Atomic Structure

1
Atomic Structure
2
Simple model of an atom

• An atom is made of a tiny nucleus with electrons
  orbiting around it.
• The nucleus is made up of protons and neutrons.
• Much of an atom is empty space

3
The protons

• Each proton has a 1charge.
• Each proton has a mass of 1 atomic mass unit.
• The number of protons in an atom is called its
  atomic number(z).

4
The neutrons

• The neutrons have no charge.
• Each neutron has a mass of 1 atomic mass unit.
• The total number of protons and neutrons in an
  atom is called its mass number (A).

5
The Electrons

• Each electron has a charge of -1.
• Electrons have negligible mass of 1/1840 that of
  a proton.

6
Convention for writing an atom

• A zX

7
Isotopes

• Isotopes are atoms of the same element which have
  the same number of protons but different number
  of neutrons.

8
Relative Isotopic Mass

• Relative Isotopic Mass Mass of one isotope of
  the element/(1/12) X Mass of one atom of 126C.
• An atom of 12C has a relative atomic mass of 12
  exactly.

9
Relative Atomic Mass(Ar)

• Ar Weighted average of the isotopic
  masses/(1/12) X mass of one 126C atom

10
Calculating the relative atomic mass of chlorine

• Chlorine has two isotopes 35Cl and 37Cl in
  relative proportions of 75 and 25 respectively.
  
• The weighted average mass of a chlorine atom is
  35X(75/100) 37 X (25/100) 26.259.2535.50(no
  unit)

11
Relative Molecular Mass(Mr)

• Mr Mass of one molecule/(1/12)X Mass of one
  126C atom.
• The relative molecular mass can be worked out by
  adding the relative atomic masses of all the
  atoms present in one molecule.

12
Mass spectrometry

• A mass spectrometer separates the isotopes of an
  element according to their masses and shows the
  relative numbers of the different isotopes
  present.
• Before the isotopes can be separated, they must
  be converted to positive ions.

13
The workings

• Evacuation of the instrument
• Vaporisation of liquid or solid samples
• Production of positive ions
• Acceleration of positive ion
• Deflection of positive ions
• Detection of positive ions according to
  mass(m)/charge(e). When charge 1, mass/charge
  mass

14
The uses of a mass spectrometer

• To find the isotopic composition of an element.
• To work out the relative atomic mass of an
  element.
• To find the relative molecular mass and the
  fragmentation pattern of a molecule.
• In forensic science.

15
The mass spectrum of Cl
Abundance

• Chlorine has two isotopes 35Cl and 37Cl with
  relative proportions of 75 and 25 respectively.

75
25
35 37 m/ e
16
First ionisation energy

• It is the energy required to remove one electron
  from each of one mole of gaseous atoms to form
  one mole of gaseous ions with single positive
  charge and one mole of electrons.
• The equation for first ionisation energy of
  element A is A(g) A(g) e

17
Successive ionisation energies

• Successive ionisation energies provide evidence
  for the existence of quantum shells or electronic
  energy levels.

18
Successive ionisation energies

• If an atom has two electrons, it will have two
  ionisation energies, first ionisation energy and
  second ionisation energy.
• If an atom has three electrons, it will have
  three separate ionisation energies.
• All these ionisation energies for each element
  are its successive ionisation energies.

19
The successive ionisation energy graph of Be

• The diagram indicates two electronic energy
  levels.
• Electrons 1 and 2 are at a higher energy level
• Electrons 3 and 4 at a lower energy level
  -nearest to the nucleus.

Log IE/kJ mol-1
x
x
x
x
1 2 3 4 Ionisation no
20
Electron configuration- key points

• Each element has a characteristic emission
  spectrum which can be used to identify it.
• The electrons in an element can exist only at
  certain energy levels - shells and sub-shells
• The region in which an electron moves for most of
  the time is called an orbital.
• An orbital can hold two electrons.

21
The Line Spectrum

• An electron can absorb sufficient energy and move
  to a higher energy level.
• When such an electron drops to a lower energy
  level, the energy absorbed is given out.
• The amount of energy given out appears as a line
  in the line spectrum of the element.

22
First ionisation energies of successive elements
- H toNe

• These provide evidence of shells and subshells.
• The first shell can have one sub-shell, s
  subshell.
• The second shell can have two subshells, s and p

1st IE/kJ mol-1
x
x
x
x
x
x
x
x
x
1 2 3 4 5 6 7 8 9 10
z
23
The aufbau principle

• Electrons always occupy the lowest available
  energy sub-level or subshell.
• Electrons pair up after a sub-level is half
  filled.
• Numbers 1, 2, 3 denote the shells. Letters s, p,
  d, f denote the subshells. A superscript
  indicates the number electrons.
• Sequence of energy levels 1s2s2p3s3p4s3d

24
Subshells and orbitals

• An s sub-shell has only one orbital.
• A p sub-shell has three orbitals.
• A d subshell has five orbitals.

25
Sub-shells and electrons

• An s sub-shell can have a maximum of two
  electrons.
• A p sub-shell can have a maximum of six
  electrons.
• A d sub-shell can have a maximum of ten electrons.

26
Shape of an s orbital

• An s orbital is spherical in shape
• The sphere is made up of a cloud of negative
  charge from the electrons

27
Shape of p-orbitals - dumb-bell shaped

• The three p orbitals, px,, py and pz.

py
Px
pz
px
28
Electron configuration of elements

• H 1s1 He 1s2
• Li 1s2 2s1 Be 1s22s2
• B 1s22s22p1 C 1s22s22p2
• N 1s22s22p3 O 1s22s22p4
• F 1s22s22p5 Ne 1s22s22p6
• Na 1s22s22p63s1 Mg 1s22s22p63s2