Atomic Structure Pre-AP Chemistry
Target Words
Atom Atomic number Atomic theory Electromagnetic spectrum Electron Electron cloud Emission spectrum Energy level Hypothesis Isotope
Law of definite proportions Lewis dot diagram Mass number Neutron Nucleus Proton Scientific law Theory Valence electron
What do you know about atoms?
Early Atomic Theory
Philosophically based from ethical, religious, or other schools of thought, not modern scientific methodology Greek
Indian
Democritus – proposed atomic theory to account for philosophical disagreements on matter versus void Multi-faceted between scientific elemental forms and religious theories concerning souls and knowledge
Islamic
Atomism reconciles the intent of the divine with the existence of the physical
Early Scientific Work
France, 1789
France, 1799
Antoine Lavoisier develops the Law of Conservation of Mass through highly accurate measurements Joseph Proust develops the Law of Definite Proportions, which states that when broken down, compounds always have the same ratio of elemental ingredients
England, 1802
John Dalton builds on previous work to develop the Law of Multiple Proportions; fixed amounts of element A will always combine with element B in proportions that reduce to whole numbers
Dalton’s Atomic Theory (1803)
All matter is made up of atoms. Atoms are indestructible and cannot be divided into smaller particles. All atoms of an element are exactly alike, but are different from atoms of other elements.
Dalton’s Work Published
Though brilliantly and empirically derived, his model was flawed and incomplete. Why?
Look closely at his illustrated plates: what do you see here?
Questions Raised
Why are there so many different elements?
What makes one atom different from another?
How do atoms combine to form compounds?
Review Slide
Early work on Atomic Theory was based in philosophy, not science. Real science-based work started in the late 1700’s in France and England; Lavoisier and Proust. Laws of; Conservation of Mass, Definite and Multiple Proportions led to Dalton formulating his Atomic Theory.
All matter is made up of atoms.
Atoms are indestructible and cannot be divided into smaller particles.
All atoms of an element are exactly alike, but are different from atoms of other elements.
Dalton’s Model; a solid ball of the ?element? that is tiny. Questions were left unanswered!
Cracking into the Mystery of the Atom!
Additional refinement was done throughout the 1800’s to further find atomic weights and relative masses of elements (Avagadro)
The inner secrets of the atom were not divulged until very recently.
J. J. Thompson, England (1897)
British physicist experimenting with electricity and vacuum tubes. Constructed a vacuum tube with positive and negative terminals at either end, and a phosphorescent (glows when energized) strip.
Science in ACTION!
Thompson’s Thoughts
Something is passing between the two metal nodes.
Is it light?
Is it something else?
How could he know?
Science, but with magnets!
Thompson’s Conclusions
The stream is NOT light; magnets do not bend light. Therefore the stream is made up of charged particles. The stream can NOT be something already in the tube, it is a vacuum. Therefore the stream MUST be coming from the metal nodes at either end. Therefore atoms must have particles that can be separated and shot as a ray, particles with an electric charge!
More Information and Conclusions
The magnet bends the stream certain ways. Therefore the charged particles had a NEGATIVE charge. The particles were EXTREMELY small, 1/1387 the mass of the smallest atom. Therefore the atom must have other stuff inside it! Atoms normally have a NEUTRAL charge. Therefore the other stuff is POSITIVELY charged.
Thompson’s Model
Thompson described the atom as a ball of positive charges with electrons drifting around somehow. Though including new subatomic particles, his model maintained the ‘solid ball/clump’ structure of Dalton’s earlier model.
Complications Plague Thompson
It was discovered that the positive terminal also emits a ray.
Thompson proposed a positive particle so this was expected.
The masses of some elements were found to be slightly variable – sometimes they weighed more, sometimes a little less
The rational explanation is a third particle, a neutral one called the neutron. Atoms that are chemically identical but of different weights are called isotopes.
