Atomic Theory.

Atomic Theory

DEMOCRITUS BC The Greek philosopher Democritus proposed that all matter was made of small, unbreakable particles he called atoms which means unbreakable. He believed that atoms were too small to be seen. Philosophers are not scientists. They do not test their ideas. Instead they use reasoning to back up their beliefs. To them, human reasoning was superior to experimentation.

ARISTOTOLE The famous philosopher Aristotle, who also lived at that time, argued that all matter was made of only four elements. For the next two thousand years, Aristotle overshadowed Democritus. Finally, in the early 1800s, the atomist’s theory was revived by John Dalton.

John Dalton 1766-1844 In 1809, Dalton by proposing the following:
a) All matter was made of atoms. b) Atoms were solid spheres. c) Atoms of different elements differed in mass. d) Atoms were indivisible and indestructible. e) Atoms combine to form compounds.

J.J. THOMSON Before you can understand Thomson’s experiment, 3 properties about electrical charges: a) There are two types of electrical charge: positive and negative. b) Opposite charges attract. c) Like charges repel. Thomson took Cathod ray tube and added two plates inside the tube and connected them with a wire. When the plates were not charged, the ray shot straight.

Thomson’s Experiment Voltage source - + Vacuum tube Metal Disks

- + Thomson’s Experiment
Voltage source - + Passing an electric current makes a beam appear to move from the negative to the positive end

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field he found that the moving pieces were negative

Cathod Ray Tube Conclusion
Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons. Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons Electrons have so little mass that atoms must contain other particles that account for most of the mass

Thomson’s model was called the Plum Pudding Model was named after a popular dessert in England at that time. It was the first model to propose that smaller charged particles make up the atom. Thomson’s model lasted less than two decades but it was first to propose the existence of subatomic particles. In 1911 another scientist who worked in Thomson’s lab improved on his atomic model.

ERNEST RUTHERFORD One type of radioactivity is when an atom throws out a positively charged particle from the nucleus. This particle was called an alpha particle (α). Rutherford used this alpha particle to investigate the structure Rutherford and Geiger in the Cavendish Lab

Rutherfold’s Gold Foil Experiement
Uranium is a radioactive element that gives off positive particles (alpha particles). Rutherford used these positive particles to invest Rutherford encased uranium in lead (which absorbs alpha particles). This produced a beam of alpha particles traveling in a straight line. He fired these positive particles at a thin piece of gold (dense metal). A screen around the gold to detect where the alpha particles were traveling.

Rutherfold’s Gold Foil Experiement
Rutherford shot alpha particles at a thin sheet of gold to observe what happened when the positive α particles passes through the gold atoms. If Thompson’s model was correct the alpha particles should pass through the diffused positive cloud with ease.

Fluorescent Screen Lead block Uranium Gold Foil

What he expected

Because

He thought the mass was evenly distributed in the atom

Since he thought the mass was evenly distributed in the atom

What he got

Rutherfold’s Conclusion
From his observations Rutherford concluded that the atom had a dense, positive central nucleus composed of + charged protons. He stated that the electrons orbited the nucleus - like planets orbiting the Sun. In 1909 Rutherford proposed his Planetary Model of the Atom. His model created positively charged protons located in the nucleus and placed electrons in orbit around the nucleus – like planets around the sun.

Almost no deflection; few greatly deflected
+

Checking for understanding
Explain Thompson’s conclusions in 3 points: Explain Rutherford’s conclusions in 3 points:

Atomic Structure

Protons Neutrons Electrons Subatomic Particles
Over the past century scientist have discovered that the atom is composed of 3 subatomic particles: Protons Neutrons Electrons

Draw this diagram. Label all subatomic particles and include their charges.

