Atomic Structure
IB Chemistry Power Points
Topic 02
Atomic Structure
2.1: The nuclear atom 2.1 The nuclear atom Understandings: • Atoms contain a positively charged dense nucleus composed of protons and neutrons (nucleons). • Negatively charged electrons occupy the space outside the nucleus. • The mass spectrometer is used to determine the relative atomic mass of an element from its isotopic composition.
• Use of the nuclear symbol notation to deduce 𝑋𝑋 𝑍𝑍 𝐴𝐴the number of protons, neutrons and electrons in atoms and ions.
• Calculations involving non-integer relative atomic masses and abundance of isotopes from given data, including mass spectra.
2.1 Application and skills
ATOM.. Atomos
• An atom is the smallest particle of an element that retains its identity in a chemical reaction.
• Although early philosophers and scientists could not observe individual atoms, they were still able to propose ideas about the structure of atoms.
Once upon a time…
Democritus atomic theory • Democritus reasoned that
atoms were indivisible and indestructible.
– Although, Democritus’s ideas agreed with later scientific theory, they did not explain chemical behavior
– They also lacked experimental support because Democritus’s approach was not based on the scientific method.
Democritus who?
John Daltons Atomic theory • By using experimental methods, Dalton transformed Democritus’s ideas
on atoms into a scientific theory.• Dalton studied the ratios in which elements combine in chemical
reactions.• All elements are composed of tiny indivisible particles called atoms.• Atoms of the same element are identical. The atoms of any one element are
different from those of any other element.• Atoms of different elements can physically mix together or can chemically
combine in simple whole-number ratios to form compounds.• Chemical reactions occur when atoms are separated from each other, joined,
or rearranged in different combinations. Atoms of one element are never changed into atoms of another element as a result of a chemical reaction.
Thomson’s Plum Pudding model
Discovered Electrons • So maybe the atom is
divisible after all • Thomson discovered the
“electron” by conducting his cathode ray experiment.
• He then proposed the plum pudding model of the atom.
Rutherford’s Gold-Foil Experiment
Rutherford’s results were that most alpha particles went straight through, or were slightly deflected.
• What was surprising is that a small fraction of the alpha particles bounced off the gold foil at very large angles.
• Some even bounced straight back toward the source.
• Which lead to the discovery of the nucleus.
Review – Basic Atomic Structure
NUCLEUS ELECTRONS
PROTONS NEUTRONS
POSITIVE
CHARGE
ATOM
POSITIVECHARGE
PROTONS
NEUTRALCHARGE
NEUTRONS
NUCLEUS
NEGATIVE CHARGE
ELECTRONS
ATOM
Subatomic components
Relative
Mass
Charge
Proton 1 +1
Neutron 1 0
Electron 5 x 10-4 -1
Review – Basic Atomic Model
A-Z notation
© Addison-Wesley Publishing Company, Inc.
126C
mass number A
atomic number Z
element symbol
The atomic number equals the number of protons. Each element has a unique atomic number.
Mass Number• mass number A = protons +
neutrons• always a whole number
© Addison-Wesley Publishing Company, Inc.
• NOT the value given on the Periodic Table!
Practice: determine the required values and write the chemical symbol in A-Z notation.
• Chlorine-37– atomic #:
– mass #:
– # of protons:
– # of electrons:
– # of neutrons:
17
37
17
17
20
Cl3717
Ions
• ions are electrically charged atoms
Neutral atom
negative ionpositive ion
lose electrons
gain electrons
p+ > e- p+ < e-
cation anion
Practice: determine the required values for the negative chloride ion 37
Cl -1
37 Cl-1
– atomic #:
– mass #:
– # of protons:
– # of electrons:
– # of neutrons:
17
37
17
18
20
Practice: determine the required values for the positive calcium ion 40
Ca +2
40 Ca+2
– atomic #:
– mass #:
– # of protons:
– # of electrons:
– # of neutrons:
20
40
20
18
20
© Addison-Wesley Publishing Company, Inc.
Isotopes: Atoms of the same element with different mass numbers.
carbon-12 and carbon-14 are
isotopes
similar chemical properties
stable
radioactive
Radioisotopes and Their Uses
Radioisotopes are unstable isotopes that undergo radioactive decay. Radioisotopes have a number of uses:
U-235 is used as fuel in nuclear reactorsCo-60 is used in cancer radiation therapyC-14 is used as a tracer and for archeological datingAm-241 is used in smoke detectors
Mass SpectrometerA mass spectrometer is used to detect, identify and measure the abundance of different atoms, molecules or molecular fragments.