Review Slide
More work on the Atomic Theory provided very important additional details. Thompson’s experiments with vacuum tubes, cathodes, and magnets demonstrated;
That atoms were made of smaller particles. The particles were negative, positive, or neutrally charged. Named the Electron, Proton, and Neutron.
Elements sometimes came in different weights (isotopes) – this was proposed to be because of having more or less neutrons inside.
Important question; How is everything arranged?
Rutherford, 1908
Another British physicist interested in working with positively charged particles, called α particles (alpha).
He was working on scattering, or finding out how these particles bounced for complex physics reasons.
Rutherford’s Experimentation
After constructing an alpha particle gun, it was aimed at a sheet of gold foil.
Undeflected particles
Gold foil
Gold can be easily flattened until it is a few atoms thick.
Deflected particles ZnS fluorescent screen
Source of fast moving, positively charged particles (α particles)
What SHOULD have happened.
What Really happened!
Exercise 1. 2. 3. 4. 5.
6.
7.
What kinds of particles are emitted by the radioactive source? What is their charge? Toward what are the alpha particles being directed? What happens when the charged particles strike the surface of the gold foil? What was the purpose of the fluorescent screen? What did Rutherford conclude from the gold foil experiment regarding the amount of empty space in the atom? Why was he able to conclude this? How does the path of a charged particle that strikes the center of a gold atom differ from the path of a particle that passes near the center? Based on this experiment, where is most of the positive charge and mass of an atom found?
Rutherford’s Model
Rutherford or Nuclear model In the early twentieth century, Rutherford showed that most of an atom's mass is concentrated in a small, positively charged region called the nucleus.
Questions post Rutherford
How are the electrons arranged?
What size are these atoms?
How does energy figure into this?
Niels Bohr
Another physicist, but Danish!
His idea was that electrons orbited the nucleus – but with quantum energy states keeping them in place instead of gravity! (This is complicated, more on it later.)
Electrons ‘orbit’ the nucleus in layers, or electron shells, which depend on the number of electrons and size of the nucleus.
Each shell can only hold a certain number of electrons
Electron Shells
Each element has a different number of electrons, and so they have different electron shell arrangements;
Hydrogen for example has one electron:
Oxygen has 8 electrons:
There can be many different layers!
Bohr
Bohr model After Rutherford's discovery, Bohr proposed that electrons travel in definite orbits, arranged in shells or levels Around the nucleus.
Review Slide
Rutherford discovered that the mass of an atom is concentrated in the center, or nucleus of an atom. He was unable to determine the exact arrangement of other particles. Bohr calculated the locations of electrons using highenergy equations and complex math. Bohr demonstrated that electrons orbit in energy layers, or shells
Atomic Model Review
? Dalton model Not much of a model, but Dalton’s understanding was of a discrete little ball of whatever made atoms different from one another.
Rutherford or Nuclear model In the early twentieth century, Rutherford showed that most of an atom's mass is concentrated in a small, positively charged region called the nucleus.
Thompson model After working with cathode ray tubes, Thompson created the ‘plum pudding’ model with electrons Embedded in an unknown positively charged substrate.
Bohr model After Rutherford's discovery, Bohr proposed that electrons travel in definite orbits, arranged in shells or levels around the nucleus.
Atomic Makeup
Nucleus
Contains protons Contains neutrons
Electron Shells
Holds all the electrons in orbits
How many subatomic particles?
Every element has different numbers of subatomic particles making up its nucleus and electron shells.
The number of protons is unique for each element and does not change.
The number of electrons can vary.
The number of neutrons can vary.
Reading the Periodic Table
The periodic table gives us all the information we need; An examples is below.
Breaking it down
Every element has a unique atomic number. The # of protons = atomic number. The mass of an atom is concentrated in the nucleus so the atomic mass = # protons + # nuetrons. The # of neutrons = atomic mass – atomic number. Atoms have no charge (ground state) so the positive charges of the protons must be canceled by the negative charges of the electrons. The # of electrons = # of protons.
