Nature of matter

Question 1

What is amu in relation to atomic weight?

Answer 1

By common agreement an atom of the most common isotope of carbon has exactly 12 atomic mass units (amu or just u for unified atomic mass unit).
1amu = 1.6606x10-27 kg

Question 2

Do different isotopes of the same element have the same atomic mass? Explain ?

Answer 2

Different isotopes of the same element will obviously have different atomic masses. When a periodic table is consulted the value that is quoted is the average atomic mass which depends on the natural abundance of the isotopes.

Question 3

Explain rutherfords scattering experiment and comment on his findings

Answer 3

Rutherford found that most of the alpha particles went through the thin foil undeflected, and some were scattered by a few degrees
….but some were scattered back at the source
In Rutherford’s own words this was equivalent to firing a 15 inch cannon ball at a piece of tissue paper to find that it bounced back and hit you!
Some alpha particles must therefore be colliding with objects much more massive than themselves
The only possible explanation was that the mass of the atom was not distributed uniformly over its volume, but that the majority of the mass was concentrated in a tiny fraction of the volume

Question 4

Explain rutherfords atom model?

Answer 4

99.99% of the mass of the atom was concentrated in the nucleus
which is only

0.000 000 000 000 001 metres

ie 10-15 m, across
The remaining mass is carried by the electrons which orbit the nucleus at a distance of approximately

0.000 000 000 1 metres (ie 10-10 m)

So a lot of empty space (99%)

Question 5

Give bullet points on rutherfords Atom

Answer 5

• A neutral atom contains Z electrons and Z protons
• -Ze charge, electronic cloud
• +Ze concentrated in the nucleus
• Mass is concentrated in the nucleus
• Nucleus contains protons (and neutrons)

Question 6

What was the problem with rutherfords atom??

Answer 6

Unfortunately, within the framework of classical physics Rutherford’s atom was unstable
Because the electron should emit energy and spiral into the nucleus

Question 7

How did Bohr fix rutherfords atom model?

Answer 7

By quantising (h/2)
E.g
L=mvr=n(h/2)

L=angular momentum
n=integer

Question 8

What is the quantum energy condition equation ?

Answer 8

hf=E= Einitial-Efinal

Question 9

What orbit does the electron have to be into for lowest energy (ground state)?

Answer 9

N=1

Question 10

How does an atom become ionised?

Answer 10

When an electron is given enough energy to escape

Question 11

What equation can be used to calculate the ionising energy?

Answer 11

Ephoton = hf = IE+KE

(
h = 6.63 x 10-34Js)

Question 12

How did sommerfields concept of elliptical orbits lead to another quantum number?

Answer 12

Sommerfeld described orbits which had different elliptical shapes but the same value of of the principal quantum number n
This gave a number of different stationary states, some with slightly smaller and some with slightly larger energies - and hence multiple spectral lines - just as observed
Only certain elliptical orbits were allowed, and these could be represented by another quantum number, k, also quantised in units of h/2
Note -
An alternative notation that is used is the quantum number l, where l is simply related to k by the relation ship l=k-1

Question 13

What letters are given to k=1, k=2, k=3 & k=4??

Answer 13

k = 1        is given the letter s (sharp)
k = 2        is given the letter p (principal)
k = 3        is given the letter d (diffuse)
k = 4        is given the letter f (fine)

Question 14

How did sommerfields fix zeemans observations that if a magnetic field was applied to a hot gas the emission gas were further split into a finer structure?

Answer 14

Sommerfeld was able to account for this orientation effect with the addition of a magnetic quantum number, ml, which had to be an integer

Question 15

What is the meaning of the magnetic quantum number ml?

Answer 15

ml describes how the direction of the angular momentum is quantised with respect to the direction of the magnetic field

Question 16

Explain Wolfgang pauli’s fourth quantum number spin?

Answer 16

quantum number which he took to represent the electron spinning, not just in its orbit, but around its own axis
Because there are only two directions of spin, clockwise or spin up, and anti clockwise or spin down, there are only two values for Pauli’s electron spin quantum number

Question 17

Why is the pauli’s exclusion principle so important?

Answer 17

It is the restrictions imposed by the Exclusion Principle - ie that no two electrons can be in the same quantum state - that stop all the electrons piling up into the lowest (n=1) energy state
The implications of the Pauli Exclusion Principle are extremely profound
….and hence stops all matter from collapsing!
The Pauli Exclusion Principle also implies that there is some sort of connectivity between the electron states in an atom - one electron must “know” which states all the other electrons are occupying in order to choose its own state!

Question 18

What are the four quantum numbers ?

Answer 18

n the principal quantum number
(quantisation of angular momentum)

ml quantisation of the orientation of the orbit
with respect to a magnetic field

k or l related to the ellipticity of the orbit

ms the electron spin quantum number

Question 19

How are the four quantum numbers dependent on each other?

Answer 19

n must be a positive integer (1,2,3,4 …..etc)

m can be a positive or negative integer
(….-2,-1, 0,1,2….etc), but must be no larger than l

l a positive integer or zero (0,1,2,3,4 …..etc), but no bigger than n -1

s must be +1/2 or -1/2

Question 20

What is the equation that de Brogile used to link the relationship between particles and waves ?

Answer 20

Wavelength =h/p

where the momentum p=mv and h is the Planck constant.

Question 21

What’s hunds rule ?

Answer 21

When electrons occupy degenerate orbitals (i.e. same n and l quantum numbers), they must first occupy the empty orbitals before doubly occupying them. Furthermore, the most stable configuration results when the spins are parallel.

Question 22

Describe briefly the structure of the atom

Answer 22

structure of the atom
The atom
•A neutral atom contains Z electrons and Z protons

•Nucleus contains protons and neutrons