Atomic Structure
Lesson 1
The Nuclear Atom
Main
Dalton’s Theory
▶ In 1808 John Dalton developed an atomic model that
was supported by experimental data
▶ This model formed the origin of atomic theory
▶ This model was refined and replaced over time
▶ Four postulates remain true today:
▶ 1. All matter (materials) consist of very small particles called atoms
▶ 2. An element consists of atoms of one type only
▶ 3. Compounds consist of atom of more than one element and are
formed by combining atoms in whole-number ratios
▶ 4. In a chemical reaction atoms are not created or destroyed
Main
Thomson’s Model
▶ In 1906 Thomson won a Nobel Prize in Physics for the discovery of the
electron
▶ Thomson proposed the “plum-pudding” model of the atom-the atom
was similar to a plum pudding (a dessert eaten on Christmas day in the
UK and Ireland) with negatively charged particles (the raisins)
embedded in a positive region (the pudding) of the atom
Main
Rutherford’s Experiment
▶ In 1909 Rutherford and his co-workers conducted the gold foil
experiment
▶ A thin gold metal foil was placed in an evacuated chamber and
bombarded with alpha particles
▶ Alpha particles are high-energy positively charged He2+ ions
▶ Most of the particles went through the gold foil and some were
deflected slightly and some greatly
▶ This led to the discovery of the
nucleus
Main
Subatomic Particles
▶ Atoms contain protons
(+) and neutrons that are
sometimes referred to
as nucleons
▶ Electrons (-) occupy
space outside of the
nucleus
Subatomic Particles
Particle Relative Mass Relative
Charge
Proton 1 amu +1
Neutron 1 amu 0
Electron 1/1836 amu -1
1 amu = 1.660539 x 10-24g
Main
Discovery of the Neutron
▶ The neutron was discovered by James Chadwick in 1932
▶ His discovery was based on the experiment in which Beryllium was placed in a
vacuum chamber and bombarded with He2+ ions
▶ Beryllium was found to emit neutrons and he was able to prove that the
particles were indeed neutrons
▶ Solved the last piece of the puzzle on atomic
structure
Main
Nuclear Symbol Notation
▶ The atomic number, Z, is the
number of protons in the
nucleus of the atom of an
element
▶ For neutral atoms, the
number of electrons equals
the number of protons
▶ The mass number, A, is the
number of protons +
neutrons in the nucleus
Radioisotopes
▶ Isotopes differ in the number of neutrons resulting in
different mass numbers
▶ Radioactive isotopes are used in nuclear medicine for:
▶ Diagnostics
▶ Treatment
▶ Research
▶ Tracers
▶ Geological clocks
Main
Isotopes
▶ Different atoms of the same element with different mass
number (different numbers of neutrons in the nucleus)
▶ Consider 35Cl and 37Cl, most naturally occurring samples of
elements are composed of mixtures of isotopes but usually
one isotope is far more abundant that the others and the
mass number of the most common isotope is quoted
▶ Isotopes have the same chemical properties (they react in
exactly the same way) but different physical properties, i.e.
melting points and boiling points
Main
PET Scans and SPECT Imaging
▶ Positron Emission Tomography (PET) scanners give 3-D images of tracers
concentration in the body and can be used to detect cancers
▶ Single-Photon Emission Computed Tomography (SPECT) imaging can be used
to detect the gamma rays emitted in iodine-131, used in the treatment of
thyroid cancer and to determine which thyroid gland is functioning normally
Main
Mass Spectrometer
▶ An instrument used to determine the relative atomic mass of an element
Main
Operation of the Mass Spectrometer
1. Vaporization: The sample is injected into the instrument where
it is heated and vaporized, producing gaseous atoms or
molecules
2. Ionization: The gaseous atoms are bombarded by high-energy
electrons, generating positively charged species:
X(g) + e- → X+(g) + 2e-
3. Acceleration: The positive ions are attracted to negatively
charged plates and accelerated in the electric field
Main
Operation of the Mass Spectrometer
4. Deflection: The positive ions are deflected by a magnetic
field perpendicular to their path. The degree of deflection
depends on the mass-to-charge ratio (m/z). The species with
the smallest mass and highest charge will be deflected the
most. Particles with no charge are not deflected in the
magnetic field.