The Proton Symbol = p+ Relative Mass = 1 Atomic Mass Unit (AMU).
Actual mass = x g Location: Inside the nucleus 5. Electrical charge: Positive. 6. Importance: The atomic number which is the identity of the element. 7. Discovered by: Ernest Rutherford in 1909

The Electron Symbol = e- Relative Mass = 1 /1836 Atomic Mass Unit.
Actual mass = 9.11 x g 4. Location: Energy level outside the nucleus 5. Electrical charge: Negative. Importance: The number of electrons located in the last energy level determine the chemical activity of the element. Discovered by: J.J.Thomson in 1897

The Neutron Symbol = n Relative Mass = 1 Atomic Mass Unit (AMU).
Actual mass = 1.675 x g 4. Location: Inside the nucleus 5. Electrical charge: Neutral. 6. Importance: Is responsible for isotopes (atoms of the same element with different numbers of neutrons. 7. Discovered by: James Chadwick in 1932

Atomic Number Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element. Element # of protons Atomic # (Z) Carbon 6 Phosphorus 15 Gold 79

Mass Number Mass number is the number of protons and neutrons in the nucleus of an isotope. Mass # = p+ + n0 Nuclide p+ n0 e- Mass # Oxygen - 10 - 33 42 - 31 15 18 8 8 18 Arsenic 75 33 75 Phosphorus 16 15 31

Isotopes are atoms of the same element having different masses due to varying numbers of neutrons.
Protons Electrons Neutrons Nucleus Hydrogen–1 (protium) 1 Hydrogen-2 (deuterium) Hydrogen-3 (tritium) 2

Composition of the nucleus
Atomic mass is the average of all the naturally occurring isotopes of that element. Atomic Masses Isotope Symbol Composition of the nucleus % in nature Carbon-12 12C 6 protons 6 neutrons 98.89% Carbon-13 13C 7 neutrons 1.11% Carbon-14 14C 8 neutrons <0.01% Carbon =

Weight Average Atomic Mass
The atomic masses given on the periodic table are WEIGHT-AVERAGED masses. This is calculated using both the masses of each isotope and their percent abundances in nature. For the purposes of simplicity, we will round weight-average mass to the THOUSANDTHS place. The weight-average mass is based on the abundance of the naturally occurring isotopes of that element

Weight Average Atomic Mass
To find the weight-average mass of an element given the mass of each isotope and each isotopes percent abundance: WAM = (massisotope 1 X % ) + (massisotope 2 X % ) + (massisotope 3 X % ) + etc…

Atomic Mass Unit (AMU) amu = atomic mass unit
the ratio of the average mass per atom of the element to 1/12 of the mass of 12C in its nuclear and electronic ground state. An atomic mass unit is actually an average mass, found by taking the mass of a C-12 nucleus and dividing it by 12 Hydrogen = 1amu, 1/12 of C

Calculate the atomic mass
Carbon has two stable isotopes Carbon-12 has natural abundance of 98.89% and amu Carbon-13 has natural abundance of 1.11% and amu Calculate the atomic mass Givens Carbon-12 m= amu Abundance= 98.89%=0.9889 Carbon-13 m = amu Abundance = 1.11%=0.0111 Formula atomic mass of carbon-avg = (mass C-12 x nat.abund) + (mass C-13 x nat.abund.) Plug in the #s (12.000amu x ) + ( amu x ) = amu = 12.0 amu

4 Types of Electron Configuration of Elements

1. Shell Configuration Shell Number (Principle Electron Level) Number of Electrons to hold 1 2 8 3 4 18 5 6 32 7 Shows how many electrons are found in each shell (principal energy level). This is the configuration Niels Bohr would have come up with as the discoverer of the energy level!

