topic 3: bonding Flashcards
ionic bonding
-ionic bonds are oppositely charged ions in a lattice, held together by electrostatic for of attraction, occurs between metals and non metals, electrons transfer from metal to non metal
-metals form positive ions and non metals form negative ions
covalent bonds
-when 2 non metal atoms share pairs of electrons and held by electrostatic forces between shared pair of electrons and nucleus
coordinate/ dative covalent bond
-one atom provides both the electrons in a covalent bond
-atom donating electrons must have a lone pair
-the charge of the receiving atom remains
what determines the shape of molecules (2 types of electron pairs)
2 TYPES OF ELECTRON PAIRS
-BONDING PAIRS: 2 shared electrons in a covalent bond
-LONE PAIRS: 2 electrons not involved in bonding on one atom
-REPULSION STRENGTH: lone pairs repel more than bonding pairs as they are closer to the nucleus of the central atom than the bonding pairs and reduce the bond angle to a small extent and therefore compact
biggest angle, second biggest angle and smallest angle
-lone pair/ lone pair - biggest angle
-lone pair/ bonding pair - second biggest angle
-bonding pair/ bonding pair - smallest angle
what is the the shape of a molecule a consequence of?
the shape of any molecule or ion is a consequence of the number of electron pairs which repel eachother as far as possible
work out the shape of a molecule
- work out central atom - atoms bonded to it
- work out number of outer electrons in central atom
- add one to this number for every atom bonded to central (add for - and substract for +)
- divide by 2 to find the number of electron pairs on the central atom
- the number of atoms bonded to central atom is the bonding pairs and the rest are lone pairs
electronegativity definition
the power of an atom to attract the pair of electron in a covalent bond
trends in electronegativity
-increases across a period
-decreases down a group
electronegativity increases across a period
-the atoms have the same shells and similair shielder
-the atoms have more protons so they have a stronger force of attraction to the pair of electrons in the covalent bond
-fluorine is the most electronegative elements
electronegativity decreases down a group
-the atoms have more shells and more shielding
-so there is a weaker force of attraction to the pair of electrons in the covalent bond
electronegativity in noble gases
noble gases do not have electronegativity values because they have a full outer shell, so they do not normally form covalent bonds
bond polarity
the unequal distribution of electrons between atoms in a covalent bond, therefore there is a difference in electronegativity
dipole
difference in charge between two atoms caused by an electron shift
permanent dipole
in a polar bond the difference in electronegativity between the two atoms
when is a covalent bond polar?
-when two different atoms are covalently bonded, so there are different electronegativityes so unequal sharing of electrons, this s a dipole moment
-delta (d+) and delta (d-)
-the greater the difference in electronegativity between the atoms, the more polar the bond
examle: H (d+) —– Cl (d-) chlorine is much more electronegative than hydrogen, so hydrogen chloride has a permanent dipole (different electronegatives)
when is a covalent bond non polar + example
-when two of the same atoms are covalently bonded, so they have the same electronegativity as electrons are both shared equally as there is an equal attraction to both atoms
example: hydrogen and carbon (hydrocarbons) have similair electronegativites
all hydrocarbons are non polar
Cl-Cl and C-H
how can a molecule be polar?
-has polar bonds, which do not cancel out since it has an uneven distribution of charge across the molecule - asymmetrical shape, so it has a permanent dipole
-asymmetrical with lone pairs
what bonds are present in polar molecules?
VAN DER WALLS, DIPOLE-DIPOLE or HYDROGEN BONDS
examples: H2o the end of the molecule is negatively charged and other end is positively charged
this is an asymmetrical shape so there is an uneven distribution of charge which do not cancel out
how can a molecule be non polar?
the polar bonds are arranged symmetrically in the molecule, then the (dipole) charges cancel out and there is no permanent dipole
what bonds are present in a non polar molecule + example
VAN DER WALLS
examples: CO2
o-c-o the positively and negative charges are spread evenly (symmetrical shape) across the molecule so the charges cancel out and there is no permanent dipole
intermolecular forces
weak forces of attraction between molecules
3 types of intermolecular forces - order from the weakest to strongest (increasing strength(
- van der walls (induced dipole-dipole forces) - weakest
- permanent dipole-dipole forces
- hydrogen bonding - strongest
van der waal forces
-van der waals are caused by the constant movement of electrons, this causes an uneven distribution of electrons and charge within the molecules
-this induces ( gives rise to) a temporary dipole in the neighbouring molecule
-this causes a weak force of attraction between delta + and - charged molecules
—> induced dipole-dipole
-van der waals are found between all molecular substances and only forces in non-polar bonds
-van der waals forces can hold molecules in a lattice
stronger van der waals forces have higher boiling piints
bigger molecules have more electrons so there are stronger van der waals (larger induced dipoles) therefore the boiling point increases
permanent dipole-dipole attraction + example
-found ONLY IN POLAR MOLECULES and substances with PERMANENT DIPOLES (in polar bonds),
-the d+ is attracted to the d- on the next molecule, there are WEAK ELECTROSTATIC FORCES OF ATTRACTION between the charges
-molecules with permanent dipole dipole attraction have a HIGHER BOILING POINT than induced dipole-dipole/ van der waals
example: hydrogen chloride gas has polar molecules:
H(d+) - Cl(d-)—-H(d+) - Cl(d-)—-H(d+) - Cl(d-)
the d- chlorine is attracted to the d= hydrogen on the next neighbouring molecule (drawn with a dotted line)
hydrogen bonding + example
-the STRONGEST intermolecular force
-occurs when hydrogen is covalently bonded to only fluorine, nitrogen or oxygen
-fluorine, oxygen and nitrogen are VERY ELECTRONEGATIVE so attract the electrons, they draw the electrons away from the hydrogen atom - the bond is so POLARISED
-a hydrogen bond is formed between a H(d+) atom and a lone pair of electrons on the fluorine, oxygen or nitrogen on the next neighbouring molecule, this forms WEAK BONDS BETWEEN THE MOLECULES
example: a lone pair of electron on the oxygen is attracted to the d+ hydrogen
boiling points of hydrogen bonding
boiling points of compounds like H2O, HF and NH3 are the highest as they have hydrogen bonding
-hydrogen bonds have HIGH BOILING POINTS as they have stronger bonds between molecules which require more energy to overcome these bonds
importance of hydrogen bonding in ice
-as a liquid, water forms hydrogen bonds which break and reform easily as the molecules are moving about
-when frozen the molecules cant move, so the hydrogen bonds hold the molecules in a fixed position
-in order to fit, the molecules are slightly less closely packed together, this mean the ice is less dense than and floats on top of it, this is why there is life (fish surviving in ponds during winter) beneath an icy surface which acts as an insulator
