Basic overview about structures of atoms and electron configuratoin which determines the chemical properties of elements

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 -- “ –