3.5
fluorine = 4.0
chlorine = 3.0
why do the noble gases not electronegative
because, in general, they do not form bonds (covalent bonds)
what does electronegativity depend on
- the nuclear charge
- the distance between the nucleus and the outer shell electrons
- the shielding of the nuclear charge by electrons in inner shells
what are the trends of electronegativity going down a group
going down a group, the electronegativity decreases
This is because there is more shells and shielding of the nucleus by electrons in inner shells
what are the trends of electronegativity going along a period
going across a perios in the Periodic Table,the electronegatibvity increases
This is because the nuclear charge increases while the number of inner main levels remain the same and the atoms become smaller
what is polarity
polarity is the unequal sharing of the electrons between atoms that are bonded together covalently
It is a property of the bond
how is the covalent bond when both of the atoms are the same
when both atoms are the same, e.g. in flourine F2, the electrons in the bond must be shared equally between the atoms
This is because both atoms have exactly the same electronegativity and the bond is completely non - polar
If you think of the electrons as being in a cloud of charge, then the clouds is uniformly spread between the two atoms
how is the covalent bond when there is two atoms that are different in electronegativity
in a covalent bond between two atoms of different electronegativity, the electrons in the bond will not be shared equally between the atoms
e.g. the molecule hydrogen fluoride HF
hydrogen has an electronegativity of 2.1 and fluorine of 4.0
This means that the electrons in the covalent bond will be attracted more by the fluorine than the hydrogen
The electron cloud is therefore distorted towards the fluorine
The fluorine end of the molecule is therefore relatively negative and the hydrogen end relatively positive, that is, electron deficient
covalent bonds like this are said to be polar - the greater the electronegativity difference the more polar the covalent bond
what kind of properties could you say HF has
although HF is covalent, it has some ionic characteristics - it is going some way toward the separation of the atoms into charge ions
It is also possible to have ionic bond with some covalent character
what are intermolecular forces
simple covalent molecules are attracted to one another by weak forces called intermolecular forces
These forces are between molecules
If the intermolecular forces are strong enough then molecules are held closely enough to be liquids or even solids
what are the three intermolecular forces
van der waals forces weakest
dipole dipole forces
hydrogen forces
strongest
when do dipole dipole forces occur
dipole forces act between molecules that have permanent dipoles ( occur when a molecule has a polar bond)
Two molecules which both have dipoles will attract one another
e.g. in the hydrogen chloride molecule; chlorine is more electonegative than hydrogen
so the electrons are pulled towards the chlorine atom rather than hydrogen
The molecule therefore has a dipole and, is written H +δ - CL-
why do molecules with dipoles sometimes flip
whatever their starting positions, the molecules with dipoles will “flip” to give an arrangement where the two molecules attract
what does the dipole dipole forces depend on
it depends on the shape of the molecule
so if the shape is symmetrical, dipoles cancel each other out
what are Van Der Waal forces
all atoms and molecules are made up of positive and negative charges even though they are neutral overall
These charges produce very weak electrostatic attraction between all atoms and molecules
These are called Van Der Waals forces
how do Van Der Waal forces work
imagine a helium atom
It has two positive charges on its nucleus and two negatively charged electrons
The atom as a whole is neutral but at any moment in time the electrons could be anywhere
This means that the distribution of charge is changing at every instant
Any of the arrangements mean that the atom has a dipole at that moment
An instant later, the dipole may be in a different direction
This dipole then affects the electron distribution in nearby atoms so that they are attracted to the original helium atom for that instant
The original atom had induced dipoles in the nearby atoms
- as the election distribution changes in the helium atom, it will induce new dipoles in the atoms around it, which will be attracted to the original one
how long is the dipole last in the atom
any particular dipole will last be instant and temporary
van der waal forces…
act between atoms or molecules at all times
they’re in addition to any intermolecular forces
the dipole caused by the changing position of the electrons cloud so the more electrons there are the larger the instantaneous dipole will be
why do noble gases and hydrocarbons boiling points increase as the as it gets larger/ the atomic numbers get larger
The size of the Van Der Waals forces increases with the number of electrons present
This means that atoms or molecules with larger atomic/ molecular masses produce stronger Van Der Waals forces than atoms or molecules with small atomic molecular masses
what is hydrogen bonding
it is a special type of intermoleculer forces with some characteristics of dipole dipole attraction and some of a covalent bond
what does a hydrogen bond consist of
it consists of a hydrogen atom “sandwiched” between two very electronegative atoms
what are the conditions that must be met in order for a hydrogen bond to occur
