Atomic and Molecular Properties

Question 1

closeness of atoms and their energy

Answer 1

attractive at a distance with zero potential energy, close are balanced with some potential energy, very close is repulsion with more potential energy

Question 2

what are bond energies? what do they depend on?

Answer 2

the amount of energy that must be supplied to break a chemical bond
- vary based on atomic properties

Question 3

bond energies as number of shared electrons increases

Answer 3

bond energy increases as the bond is strengthened

Question 4

bond energy as electronegativity difference (delta X 0-1 scale) increases

Answer 4

increases as polar bonds gain stability from the electrostatic attraction between partial charges

Question 5

what happens to bond energy as bonds become longer?

Answer 5

bond energy decreases (single bonds are longer than triple bonds). As atoms become larger, electron density is spread out, affecting attraction to nuclei

Question 6

properties of covalent molecules (electrically neutral)

Answer 6

• lower melting and boiling points
• many are liquids or gases at room temp
• softer in solid state
• generally insoluble in water
• poor conductors

Question 7

endothermic bonds

Answer 7

energy is added to break bonds

Question 8

exothermic bonds

Answer 8

forming bonds releases energy

Question 9

pure covalent bonds

Answer 9

diatomic molecules such as H2

the electrons have equal probability of being near either atom

Question 10

polar covalent bonds

Answer 10

• no diatomic
• electrons may be shared unequally
• partial positive and partial negative charge

Question 11

electronegativity, patterns

Answer 11

tendency for an atom to attract electrons

• the greater the difference between atoms, the more polar the bond
• increases bottom to top and left to right
• arbitrary, relative, scale 0-4

Question 12

electron affinity

Answer 12

• measurable physical quality
• energy released or absorbed
  when an atom acquires an electron

Question 13

less than 0.4 electronegativity diff

Answer 13

pure covalent

Question 14

0.4-1.8 electronegativity diff

Answer 14

polar covalent

Question 15

greater than 1.8 electronegativity diff

Answer 15

ionic

Question 16

homonuclear

Answer 16

same element, covalently bonded such as F2

Question 17

heteronuclear

Answer 17

more than one element type covalently bonded such as CH4

Question 18

resonance structures

Answer 18

sometimes atoms break the octet rule or there are more than one way that electrons move around a central atom. theoretically might have two double bonds and two single bonds but might be that electrons are spread out and there are 4 bonds of order 1.5

Question 19

dipole moment

Answer 19

assymentrical distribution of electrons. one end is slightly negative and the other is slightly positive.

• symbolised by mu, u
• greater the difference in delta X the larger the mu

Question 20

how are dipole moments measured?

Answer 20

• electrical field is applied across two plates (- and +)

- molecules alight with positive end towards negative plate

Question 21

how is dipole moment calculated?

Answer 21

mu = q x e x r
q x e is the atomic charge 
e is electron charge (1.602e-19 C)
C is the coulomb constant 
Cm (unit of mu) is coulomb meters 
D is Debye (1D = 3.336e-30 Cm)

Question 22

what is a polar molecule?

Answer 22

where all the dipole moments from each bond produce an unequal sharing across the molecule

Question 23

how to indicate dipole moments?

Answer 23

• add multiple arrows from the central atom towards the more positive atom
• arrows may cancel and the molecule is nonpolar
• simplify arrows by pointing towards the average

Question 24

lewis structures with free radicals

Answer 24

put the electron on the most electronegative atom

- if possible, move electrons around so that this atom is close to getting a full shell

Question 25

electron deficient molecules

Answer 25

central atoms do not have a full valence shell

• central atom is usually from group two or 13
• outer atoms are hydrogen or atoms that don’t form multiple bonds

Question 26

hypervalent molecules

Answer 26

• elements from period 2 can only have 8 electrons in their outer shell
• elements in period 3+ can share electrons because of empty d orbits

Question 27

cohesion

Answer 27

water molecules stick together because of hydrogen bonding

Question 28

adhesion

Answer 28

water molecules stick to other things

Question 29

surface tension

Answer 29

hydrogen bonds form a film on top of water on which some bugs can walk

Question 30

water freezing

Answer 30

• hydrogen bonds space out water molecules, making ice less dense so it can float
• aquatic life can survive underneath