Chemical bonding lecture 2

Question 1

What is the valance shell electron-pair repulsion theory (VSEPR)

Answer 1

it is a theory that states that valence electrons arrange themselves as far as possible to minimize their repulsion.

Question 2

What is the steric number SN

Answer 2

The steric number of a central atom is defined as:

SN = [Number of atoms bonded to the central atom] + [ Number of lone pairs on a central atom]

SN helps us determine the shape of various compounds

Question 3

why can molecules of the same steric number SN have different structures

Answer 3

1. it is because SN is dependent on both the bonding pair and lone pair, in which the lone pair tends to occupy more space since the bonding pair is held closer to the central atom, therefore there can be different repulsions at the same SN giving us different shapes
2. Electropositive substituents repel more than electronegative substituents so in a compound different substituent atoms might lead to an unpredicted shape

Question 4

What different shapes can form as we start from a fully substituted molecule and slowly remove atoms to form lone pairs? (For SN :(1 to 6))

Answer 4

Notes

Question 5

What is electromagnetic radiation?

Answer 5

it is a beam of light consisting of electrical and magnetic fields perpendicular to each other in the direction in which light is propagating

Question 6

Why was rutherfords planetry model of the atom wrong?

Answer 6

because it goes against the fundamental rules of physics where Maxwell’s electron magnetic theory suggests that when a charge accelerates it emits radiation and loses energy and according to Ruther’s theory the electron is constantly accelerating around the nucleus which will then lead it to lose energy and collide into the nucleus.

Question 7

What are the discoveries that Plank has made regarding energy

Answer 7

1. The energy of a system can only take discrete values, which is represented on its energy level diagram
2. quantized oscillators can only gain or lose energy in discrete amounts represented by E= hf
3. to emit energy from higher energy states , the temp of the quantized system must be sufficiently high to excite those states

Question 8

what is the formula to calculate the intensity of radiation

Answer 8

I = 8piKTv^2 / c^3

I - intensity of radiation
k - Boltzmann constant
T - temp i K
v- frequency
C - speed of light

Question 9

What is an energy-level diagram?

Answer 9

it is a series of horizontal lines where each line represents an allowed energy value of a system

Question 10

What is a spectrograph?

Answer 10

it is a box-like container used to exclude stray raws, where the light to be analyzed is placed through a small slit in the box which then causes the light to pass through a prism splitting into its components which then are displayed on a screen

Question 11

what is the model that Bohr suggested for the absorption of light by atom

Answer 11

his model suggested that the light absorption resulted in the transition of atoms from two discrete energy levels where the frequency of the light absorp is related to the initial energy and final energy by the equation:

Ef = Ei + hv, where hv is the energy absorbed from the light

Note if absorption hv is positive and the change in energy is positive if the emission of light hv is negative and the change in energy is also negative

Question 12

What is Bohr’s model of discrete energy levels?

Answer 12

notesss

Question 13

what is ionization energy? how is it interpreted in the Bohr model of discrete energy? How can you calculate it using energy change

Answer 13

Ionization energy is the minimum energy required to remove an electron from an atom.

In Bohr’s model, the ionization is represented by a change from n=1 to n= infinite meaning Ef = 0 therefore the change in energy is = Ef - Ei = 0 - R = R

From the change in energy, we get the ionization energy via the following equation:
(change in energy) * ( Avogadro’s constant)

Question 14

What is the equation that represents the wavelength of the energy emitted or absorbed by a one-electron atom?

Answer 14

1/wavelentgh = R x Z^2 * [1/nh^2 - 1/ns^2]

nb is the higher quantum level

ns is the smaller quantum level

Question 15

what is the photoelectric effect? What is the equation denoted to it?

Answer 15

it is when a beam of light with sufficient energy (to meet the frequency threshold of a metal) is shone onto that metal causing the release of electrons and generating an electric current

The equation:

Emax = 1/2mv^2 = hv - ϕ

where ϕ is the work function defined as the energy barrier that electrons must overcome in order to escape the surface of the metal after they have absorbed a photon and it is equal to h * threshold frequency

Question 16

what is a standing wave

Answer 16

it is wave oscillates in time but its peak amplitude profile does not move in space.

Since the ends are fixed the oscillations that can persist are those an internal number of half wavelengths that can fit in length L: nλ/2 = L

Question 17

what are the fundamental and higher harmonics? What happens as we get into higher harmonics?

Answer 17

a fundamental harmonic is an oscillation at n = 1 and higher harmonics are oscillations at n > 1 as n increases the number of nodes increases the wavelength decreases and the energy and frequency of a wave increase

Question 18

what is the de Broglie’s wavelength? How did he reach that prediction?

Answer 18

Notes