atom and bonding

Question 1

what is the criteria for a solid?

Answer 1

• can sustain shear stress with out yielding (giving way under pressure)
• building blocks such as atoms or molecules have well defined average positions

Question 2

describe the potential energy of between 2 atoms/molecules for large r

Answer 2

potential energy approaches 0

Question 3

describe the potential energy of between 2 atoms/molecules for small r

Answer 3

• potential energy is large and positive since 2 atoms/molecules cannot be superimposed (Pauli exclusion principle)
• this is repulsion

Question 4

describe the potential energy at Ro (equilibrium bond length)

Answer 4

• for a bond to form between 2 atoms/molecules there is a minimum potential energy
• this potential energy is negative and describes attraction

Question 5

what is the equation for U(r) derived from taylor expansion?

Answer 5

U(r) =U(r0) + 0.5k(r -r0)^2
potential energy of a spring: 0.5k(r -r0)^2
k = d^2 U(r0) / dr^2
so interactions act like a spring for small deformation?

Question 6

what are the four quantum numbers?

Answer 6

• principle quantum number, n
• orbital quantum number, l
• magnetic quantum number, ml
• spin quantum number, ms

Question 7

what is Pauli exclusion principle?

Answer 7

only one electron is allowed in each state characterised by all four quantum numbers

Question 8

what is Hund’s rule?

Answer 8

The electronic configuration in an atomic ground state maximises the number of unpaired electrons

Question 9

what is Aufbau principle?

Answer 9

• fill the orbitals starting at the lower energies
• only 2e- allowed in each orbital (one spin-up and one spin-down)
• remember Hund’s rule

Question 10

what are valence electrons?

Answer 10

• the electrons in the outermost occupied shells of an atom

- The valence electrons are the electrons that take part in bonding and control the properties of a material

Question 11

what is the octet rule?

Answer 11

An outermost shell consisting of a filled s and p subshell is a very stable configuration leading to chemical stability. To fill the s and p subshells requires 8 electrons. The striving for this stable condition is called the ”octet rule”

Question 12

define a ‘filled’ or ‘closed’ electronic shell

Answer 12

if a shell contains the maximum number of allowed electrons it is termed “filled” or “closed”. However, when the outermost shell consists of a filled s and p orbital it is also generally termed “filled” even if more electrons are allowed in higher orbitals.

Question 13

1s2 2s2 2p6 3s2 3p2. Explain what the notation means

Answer 13

• The first number is the principal quantum no. n, shell number
• The letter is the orbital quantum no. l, where l=0 is s and l=1 is p.
• last number describes how many electrons is sitting in this particular sub shell

Question 14

define electronegativity

Answer 14

a measure of the tendency of an element to attract electrons

Question 15

how does electronegativity change across the period?

Answer 15

increases, since the nuclear charge increases across the period so there is a stronger attraction of the bonding electron

Question 16

define electropositivity

Answer 16

a measure of the tendency of an element to donate electrons

Question 17

how does electropositivity change across the period?

Answer 17

decreases from left to right, since the nuclear charge increases across the period

Question 18

how does atomic radius change across the period?

Answer 18

decreases due to electrons being pulled by a stronger nuclear charge, as nuclear charge increases across the period

Question 19

how does atomic radius change down a group?

Answer 19

increases, due to more electron shells

Question 20

how does electronegativity change down the period?

Answer 20

decreases as the bonding electron is further from the nucleus

Question 21

what does the attraction between a bonding pair of electrons depend on?

Answer 21

• atomic number
• electron nucleus distance (atomic radius)
• electron shielding from inner electron shells

Question 22

define ionization energy

Answer 22

the energy required to remove outermost electrons from an element in its ground state

Question 23

describe covalent bonding

Answer 23

definition: sharing of electrons between atoms resulting in a more stable electron configuration
- bonds are determined by valence electrons
- bonding between elements of similar electron configuration
- directional bonding
- bond energy varies from very strong to weak

Question 24

describe ionic bonding

Answer 24

definition: electrostatic attraction between oppositely charged ions that pack together in a lattice structure
- bonding is between electrons with different electronegatvities due to coulomb attraction between positive and negative ions
- non directional bonding
- large bond energies

Question 25

describe metallic bonding

Answer 25

definition: ion core in a ‘sea’ of free electrons
- valence electrons not associated with a particular atom
- free electrons mean material is a good conductor of heat and electricity
- non directional bonding – free electrons shield the ion cores from electrostatic forces
- bond strength varies – high melting point normally means strong bonds

Question 26

describe van der waals bonding

Answer 26

• dipoles form in atoms/molecules when there is a separation of +ve and -ve charges
• dipoles attract oppositely charged dipoles from other atoms/molecules
• non directional bonding
• weak bonds

Question 27

van der waals interactions occur between…

Answer 27

• induced dipoles
• induced dipole and polar molecule
• polar molecules

Question 28

describe the effect increasing shells has on van der waals forces

Answer 28

• increasing the atomic radius means larger atoms with more electrons.
• larger dipole moments are more easily formed so stronger van der waals forces
• result: boiling point higher

Question 29

describe hydrogen bonding

Answer 29

• hydrogen atom covalently bonds to a strongly electronegative atom (like N, O, F)
• hydrogen is attracted to the lone pair of an electronegative atom forming a hydrogen bond
• H bonds are weaker than covalent and ionic bonds
• H bonds are stronger than van der waals