5. Detection: The detector detects species of a particular m/z
ratio. The ion hits the counter and electrical signal is generated.
Main
Mass Spec Graph
B-80.1
B-19.9
Each peak represents an isotope
Main
Lesson 2
Electron Configuration Part A
Main
The Electromagnetic Spectrum
▶ The electromagnetic spectrum (EMS) is a spectrum of wavelengths that
comprise the various types of electromagnetic radiation
Main
The Electromagnetic Spectrum
▶ Energy is inversely proportional to the wavelength
E 1/ƛ
▶ Wavelength is directly related to frequency:
▶ c=ʋƛ
▶ c=3.00 x 108 ms-1
▶ SI units:
▶ Energy: joules, J
▶ Wavelength: meter, m
▶ Frequency: hertz, Hz or s-1
Main
Emission Spectra
▶ Emission spectra are produced when photons are emitted from atoms as
excited electrons return to a lower energy level.
▶ In the absorption spectrum, gaseous atoms absorb certain wavelengths of light
from the continuous spectrum
Main
Hydrogen Atom
▶ If a pure gaseous element is subjected to a high voltage under reduced
pressure, the gas will emit a certain characteristic color of light
▶ The line emission spectrum of hydrogen provides evidence for the existence of
electrons in discrete energy levels, which converge at higher energies
Main
Quantization of Energy
▶ The precise lines in the line emission of an element
have specific wavelengths
▶ Each characteristic wavelength corresponds to a
discrete amount of energy
▶ Quantization is the idea that electromagnetic radiation
comes in “parcels” or quanta
Main
Quantization of Energy
▶ A photon is a quantum of radiation, and the
wavelength and energy of a photon are related
▶ h: Plank’s constant = 6.63 x 10-34J s
▶ ʋ: frequency of radiation
▶ c: speed of light = 3.00 x 108 ms-1
Main
Explanation of Spectra
▶ In 1913, Neils Bohr proposed a theoretical explanation
for the emission spectrum of the hydrogen atom
1. The hydrogen atom consists of protons at the center, while
electrons orbit in a circular path. The attraction between the two
is balanced by the acceleration of the electron moving in its orbit
2. Each orbit has a definite energy associated with it. The energy in
a particular orbit is fixed or quantized
E = -Rn(1/n2)
R: Rydberg constant = 2.18 x 10-18J
n: principle quantum number’ positive integers
Main
Electron States
▶ When an electron in its ground-state is excited, it moves to a higher
energy level and stays in the excited-state for a fraction of a second
▶ When the electron falls back down from the excited-state to a lower
energy level it emits a photon, a discrete amount of energy, which
corresponds to a particular wavelength
Main
Electron States
▶ An electron can be excited to any energy level higher
than its current level
▶ The electron can also fall back down to any lower
energy level
ΔE
Main
Hydrogen Atom
▶ The Balmer series
comprises lines
associated with
electronic transitions
from upper energy
levels back down to the
n=2 energy level
Line Series
Series nf ni Region of EMS
Lyman 1 2,3,4,5... UV
Balmer 2 3,4,5,6... Visible and UV
Paschen 3 4,5,6,7,... IR
Quantization and Atomic Structure
▶ The line emission spectrum of hydrogen provides evidence for the
existence of electrons in discrete energy levels, which get closer
together (converge) at higher energy levels
▶ At the limit of this convergence the lines merge, forming a continuum
▶ Beyond this continuum electrons can have any energy; it is no longer
under the influence of the nucleus and is outside of the atom; called a
free electron
Main
Lesson 3
Electron Configuration Part B
Main
Electron Arrangement
▶ Electron arrangements are useful for explaining and
predicting the chemical properties of an element
▶ Heisenberg’s uncertainty principle allows us to
calculate the probability of finding an electron in a
given region of space within the atom
▶ Schrӧdinger’s equation describe possible energy states
the electron can occupy
▶ An atomic orbital is a region in space where there is a
high probability of finding an electron
Main
Atomic Orbital
▶ Any orbital can hold up to two electrons
▶ Each atomic orbital, s, p, d, f, have characteristic