Mass spectrometer studies are used to determine the average atomic mass for an element. The operation of a mass spectrometer can be divided into 5 steps:
1. Vaporization2. Ionization3. Acceleration4. Deflection5. Detection
Chapter 12 19=>
Vaporization: the element to be analyzed is heated and vaporized (gaseous form).
http://www.magnet.fsu.edu/education/tutorials/java/singlesector2/index.html
Chapter 12 20=>
Ionization: the gaseous element is injected slowly into a vacuum chamber where the atoms are bombarded by electrons. This forms ions positive ions X (g) + e- X+
(g) + 2 e-
http://www.magnet.fsu.edu/education/tutorials/java/singlesector2/index.html
Chapter 12 21=>
Acceleration: the gaseous ions are accelerated through an electric field (towards a negative plate)
http://www.magnet.fsu.edu/education/tutorials/java/singlesector2/index.html
Chapter 12 22=>
Deflection: Ions are deflected in an adjustable magnetic field oriented at right angles to the path. Heavier ions are deflected less.
http://www.magnet.fsu.edu/education/tutorials/java/singlesector2/index.html
Chapter 12 23=>
Detection: ions of a specific mass are counted
http://www.magnet.fsu.edu/education/tutorials/java/singlesector2/index.html
A sample mass spectrograph
Output provides the abundances of the elemental isotopes of different relative mass
Atomic Mass is Relative
• 12C atom = 1.992 × 10-23 g
• 1 p = 1.007276 amu1 n = 1.008665 amu1 e- = 0.0005486 amu
© Addison-Wesley Publishing Company, Inc.
• atomic mass unit (amu)
• 1 amu = 1/12 the mass of a 12C atom
Average Atomic Mass• a weighted average
of all isotopes of an element
100
(%)(mass(mass)(%) )
Avg.AtomicMass
• this value is found on the Periodic Table
• based on the % abundance data from mass spectrometer
Avg.AtomicMass
Average Atomic Mass
• EXAMPLE: Calculate the average atomic mass of chlorine if its abundance in nature is 75.77% 35Cl, and 24.23% 37Cl.
(35)(75.77) (37)(24.23)
10035.48amu
Average Atomic Mass
equation 1
equation 2
(68.9257)(x) (70.9249)(y)69.7231=
100x + y = 100
Average relative mass of Ga 69.7231 amu
Gallium has two naturally occurring isotopes, Ga-69 and Ga-71, with masses of 68.9257 amu and 70.9249 amu, respectively. Calculate the percent abundances of these isotopes
Solve to get 60.1% Ga-69 and 39.9% Ga-71
2.2 Electron Configuration
• Understandings • Emission spectra are produced when photons are emitted
from atoms as excited electrons return to a lower energy level.
• The line emission spectrum of hydrogen provides evidence for the existence of electrons in discrete energy levels, which converge at higher energies.
• The main energy level or shell is given an integer number, n, and can hold a maximum number of electrons, 2n2.
• A more detailed model of the atom describes the division of the main energy level into s, p, d and f sub-levels of successively higher energies.
• Sub-levels contain a fixed number of orbitals, regions of space where there is a high probability of finding an electron.
• Each orbital has a defined energy state for a given electronic configuration and chemical environment and can hold two electrons of opposite spin.
• Applications -and Skills
• Description of the relationship between colour, wavelength, frequency and energy across the electromagnetic spectrum.
• Distinction between a continuous spectrum and a line spectrum.
• Description of the emission spectrum of the hydrogen atom, including the relationships between the lines and energy transitions to the first, second and third energy levels.
• Recognition of the shape of an s atomic orbital and the px, py and pz atomic orbitals.
• Application of the Aufbau principle, Hund’s rule and the Pauli exclusion principle to write electron configurations for atoms and ions up to Z = 36.
All EM radiation is fundamentally the same. The only difference between a gamma ray and a radio wave is the frequency/wavelength/energy.
Visible light is one category of EM radiation. The visible light spectrum is subdivided into six “colors”.
White LightPrism
REDORANGEYELLOWGREENBLUE
VIOLET
A continuous spectrum includes all wavelengths of radiation in a given range.