Shell Configuration (Bohr Diagrams)
Draw a nucleus with the element symbol inside. Carbon is in the 2nd period, so it has two energy levels, or shells. Draw the shells around the nucleus. C

Add the electrons. Carbon has 6 electrons. The first shell can only hold 2 electrons. C

Since you have 2 electrons already drawn, you need to add 4 more. These go in the 2nd shell. Add one at a time -starting on the right side and going counter clock-wise. C

Check your work. You should have 6 total electrons for Carbon. Only two electrons can fit in the 1st shell. The 2nd shell can hold up to 8 electrons. The 3rd shell can hold 18, but the elements in the first few periods only use 8 electrons. C

2. Sublevel Electron Configuration
Principal energy levels are made up of sublevels, much as a town is made up of streets. The expanded configuration tells you how many electrons are found in each sublevel of each PEL. Most of the time (and for all of the configurations you will be responsible for), one sublevel must fill up completely before the next one can get any electrons.

Arrangement of Electrons in an Atom
Electrons in atoms are arranged as SHELLS (n) SUBSHELLS (l) ORBITALS (ml) Each orbital can be assigned no more than 2 electrons! row # shell # possibilities are 1-7 7 rows subshell possibilities are s, p, d, or f 4 subshells group # # e- s subshell : 1 orbital , total 2 e- p subshell : 3 orbital, total of 6 e- d subshell :5 orbital, total of 10 e- f subshell: 7 orbital, total of 14 e-

s , orbital shapes

p orbitals are peanut or dumbbell shaped.

d orbitals

f orbitals

s p d f 1 2 3 4 5 6 7 1A 2A 3B 4B 5B 6B 7B 8B 1B 2B 3A 4A 5A 6A 7A 8A
group # = # valence (outside) e- s 1 2 3 4 5 6 7 p Row = # shells d f

Subshells d and f are “special”
group # = # valence e- 1 2 3 4 5 6 7 3d d period # = # e- shells 4d 5d 6d f 4f 5f

Electron Configuration – Spdf notation
HELIUM – 2 electrons Is2 group # # valence e- possibilities are: s: 1 or 2 p: 1-6 d: 1-10 f: 1-14 Total e- should equal Atomic # row # shell # possibilities are 1-7 7 rows subshell possibilities are s, p, d, or f 4 subshells

3. Orbital Box Diagram Shows how many electrons are in each ORBITAL of each sublevel, and what each electron’s SPIN is. Orbitals are all the same size, they can all fit up to two electrons in them. The spin of electrons is indicated by arrows up and down. If the orbital has two electrons in it, the first will have an up spin, and the second will have a down spin. The number of arrows will equal the number of electrons in the sublevel. . much as streets of a town can be broken down into individual houses. Just as different sized streets can have different numbers of houses on them, the more electrons a sublevel can hold, the more orbitals it can be broken down into.

Guide to Drawing Orbital Diagrams

Drawing Orbital Diagram
Draw the orbital diagram for nitrogen. Step 1 Draw boxes to represent the occupied orbitals. Nitrogen has an atomic number of seven, which means it has seven electrons. Draw boxes to represent the 1s, 2s, and 2p orbitals.

Step 2 Place a pair of electrons in the last occupied sublevel in separate orbitals. We place the remaining three electrons in the 2s orbitals.

Step 3 Place remaining electrons with opposite spins in each filled orbital. First we place a pair of electrons with opposite spins in the 2p orbitals, with arrows in the same direction.

HONORS CHEMISTRY ONLY 3a. Quantum Numbers
Electron energies are addressed in a similar way to a ZIP code. Many addresses in Ulster and northern Orange county have 125 as the prefix, with the last two digits signifying the actual postal box.\ For example, New Paltz is 12561, Wallkill is 12589, Newburgh is 12550, Pine Bush is

3a. Quantum Numbers There are four identifying characteristics of the energy of a specific electron in an atomic, each more specific than the last. They are: n (principal quantum number) = Principal Energy Level (1, 2, 3, 4, etc.) l (levarotary) = Sublevel (s, p, d, f) m (magnetic) = Orbital s (spin) = Spin (+ 1/2, - 1/2)

3a. Quantum Numbers n , principal quantum number
based on Bohr’s observations of line spectra for different elements ‘n’ relates to the main energy of an electron allowable values: n = 1, 2, 3, 4, … electrons with higher ‘n’ values have more energy

