chem bonding Flashcards
what is electronegativity?
the ability of an atom to attract electrons in a covalent bond to itself
how does electronegativity change across the period?
electronegativity increases across the period.
across the period, nuclear charge increases + shielding effect remains relatively constant -> effective nuclear charge increases -> electronegativity increases
how does electronegativity change down the group?
electronegativity decreases down the group.
in the same grp, nuclear charge and shielding effect increases, so elements have roughly the same effective nuclear charge. BUT no. of quantum shells increases -> atomic radius increases -> electronegativity decreases
what are the 3 most electronegative atoms
F, O, N
describe metallic bonding.
metals have a giant metallic lattice structure, which is held together by strong electrostatic forces of attraction between metal cations and the sea of delocalised electrons
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what are the factors that affect the strength of metallic bonding?
number of valence electrons contributed per metal atom AND charge and radius of metal cation
how does the number of valence of electrons contributed per metal atom affect strength of metallic bonding?
number of valence electrons contributed per metal atom increases, metallic bonding stronger
larger number of valence electrons contributed per atom -> greater number of delocalised electrons -> stronger metallic bonding
how does charge and radius of metal cation affect the strength of metallic bonding?
increased charge and decreased atomic radius leads to stronger bonding
charge density = ionic charge/ionic radius
higher charge + smaller radius = higher charge density -> stronger metallic bonding
why do metals have high melting and boiling points?
a large amount of energy is required to overcome the strong electrostatic forces of attraction between cations and the sea of delocalised electrons
stronger metallic bonding -> more energy required to overcome metallic bonding -> higher melting & boiling point
why are metals good electrical & thermal conductors?
metals have delocalised electrons that act as mobile charge carriers
why are metals malleable and ductile?
when a large stress is applied to a piece of solid metal, the layers of ions will slide over one another into new positions. the overall shape of the metal changes but doesn’t break apart as the sea of delocalised electrons prevent repulsion
describe ionic bonding
ionic compounds have a giant ionic lattice structure which is a lattice of cations and anions held together by strong electrostatic forces of attraction between oppositely charged ions
what is lattice energy?
lattice energy is the heat evolved when 1 mole of pure ionic solid is formed from its constituent gaseous ions
why do ionic compounds have high melting and boiling points?
a large amount of energy is needed to overcome the strong electrostatic attractions between oppositely charged ions
can ionic compounds conduct electricity?
ionic compounds can conduct electricity in molten or aqueous state but NOT in solid state
in solid state, ions are in fixed positions and are not free to move. in molten/aqueous state, ionic compounds are good electrical conductors as ions can act as mobile charger carriers. the higher the concentration of the compound, the better its conductivity
why are ionic compounds hard?
oppositely charged ions are held in fixed positions by strong ionic bonding. large amounts of energy are required to move the ions out of position is ionic lattices are quite hard
why are ionic compounds brittle?
repulsion between ions of like charges causes the lattice to shatter
what is a covalent bond?
a covalent bond is formed when two atoms share a pair of electrons which are found in the valence orbitals of the atom
what does bond energy (strength) depend on?
bond length and bond order
- shorter bonds (usually formed by smaller atoms) are stronger as the overlap between orbitals is more effective
- multiple bonds are shorter and stronger than single bonds. when more electrons are being shared, the attraction between the 2 positively charged nuclei is stronger
what elements can have more than 8 electrons in their valence electron shell?
elements from period 3 onwards !!!!!
what is VSEPR theory?
valence shell electron pair repulsion theory
what are the 2 main principles of the VSEPR theory?
- electron pairs around they central atom of a molecule arrange themselves as far apart as possible to minimise their mutual repulsion
- the repulsion between 2 lone pairs > lone pair & bond pair > 2 bond pairs
why is the repulsion between 2 lone pairs > lone pair & bond pair > 2 bond pairs
a lone pair is attracted by only 1 nucleus while a bond pair is attracted by two nuclei -> electron cloud of a lone pair is less ‘elongated’ than a bond pair so a lone pair of a central atom takes up more space around the atom -> it is closer to the central atom so it repels other electron pairs around it more than a bond pair would
how does the percentage of s character in hybrid orbitals affect bond strength?
the higher the percentage of s character, the stronger the hybrid orbital is.
since the p orbital is more elongated compared to a spherical s orbital, a hybrid orbital that has more p character tends to form a longer and weaker bond
how does difference in electronegativity affect the type of bond formed?
small difference in electronegativity -> covalent/nonpolar bond
large difference in electronegativity -> ionic/polar bond
what are the 3 types of intermolecular forces of attraction?
instantaneous dipole-induced dipole attractions (dispersion forces), permanent dipole-permanent dipole attractions and hydrogen bonding
where do dispersion forces occur?
between all molecules or between noble gas atoms
what factors affect the strength of dispersion forces?
number of electrons and surface area of contact between adjacent molecules
how do the number of electrons affect the strength of dispersion forces?
larger number of electrons = more polarisable = stronger dispersion forces
how does the surface area of contact between adjacent molecules affect the strength of dispersion forces?
larger surface area of contact = dipoles are more easily induced = stronger dispersion forces
where do permanent dipole-permanent dipole attractions occur?
between polar molecules that do not have hydrogen bonding. the attraction is between the oppositely charged ends of 2 polar molecules
where does hydrogen bonding occur?
between polar molecules that contain a hydrogen atom bonded to F/O/N and an atom (F/O/N) with a lone pair of electrons
how does hydrogen bonding occur?
the hydrogen atom is highly electron deficient so the lone pair on F/O/N is able to attract the electron deficient H atom strongly
what factors affect the number of hydrogen bonds that can be formed?
number of hydrogen atoms and the number of lone pairs available
what factor affects the strength of hydrogen bonding?
the electronegativity of the atom that H is bonded to. the more electronegative the atom is (eg F), the stronger the hydrogen bonding
how does hydrogen bonding affect boiling point?
- more electronegative = larger electronegativity difference = stronger hydrogen bonding = more energy required to overcome = higher boiling point
- molecule can form more hydrogen bonds = more extensive hydrogen bonding = more energy required to overcome = higher boiling point
how does intramolecular hydrogen bonding affect boiling point?
more intramolecular hydrogen bonding = less intermolecular hydrogen bonding = less energy required to overcome = lower boiling point
why do simple molecular substances have low melting and boiling points?
the simple molecular lattices are held together by weak intermolecular forces of attraction between molecules and little thermal energy is required to overcome them
why is diamond unable to conduct electricity?
it has no mobile charger carriers as all valence electrons are held between the atoms are not free to move
why is graphite a good conductor of electricity?
each carbon atom in graphite is covalently bonded to 3 other carbon atoms so there are delocalised electrons that are free to move along the layers
why is graphite soft and slippery?
layers of graphite are held together by weak dispersion forces so they can slide over each other easily
rank solute-solvent interactions in terms of how strong they are
- ion-dipole interaction (strongest)
- hydrogen bonding
- pd-pd interaction (between 2 polar molecules that cannot form hydrogen bonding)
- pd-id interaction (between 1 polar and 1 non-polar molecule)
- dispersion forces (weakest)
what interactions can be formed between water and a solute?
hydrogen bonding, pd-id interactions and ion-dipole interactions