When white light is passed through a prism a continuous spectrum is produced.
Colored lights do not emit all the wavelengths of the visible light spectrum. For example, a red light emits mostly wavelengths from the red end of the spectrum.
An energized gas sample will emit light of specific wavelengths characteristic of the gas. This is called a line spectrum
Emission spectra are unique for each element
The Bohr model of the atom was developed using information from hydrogen emission spectrum studies. Bohr envisioned an atomic model with:
• a central dense positive nucleus composed of protons and neutrons.
• negative electrons at specific energies orbit the nucleus
• mostly empty space. Nucleus is 10-5 times smaller than atom.
Bohr further stated that the orbiting electrons occupy discrete energy levels. Electrons can only “jump” between energy levels if they absorb or emit a specific amount of energy.
Bohr saw the line spectrum of hydrogen as a direct result of energized electrons releasing a specific amount of energy by emitting a photon of light at a certain wavelength.
The different lines in the hydrogen spectrum were evidence for a number of different energy levels.
lower energylonger
wavelength
higher energyshorter
wavelength
Visible spectrum for
hydrogen atom convergence
Lower energy = more stable electron orbit
Electrons fill the lowest energy orbitals first.
Each orbital has a maximum possible number of electrons.
As you should recall:
1st energy level (ground state) = 2 electrons
2nd energy level = 8 electrons
1
2
3
4
5
6
7
6
7
1A2A
3B4B5B6B7B8B8B8B1B2B
3A4A5A6A7A8Agroup # = # valence (outside) e-
d p
f
sRow
=# shells
Electron Configuration
1s1
row #shell #
possibilities are 1-77 rows
subshellpossibilities are
s, p, d, or f4 subshells
group ## valence e-
possibilities are:s: 1 or 2
p: 1-6d: 1-10f: 1-14
Total e- should equalAtomic #
What element has an electron configuration of 1s1?
Practice:Ask these questions every time you have to write an electron
configuration• Lithium:
1. find the element on the periodic table2. what is the period number?3. how many shells?4. what is the group number?5. how many valence electrons?6. what subshell(s) does Li have? 7. what is the electron configuration?
atomic # = 32
2
11
s
1s2 2s1
Practice:Ask these questions every time you have to write an electron
configuration• Boron:
1. find the element on the periodic table2. what is the row #?3. how many shells?4. what is the group #?5. how many valence electrons?6. what subshell(s) does B have? 7. what is the electron configuration?
atomic # = 52
2
33
p
1s2 2s2 2p1
Order of Electron Subshell Filling:It does not go “in order”
1s2
2s2 2p6
3p6
4p6
5p6
6p6
7p6
3s2
4s2
5s2
6s2
7s2
3d10
4d10
5d10
6d10
4f14
5f14
1s2 2s22p6 3p63s2 4s2 4p65s23d10 5p6 6s24d10 6p67s25d104f14 7p66d105f14
1
2
3
4
5
6
7
6
7
per
iod
# =
# e
- sh
ells 1A
2A
3B4B5B6B7B8B8B8B1B2B
3A4A5A6A7A8Agroup # = # valence e-
d
f
3d
4d
5d
6d
4f
5f
Subshells d and f are “special”
Electron Configuration
1s1
row #shell #
possibilities are 1-77 rows
subshellpossibilities are
s, p, d, or f4 subshells
group ## valence e-
possibilities are:s: 1 or 2
p: 1-6d: 1-10f: 1-14
Total e- should equalAtomic #
What element has an electron configuration of 1s1?
Practice:Ask these questions every time you have to write an electron
configuration• Lithium:
1. find the element on the periodic table2. what is the row #?3. how many shells?4. what is the group #?5. how many valence electrons?6. what subshell(s) does Li have? 7. what is the electron configuration?
Practice:Ask these questions every time you have to write an electron
configuration• Boron:
1. find the element on the periodic table2. what is the row #?3. how many shells?4. what is the group #?5. how many valence electrons?6. what subshell(s) does B have? 7. what is the electron configuration?
Order of Electron Subshell Filling:It does not go “in order”
1s2
2s2 2p6
3p6
4p6
5p6
6p6
7p6
3s2
4s2
5s2
6s2
7s2
3d10
4d10
5d10
6d10
4f14
5f14