3a. Quantum Numbers l , The Secondary Quantum Number
based on the observation that lines on line spectra are actually groups of multiple, thin lines ‘l ’ relates to the shape of the electrons’ orbits allowable values: l = to l = n - 1 i.e. for n = 4: l = 0, 1, 2, or 3 the ‘l ’ values 0, 1, 2, and 3 correspond to the shapes we will call s, p, d and f, respectively

3a. Quantum Numbers ml , the Magnetic Quantum Number
based on the observation that single lines on line spectra split into new lines near a strong magnet ‘ml ’ relates to the direction/orientation of the electrons’ orbits allowable values: ml = - l to + l i.e. for l = 2: ml = -2, -1, 0, 1, or 2 electrons with the same l value but different ml values have the same energy but different orientations

3a. Quantum Numbers ms , The Spin Quantum Number
based on the observation that magnets could further split lines in line spectra, and that some elements exhibit paramagnetism ‘ms ’ relates to the ‘spin’ of an electron allowable values: ms = - ½ or + ½ i.e. for any possible set of n, l, and ml values, there are two possible ms values when two electrons of opposite spin are paired, there is no magnetism observed; an unparied electron is weakly magnetic

ms , The Spin Quantum Number

4. Electron (Lewis) Dot Diagram
VALENCE ELECTRONS the electrons in the outermost shell (furthest energy level from the nucleus), which is also called the valence shell. The number of valence electrons that an atom has can be determined by the last number in the basic electron configuration. The number of valence electrons that an atom has determines its physical and chemical properties

Group 1 (alkali metals) have 1 valence electron

Group 2 (alkaline earth metals) have 2 valence electrons

Group 13 elements have 3 valence electrons

Group 17 (halogens) have 7 valence electrons

Group 18 (Noble gases) have 8 valence electrons, except helium, which has only 2

Transition metals (“d” block) have 1 or 2 valence electrons

Lanthanides and actinides
(“f” block) have 1 or 2 valence electrons

Lewis Dot Diagram using dots in groups of 2 around the symbol of the atom to represent the valence electrons. For every atom, the valence electrons will occupy only s and p orbitals. The s electrons fill up first, then the p electrons fill, up electrons first, followed by the downs, just like in the box diagram.

The Electron Dot diagram for Nitrogen
Nitrogen has 5 valence electrons. First we write the symbol. N Then add 1 electron at a time to each side. Until they are forced to pair up.

Draw orbital diagrams Draw Lewis dot diagrams Carbon Helium Fluorine

Nuclear Chemistry

Nuclear Chemistry Nucleus of an atom contains protons and neutrons
Strong forces (nuclear force) hold nucleus together Protons in nucleus have electrostatic repulsion however, strong force overcomes this repulsion Strong force: the interaction that binds nucleus together Nuclear force (strong force) is MUCH stronger than electrostatic force Strong force increases over short distances

Radioisotopes Radioisotopes- unstable isotopes that gain stability by releasing particles. alpha Unstable isotope stable beta gamma

Characteristics of Some Radiation
Property Alpha radiation Beta radiation Gamma radiation Composition Alpha particle (He nucleus) Beta particle (electron) High energy EM radiation Symbol , 42 He , 0-1 e  Penetrating power low moderate Very high Shielding Paper, clothing Metal foil Lead, concrete

X Review U A Z Mass Number Element Symbol Atomic Number 235 92 238
Atomic number (Z) = # protons in nucleus Mass number (A) = # protons + # neutrons = atomic # (Z) + # neutrons Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei Mass Number X A Z Element Symbol Atomic Number U 235 92 238

Alpha emission (decay)
4 2 He Identify the product formed when uranium-238 alpha decays 4 2 He Th U Determines which atom from the periodic table

0 -1 e + 14 7 N 14 6 C Beta emission (decay) 0-1 e
Identify the product formed when carbon-14 emits beta particle 0 -1 e N 14 6 C