a hydrogen atom that is bonded to a very electronegative atom
- This will produce a strong partial positive charge on the hydrogen atom
a very electronegative atom with a lone pair of electrons
These will be attracted to the partially charged hydrogen atom in another molecule and forms the bond (hydrogen)
only atoms that are electronegative enough can form hydrogen bonds are N,O,F
e.g. NH3 form hydrogen bonds with O2
why are hydrogen bonds more stronger than dipole-dipole forces
it is stronger for two reasons:
consider water for example
- in water the hydrogen atoms are highly elctron deficient
This is because the oxygen is very electronegative and attracts the shared electrons towards it
The hydrogen atoms in water are positively charged and very small. These exposed protons have a very strong eclectic field around them because of their small size - the oxygen atom in water have lone pairs of electrons
the lone pair of electrons on the oxygen atom of another water molecule is strongly attracted to the electron-deficient hydrogen atom - this strong intermolecular force is called a hydrogen bond
what are hydrogen bonds represented by
they are represented by: —
why is the shape of a molecule with hydrogen bonds always linear
take H2O with NH3 for example
the pair of electrons in the N-H covalent bond repels those in the hydrogen bond between nitrogen and hydrogen
This linearity is always the case with hydrogen bonds -
what is the effect of hydrogen bonding on the boiling point of the hydrides in different groups
the effect of hydrogen bonding between molecules can be seen if you look at the boiling points of hydrides of elements of group 4,5,6,7, against the period number
The noble gases show a gradual increase in boiling points because the only forces between the storms are Van der Waals forces and these increase with the number of electrons
the boiling points of water H20, hydrogen fluoride HF and ammonia NH3, are all higher than those of the hydrides of the other elements in their group where you would expect to lower as they only have Van der Waals forces operating.
This is because hydrogen bonding is present in each of these compounds making them harder to separate
what is the importance of hydrogen bonding
although hydrogen bonds are only 10% if the strength of covalent bonds, their affect can be significant especially when there are a lot of them
- the very fact that they are weaker than covalent bonds and can break or make under conditions where covalent bonds are unaffected is very significant - e.g. water forming ice
what happens when water is in its liquid state
when water is in its liquid state, the hydrogen bonds break and reform easily as the molecules are moving about
what happens when water freezes
when water freezes, the water molecules are no longer free to move about and the hydrogen bonds hold the molecules in fixed positions
This results in a three - dimensional structure resembling the structure of diamond
what must water molecules do in order to fit into the diamond structure when it freezes
in order to fit into this structure, the molecules are slightly less closely packed than in liquid water
This means that ice is less dense than water and forms on top of ponds rather than at the bottom
This insulated the pond and enables fish to survive through the winter - this feature must have helped life to continue in the relative warmth of the water under the ice during the Ice Ages
simple covalent compounds
Compounds that are made up of lots of individual molecules are called simple covalent compounds
The atoms in the molecules are held together by a strong covalent bond, but the molecules within the simple covalent are geld together by intermolecular forces
-low melting points and electrical insulators
giant covalent structures
sometimes called macromolecules
Carbon atoms can form this type of structure because they can each form four, strong covalent bonds
There are two types of ain’t covalent carbon structure you need to know about:
- graphite
- diamond
graphite
the carbon atoms in graphite are arranged in sheets of flat hexagons covalently bonded with three bonds
The fourth outer electron of each carbon atom is delocalised
The sheets of hexagons are bonded together by weak van der Waals forces
what are the properties of graphite
the weak bonds between the layers in graphite are easily broken so the sheets can slide over each other - graphite feels slippery and can be used as a lubricant and in pencils
the delocalised electrons in graphite are free to move also along the sheets so an electric current can flow
the layers are quite far apart compared to the length of the covalent bonds, so graphite has a low density and is used to make strong lightweight sports equipment
because of the strong covalent bonds in the hexagon sheet, graphite has a very high melting point (it sublimes at over 3900K)
graphite is insoluble in any solvent
the covalent bonds in the sheets are too difficult to break
diamond
diamond is also made up of carbon atoms
each carbon atom is covalently bonded to four other atoms
The atoms arrange themselves in a tetrahedral shape - it is a crystal lattice structure
what are qualities of diamond
because of its strong covalent bonds
diamond has a high melting point - it sublimes at over 3800K
diamond is extremely hard - it is used in diamond tripped drills AND SAWS
VIBRATIONS TRAVEL EASILY THROUGH THE STIFF LATTICE SO IT IS A GOOD THERMAL CONDUCTOR
IT CANT CONDUCT ELECTRICITY 0 ALL THE OUTEWE ELECTRONS ARE HELD IN LOCALUISED BONDS
LIKE GRAPHITE, DIAMOND WON’T DISSOLVE IN ANY SOLVENT