For Example;
Aluminum
Phosphorus
Protons = 13 (Atomic Number) Neutrons = 13 (26[atomic mass]-13[atomic number]) Electrons = 13 (To cancel the positively charged protons.) Protons = 15 (Atomic Number) Neutrons = 15 (30[atomic mass]-15[atomic number]) Electrons = 15 (To cancel the positively charged protons.)
Zinc
Protons = 30 (Atomic Number) Neutrons = 35 (65[atomic mass]-30[atomic number]) Electrons = 30 (To cancel the positively charged protons.)
Electron numbers can change?
Yes! Atoms can lose or gain electrons, which means they gain/lose a charge that is either positive or negative.
As electrons are lost or gained the +/- balance shifts.
More electrons makes the atom become negatively charged. Fewer electrons makes the atom positively charged.
The newly charged atoms are called ions.
Positive ions are cations. Negative ions are anions.
Notation and Ions
Al+2
P-1
Protons = 13 (Atomic Number) Neutrons = 13 (26[atomic mass]-13[atomic number]) Electrons = 11 (Two protons are not being cancelled.) Protons = 15 (Atomic Number) Neutrons = 15 (30[atomic mass]-15[atomic number]) Electrons = 16 (There is an extra negative charge.)
Zn+4
Protons = 30 (Atomic Number) Neutrons = 35 (65[atomic mass]-30[atomic number]) Electrons = 26 (Four protons are not being cancelled.)
Neutron numbers shift as well?
Yes. Atoms naturally exist with variable numbers of neutrons, which can change their overall atomic mass.
An atom with the same atomic number but a different atomic mass is called an isotope.
One of the most common ways to identify an isotope is to write the symbol then the mass;
Carbon -14 Neon – 21 Oxygen -16
The atomic mass on the periodic table is a weighted average of all isotopes of that element.
Isotope Examples
Aluminum - 27
Phosphorus - 31
Protons = 13 (Atomic Number) Neutrons = 14 (27[atomic mass]-13[atomic number]) Electrons = 13 (To cancel the positively charged protons.) Protons = 15 (Atomic Number) Neutrons = 16 (31[atomic mass]-15[atomic number]) Electrons = 15 (To cancel the positively charged protons.)
Zinc - 64
Protons = 30 (Atomic Number) Neutrons = 34 (64[atomic mass]-30[atomic number]) Electrons = 30 (To cancel the positively charged protons.)
Atomic Mass
The atomic mass of an element represents the average mass of all the isotopes found in nature. No element exists with only one possible isotope. Hydrogen has the smallest number of isotopes: H-1 protium, H-2 deuterium, H-3 tritium.
Calculating Atomic Mass
If you look at your periodic table for hydrogen, the atomic mass is 1.0079 amu (atomic mass units). The atomic mass is calculated by adding the % of H1 mass found in nature to the % of H-2 mass found in nature plus the % of H-3 mass. % H-1 + % H-2 + % H-3 = average mass (atomic mass) Generally the formula used is: % X + % Y + % Z… = atomic mass.
Sample Problem
Silver is found to have two stable isotopes, one has an atomic mass of 106.904 amu and the other weighs 108.905 amu.
The first isotope represents 51.82 % of the mass of the element and the second represents 48.18 %.
What is the atomic mass of the element silver?
Solution The equation to use is: %X + % Y = average atomic mass And remember to convert your percentage amounts into fractions (by dividing by 100) before you begin anything! (0.5182) 106.904 amu + (0.4818) 108.905 amu =
mass 55.398 amu + 52.470 amu = 107.868 amu
Now look at the periodic table to verify the answer.
Try This: A sample of neon contains three isotopes, neon20 (with an isotopic mass of 19.9924 amu), neon21 (20.9939 amu) and neon-22 (21.9914 amu). The natural abundances of these isotopes are 90.92%, 0.257 %, and 8.82 %. Calculate the atomic weight of neon. 20.17 amu