Gamma emission (decay)
00 γ Identify the product formed when carbon-14 releases gamma rays 0 0 γ C 14 6 C

Nuclear Reaction

Nuclear Fission Reaction splits a large nucleus apart to form two smaller ones. Reaction is unknown in the natural world, is a form of artificial transmutation Reaction can take place at any temperature or pressure Reaction is currently being used to produce electricity for our use Requires mining to extract uranium ore Produces THOUSANDS of times more energy than conventional chemical explosives Produces radioactive wastes

Nuclear Fusion Reaction combines two small nuclei together to form one larger one. All stars are powered by nuclear fusion Reaction requires temperatures of millions of degrees and vast pressures Hydrogen is the most abundant element in the universe Produces MILLIONS of times more energy than conventional chemical explosives Produces essentially no radioactive waste

Common to Both Fission and Fusion
Both generate their energy the same way by converting small amounts of mass (MASS DEFECT) into extraordinary amounts of energy.

Compare and contrast nuclear fusion and fission

Half - Life

Half- Life The half-life of a radioactive isotope is defined as the period of time that must go by for half of the nuclei in the sample to undergo decay. - Half of the radioactive nuclei/isotope in the sample decay into new, more stable nuclei/isotope After one half-life, half (50%) of the original amount of the sample will have undergone radioactive decay. After a second half-life, one quarter (25%) of the original sample will remain undecayed. After a third half-life, one eighth (12.5%) of the original sample will remain undecayed.

The blue grid below represents a quantity of C14. Each time you click,
one half-life goes by and turns red. C14 – blue N14 - red Half lives % C14 %N14 Ratio of C14 to N14 100% 0% no ratio As we begin notice that no time has gone by and that 100% of the material is C14

1 The grid below represents a quantity of C14. Each time you click,
one half-life goes by and you see red. C14 – blue N14 - red Half lives % C14 %N14 Ratio of C14 to N14 100% 0% no ratio 1 50% 1:1 After 1 half-life (5730 years), 50% of the C14 has decayed into N14. The ratio of C14 to N14 is 1:1. There are equal amounts of the 2 elements.

one half-life goes by and you see red . C14 – blue N14 - red Half lives % C14 %N14 Ratio of C14 to N14 100% 0% no ratio 1 50% 1:1 2 25% 75% 1:3 Now 2 half-lives have gone by for a total of 11,460 years. Half of the C14 that was present at the end of half-life #1 has now decayed to N14. Notice the C:N ratio. It will be useful later.

one half-life goes by and you see red. C14 – blue N14 - red Half lives % C14 %N14 Ratio of C14 to N14 100% 0% no ratio 1 50% 1:1 2 25% 75% 1:3 3 12.5% 87.5% 1:7 After 3 half-lives (17,190 years) only 12.5% of the original C14 remains. For each half-life period half of the material present decays. And again, notice the ratio, 1:7

Radioactive Dating

Radioactive Dating Radioactive Decay is a RANDOM process. It is not possible to predict when a particular nucleus will decay, but we can make fairly accurate predictions regarding how many nuclei in a large sample will decay in a given period of time.

Age of Sample = # Half-Lives X Half-Life Duration
Radioactive Dating used to determine the age of a substance that contains a radioactive isotope of known half-life. Step 1: Determine how many times you can cut your original amount in half in order to get to your final amount. This is the number of half-lives that have gone by. Step 2: Multiply the number of half-lives by the duration of a half-life Age of Sample = # Half-Lives X Half-Life Duration See Reference chart

Reference Chart

The oldest rocks on Earth have been found to contain 50% U-238 and 50%Pb-206 (what U-238 ultimate decays into). What is the age of these rocks? First, find out how many half-lives have had to go by so that you have gone from 100% U-238 to 50% U-238: 100  50 ONE half-life has gone by! Age of Sample = # Half-Lives X Half-Life Duration = 1 half-life X (4.51 X 109 years) = 4.51 X 109 years old!

Atomic Theory.

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