shapes and associated energies
Main
Atomic Orbitals
▶ S-orbital: spherically symmetrical
▶ P-orbital: dumbbell shaped
Main
Energy Levels
▶ Main energy levels, principle quantum numbers, n, are
positive integer values such as 1, 2, 3, 4…
▶ Energy levels increase as n increases
▶ Each main energy level or shell can hold a maximum
number of electrons by 2n2
Main
Sublevels
Sublevel Number of orbitals in Maximum Number of
sublevel Electrons in Sublevel
s 1 2
p 3 6
d 5 10
f 7 14
Orbital Diagrams
▶ Arrows represent electrons and boxes represent orbitals
▶ Two electrons in the same orbital must have opposite spins
▶ Electrons behave as magnets facing in opposite directions
Main
Quantum Numbers
*Up arrow = +½, down arrow = -½
Main
Electron Configurations
1. Aufbau Principle: electrons fill the lowest-energy orbital that is available
2. Pauli Exclusion Principle: any orbital can hold a maximum of two electrons with
opposite spins
3. Hund’s Rule: when filling degenerate (of equal energy) orbitals, electrons fill all
the orbitals singly before occupying them in pairs
Main
Electron Configurations
▶ Full electron configuration
Main
Electron Configurations
▶ Condensed electron configuration: convenient way of
representing valence electrons
[Nearest Noble Gas] valence electrons
Main
Electron Configurations
▶ Orbital diagrams: make use of arrows in boxes
Main
Electron Configuration Exceptions
▶ A completely full or half full d sublevel is more stable than
a partially filled d sublevel
▶ For Cu and Cr, an electron from the 4s orbital is excited and
rises to a 3d orbital.
Main
Lesson 4
Electrons in Atoms
Main
Emission Spectra and Ionization
▶ Ionization energy is the energy required to remove an electron
from a neutral gaseous atom or molecule in its ground-state
Main
Emission Spectra and Ionization
▶ Trends in first ionization energy across periods account for
the existence of main energy levels and sublevels in atoms.
▶ Successive ionization energy data for an element give
information that shows relations to electron configurations.
Main
Emission Spectra and Ionization
▶ First Ionization Energy:
▶ X(g) → X+(g) + e-
▶ Second Ionization Energy:
▶ X+(g) → X2+(g) + e-
th
▶ n Ionization Energy:
▶ X(n+1)+(g) → Xn+(g) + e-
IE1<IE2<IE3<IE4...
Main
Calculating Ionization Energy
▶ Solve for energy
▶ Multiply by Avogradro’s constant to get the energy
required to remove one mole (J mol-1)
Main
Convergence
▶ At the limit of convergence the lines merge forming a continuum
▶ Beyond this continuum the electrons can have any energy; the
electron is outside of the atom and ionization has occurred
Main
Periodic Trends in Ionization Energies
Ionization of Unknown Element
▶ The first seven ionization energies of an unknown
element are given. Which group might this element
come from?
Main
Ionization of Unknown Element
▶ There is a large jump from the 6th to the 7th ionization
energies. The element is found in the 6th group.
Main
Successive Energy Levels
▶ Aluminum
▶ 1s22s22p63s23p1
▶ The electrons are removed
from the energy level
farthest from the nucleus
(n=3)
▶ Electrons in the outer energy
level require less energy to
remove because they are
further away from the
nucleus
Successive Energy Levels
▶ Aluminum
▶ 1s22s22p63s23p1
▶ The electrons are removed
from the second energy
level (n=2)
▶ The jump in ionization
energy from the 3rd and 4th
ionization energies is
evidence of the existence of
energy levels within the
atom
Successive Energy Levels
▶ Aluminum
▶ 1s22s22p63s23p1
▶ The electrons are removed from
the energy level closest to the
nucleus (n=1)
▶ Electrons in the inner energy level
require more energy to remove
because they are closer to the
nucleus
▶ The jump in ionization energy from
the 11th and 12th ionization
energies is evidence of the
existence of energy levels within
the atom
Successive Energy Levels
▶ Aluminum
▶ 1s22s22p63s23p1
▶ The larger ionization energy
between the 9th and 10th value
occurs because the 10th
electron is removed from the
2s sublevel which experiences a
stronger electrostatic attraction
from the nucleus
▶ Provides evidence of sublevels
in atoms
