This document discusses atomic structure, emphasizing the composition and properties of atoms, including mass defect and binding energy in carbon-12. It explains quantum mechanics related to electrons, including their wave-particle duality, energy levels, orbitals, and electron shielding effects on chemical properties. Additionally, it covers the significance of isotopes, the emission spectra of elements, and the principles governing electron configurations in multi-electron atoms.

1. ATOMIC STRUCTURE
Lesson by Dr.Chris
UP, May 2014

2. WHAT WE WILL LEARN … PART 1:

3. STRUCTURE OF ATOMS
 The space is made up of electrons
 The mass is in the core

4. ATOMIC SCALES
1 mm

5. ATOMIC MASS
 1 atomic mass unit “amu”
= 1/12 of the mass of 12C atom
The carbon-12 atom has a mass of 12.000 u, and yet it contains
12 objects (6 protons and 6 neutrons) that each have a mass
greater than 1.000 u, not to mention a small contribution from
the 6 electrons.

6. MASS DEFECT
 Mass of nucleus < sum of p + n + e !
Why ?
 Some of the mass is converted to be the BINDING
ENERGY of the nucleus that holds it together ->
“mass defect”
 For 12C:
Mass defect = Dm = 6 * 1.008664 u + 6 * 1.007276 u +
6 * 0.00054858 u - 12.000 u = 0.098931 u
 The binding energy in the carbon-12 atom is
therefore 0.098931 u * 931.5 MeV/u = 92.15 MeV
(binding energies for electrons is just some eV !)

7. PROTONS AND NEUTRONS ARE NOT
FUNDAMENTAL PARTICLES

8. ATOM SYMBOLS
Ne
20
10
Atomic number Z
= no. of protons
= no. of electrons
Mass number A
= no. of protons
+ neutrons

9. EXAMPLES
 How many protons, electrons and neutrons
are in:

10. SOLUTION
 Cl can have 18 or 20 neutrons
 35.45 is a mix of 2/3 35Cl and 1/3 37Cl
Element number
= no. of protons
= no. of electrons
Mass number, not integer !
=> mix of ISOTOPES with
different no. of neutrons !
Z
A

11. ISOTOPES
 Nearly all elements have isotopes,
that means the same elements
(no. of protons = Z) has different no. of
neutrons, and therefore different mass
 Example:
Copper exists to 69.2% of 63Cu and the rest
of 65Cu with masses 62.93 and 64.93
 what is the atomic mass of the mixture ?

12. ISOTOPES AND MS
 Isotope patterns are important to identify
fragments in MS
 For example:

13. ***** BREAK *****

14. WHERE ARE THE ELECTRONS ?
 3 kinds of spectra:

15. WHERE DO THE LINES COME FROM ?
 Bohr (1913)
emission spectra of hydrogen gas
 Lines correspond to energies that are
emitted by electrons:
emitted

17. ELECTRONS ARE “FIXED” ON ORBITS !
 Electrons can move between distinct
energy levels, they cannot exist just
anywhere in the atom = quantum

18. THE HYDROGEN EMISSION SPECTRUM
 Electrical charged hydrogen gas will emit
a blue color and also UV radiation

19. QUESTION

21. How many lines in the emission spectrum
and at which energies (in cm-1) ?

22. Solution: 3 levels  3 lines
Transition A:
∆E = E3 – E2 =
-20’000 + 50’000 cm-1 =
30’000 cm-1 =
λ = 1/30’000cm-1 * 107 nm/1 cm = 333 nm
We can express energy as wavenumber,
because h and c are constant:
= const * 1/λ = const * ν

23. RYDBERG EQUATION
 From which energy level does an electron
come to n=2 when visible light of 410 nm
is emitted ?
 What is the Ionization energy of hydrogen
from this formula ?
-1

24. ELECTRONS AS WAVES
 Electrons can be regarded to be waves
instead of particles.
 DeBroglie:

25. EXPLAINS WHY ELECTRONS CAN ONLY
EXIST ON CERTAIN ORBITS

26. ENERGY OF “WAVE-ELECTRONS”
 We can model the behavior of an electron
in a restricted area
(“particle in a box”)
 Quantization comes from
the fact that a wave has
to “fit” into the boundaries

