Electronegativity & Intermolecular forces Flashcards
What is electronegativity?
the ability of an atom to attract a pair of electrons in a covalent bond
What is the most electronegative atom?
Fluorine
What is electronegativity dependent on?
- atomic radius of atom (the smaller the greater)
- number of shells the atom has (the fewer the greater)
- more shells = bigger radius = lower EN -
How does electronegativity increase on the periodic table?
- increases as you move up and to the right
What is meant by a polar bond?
a covalent bond in which there is an unequal share of electrons due to differing electronegativities of the atoms involved
How to tell if a molecule is non-polar (symmetrical)
- central atom bonded to same thing
- no lone pairs affecting bond angle
How can some molecules have polar bonds but be non-polar molecules?
They are symmetrical and dipoles cancel out
What are intermolecular forces?
Forces BETWEEN molecules (only exist in solid simple molecular substances)
Are IMFs weaker or stronger than covalent bonds?
weaker
What is the order of strength for the 3 types of IMFs?
IDD<PDD<Hydrogen
If molecule(s) are non-polar, what will be the only IMF they have between them?
Induced dipole-dipole (exists in all molecules)
If molecule(s) are polar, what IMF will occur in addition to IDDs?
- permanent dipole-dipole
- if NOF bonded to H, hydrogen bonds
How do induced dipole-dipole forces arise?
- electrons constantly moving means charge distribution can achange at any time
- one side of a molecule could suddenly become more negative (if electrons moved closer to that side)
- induces a temporary dipole due to uneven distribution of electron density
How does an induced dipole in a molecule affect neighbouring atoms and their charge spread?
- a temporary dipole in one molecule induces a dipole in another molecule
- there is an attraction between a + on one molecule and a - on a neighbouring molecule
What affects the strength of induced dipole-dipole forces?
- size/Mr of molecule (bigger = more forces)
- surface area contact (more SA contact = more forces)
- only compare SA if molecules same size / isomers -
How does strength of induced dipole-dipole forces influence melting/boiling points?
the larger the molecule, the stronger the IMFS, the higher the melting/boiling points
When and how do permanent dipole-dipole forces occur?
- occurs when molecules are polar (difference in electronegativity between atoms)
- attraction between + on one molecule and - on second molecule
When do hydrogen bonds occur?
- in POLAR molecules when hydrogen is bonded to N, O, or F
- lone pair(s) present
Why does hydrogen bonding occur?
- NOF are most electronegative atoms
- big difference in electronegativity between these atoms and hydrogen so pulls apir of electrons in bond very strongly towards themselves
- leaves hydrogen nucleus very +
- lone pair of electrons on neighbouring molecule (containg NOF) attracted to H
Is hydrogen bonding a type of bond?
No, it is an attractive force between 2 molecules
What are the features of a hydrogen bond diagram?
- partial charges on all atoms
- all lone pairs clearly shown
- hydrogen bond shown clearly (drawn linear) between lone pair (NOF) and + H on other molecule
Why is ice less dense than water (despite being solid)?
- there are fewer particles of water per unit of volume in solid state than liquid state
- water molecules further apart in ice due to more gaps/spaces
- in liquid, hydrogen bonds broken so less gaps between molecules
Why do simple molecular structures have low boiling/melting points?
- there are weak IMFs between molecules which require very little energy to break
- actual value determined by strength of IMFs
Why can simple molecular structures not conduct electricity in solid/molten/aqueous state?
- no mobile charged particles
Are non-polar molecules generally soluble?
No because they need to form hydrogen bonds to become soluble
- polar dissolves in polar -