27.  Only wavelengths are allowed:
 Use in DeBroglie:
 Therefore the kinetic energy is:
 n is the main quantum number
indicating the energy level

28. 3 QUANTUM NUMBERS
 Because a particle is moving in 3D, we
also get 3 quantum numbers
 n: main quantum number (start with 1)
 l : angular “ ( 0,1 .. n-1)
 m: magnetic “ ( -l … 0 … +l )
 Electrons can live only in these “orbitals”
(spaces) defined by 3 quantum numbers
 Up to 2 electrons can exist in one orbital

29. WATCH A DEMO VIDEO
http://www.youtube.com/watch?v=Fw6dI7cguCg

30.  Main spectral lines = n
 Fine structure = l
 With magnetic field:
Zeeman effect
magnetic quantum no. m

31. QUESTIONS
 How many orbitals are possible for the
energy level n = 2
and how many electrons can live there
maxium ?
 n = 2
 l = 0 and 1 (“s” and “p” level)
 m = 0 and -1, 0, +1 (px, y and z)

33. ***** BREAK *****

34. Part 2:

35. ELECTRONIC SHIELDING
AUFBAU PRINCIPLE (PERIODIC TABLE)

36. ELECTRON SHIELDING
 The nuclear charge experienced by an
electron is reduced by shielding by other
electrons.
 Trends in effective nuclear charge can be
used to rationalize the trends in many
properties.
 As a result of the combined effects of
penetration and shielding, the order of
energy levels in a shell of a many electron
atom is s < p < d < f
(Atkin p.16)

37. ELECTRON SHEILDING
Different from
Hydrogen, there are
electron-electron
interactions in the
other elements
Each electron experiences an effective
nuclear charge: Zeff = Z - σ

38. EXAMPLE LI (1S2 2S1)
The PAULI-EXCLUSION PRINCIPLE says
that no 2 electrons can have the same 4
quantum numbers (n l m and s)
=> the 1s orbital can not have 3 electrons,
but max. 2 (n=1, l=0, m=0, s= +/- ½)
=> The 3rd electron goes to the next energy
level n=2, but now, into s or p level ?

39. LI ELECTRON CONFIGURATION
Because of the form
of the function of 2s
it has electron
density closer to the
nucleus than 2p
 less shielding by
1s electrons
 lower energy

40. Z EFF FOR LI ELECTRONS
The 2 1s electrons influence
each other and shield the
pos. charge
The 2s electron experiences
the shielding by the 2
“inner” electrons very
strongly, but not as full 2
neg. charges

41. CONSEQUENCE FOR ALL ELMENTS
As a result of penetration and shielding,
the order of energies in many-electron
atoms is typically
ns < np < nd < nf
because, in a given shell,
s orbitals are the most penetrating
and f orbitals are the least penetrating.

42. The order of energy
changes at Ca – Sc !

43. SLATER’S RULES
ZEFF = Z – S ESTIMATION OF S:

44. EXAMPLE: K – WHERE IS THE 19TH ELECTRON ?

45. HOMEWORK (PRESENT NEXT LESSON)
Calculate the shielding for the valence electron(s) of:
Ca compare 4s2 <-> 3 d2
Sc compare 3d1 <-> 4 p1
Cu (1) compare 4s1 <-> 4 p1
Cu (2) compare 3d10 4s1 <-> 3d9 4s2
Mn compare 3d5 4s2 <-> 3d7
Co (1) compare 3d7 4s2 <-> 3 d9
Co (2) compare 3d7 4s2 <-> 3d8 4s1
Cr (1) compare 3d5 4s1 <-> 3d4 4s2
Cr (2) compare 3d5 4s1 <-> 3 d6
Questions: explain
1. How shielding determines the AUFBAU
principle
2. trend of atomic radius in PT (left to right)
3. -”- ionization energies -- “ --
4. - “ - electronegativities -- “ –