Presentation on theme: "Unit 2. Atomic Theory John Dalton Law of Conservation of Mass Law of Definite Proportions Law of Multiple Proportions Ernest Rutherford Robert."— Presentation transcript:
Atomic Theory John Dalton Law of Conservation of Mass Law of Definite Proportions Law of Multiple Proportions Ernest Rutherford Robert Millikan J.J. Thompson Atomic Structure Protons, neutrons, electrons Atomic number Isotopes Mass number Average atomic mass Wave nature of light Electromagnetic Spectrum C = λv Bohr Models Photoelectric effect Absorption/emission E = hc/ λ Heisenberg Uncertainty Principle Configurations (orbital, electron, noble gas) Pauli Exclusion Principle Hund’s Rule Paramagnetism/diamagnetism Exceptions
B.C. 400 B.C. Democritus and Leucippos use the term "atomos” 1500's Georg Bauer: systematic metallurgy Paracelsus: medicinal application of minerals 1600's Robert Boyle:The Skeptical Chemist. Quantitative experimentation, identification of elements 1700s' Georg Stahl: Phlogiston Theory Joseph Priestly: Discovery of oxygen Antoine Lavoisier: The role of oxygen in combustion, law of conservation of mass, first modern chemistry textbook 2000 years of Alchemy
1800's Joseph Proust: The law of definite proportion (composition) John Dalton: The Atomic Theory, The law of multiple proportions Joseph Gay-Lussac: Combining volumes of gases, existence of diatomic molecules Amadeo Avogadro: Molar volumes of gases Jons Jakob Berzelius: Relative atomic masses, modern symbols for the elements Dmitri Mendeleyev: The periodic table J.J. Thomson: discovery of the electron Henri Becquerel: Discovery of radioactivity 1900's Robert Millikan: Charge and mass of the electron Ernest Rutherford: Existence of the nucleus, and its relative size Meitner & Fermi: Sustained nuclear fission Ernest Lawrence: The cyclotron and trans-uranium elements
400 BC Democritus Matter consists of small particles Called them “atomos” Idea rejected by peers No scientific proof
Aristotle All matter continuous 4 elements = earth, water, air, and fire No scientific proof Idea endured for 2000 years
School Teacher Atomic Theory 1. All matter is composed of extremely small particles called atoms. There are different kinds called elements. 2. Atoms of the same element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties. 3. Atoms cannot be subdivided, created, or destroyed. 4. Atoms of different elements combine in simple, whole number ratios to form chemical compounds. 5. In chemical reactions, atoms are combined, separated, or rearranged but never destroyed/created.
Law of Conservation of Mass Total mass present before chemical reaction is same as mass after chemical reaction 2H 2 O 2H 2 + O 2 If you have 10 grams of water to start, you will get 1.12 g of hydrogen and 8.88 g of oxygen Law of Constant Composition (definite proportions) Relative numbers and kinds of atoms are constant Water is 88.8% oxygen and 11.2% hydrogen by mass no matter how much you have Law of Multiple Proportions If two elements combine to form more than one compound, the masses of the two elements are in the ratio of small whole numbers CO 2 versus CO (mass ratio is 2 to 1 for oxygen)
British Physicist Discovered electron Cathode-ray experiment Plum pudding view of atom
Electric current sent through gases in glass tube called cathode-ray tube Surface of tube opposite the cathode glowed – caused by stream of particles Ray traveled from cathode to anode Cathode rays deflected by magnetic field away from negatively charged object (like a magnet) Cathode rays concluded to have negative charge
American Physicist Charge on each electron is same Charge of electron is -1.6022 x 10 -19 C Calculated mass of electron as 9.10x 10 -31 kg Oil drop experiment
Drops of oil that had picked up extra electrons allowed to fall between two electrically charged plates Measured how voltage on plates affected rate of fall Calculated charges of drops then deduced charge of a single electron on the drops
Discovered nucleus Planetary model of the atom
Bombarded thin piece gold foil with alpha particles (positively charged particle 4 times mass of hydrogen atom) Expected to pass right through gold foil 1 in 8000 particles deflected back toward source “As if you fired 15-inch artillery shell at a piece of tissue paper and it came back and hit you” Concluded most of atom is empty space except for a very small force within atom Called positive bundle of matter the “nucleus”
Atom consists of proton, neutron, and electron Proton charge = +1 Neutron charge = 0 (neutral) Electron charge = -1 Protons and Neutrons located in nucleus 99.9% of atom’s mass is in nucleus Electrons located outside the nucleus
Ag 107.87 Silver 47 Atomic number Name of the element Element Symbol Atomic mass
Atomic Number equal to number of protons in an atom Element Symbol First letter always capitalized If second letter exists, it is lowercase
Too difficult to measure elements in “grams” so we use the atomic mass unit Approximately the mass of 1 proton or 1 neutron Relative to the carbon atom 1 amu is 1/12 the mass of the carbon atom
Isotopes are atoms of the same element having different masses due to varying numbers of neutrons.
average Atomic mass is the average of all the naturally isotopes of that element. Carbon = 12.011
Mass Number = Protons + Neutrons Not found on periodic table Isotopes have different mass numbers (due to neutrons)
JJ Thomson won the Nobel prize for describing the electron as a particle His son, George Thomson won the Nobel prize for describing the wave-like nature of the electron. The electron is a particle! The electron is an energy wave!
Much of what has been learned about atomic structure has come from observing the interaction of visible light and matter.
1924 1924 De Broglie suggested that electrons have wave properties to account for why their energy was quantized. He reasoned that the electron in the hydrogen atom was fixed in the space around the nucleus. He felt that the electron would best be represented as a standing wave. As a standing wave, each electron’s path must equal a whole number times the wavelength.
Louis deBroglie The electron propagates through space as an energy wave. To understand the atom, one must understand the behavior of electromagnetic waves.
Wavelength, Wavelength, The distance for a wave to go through a complete cycle. Amplitude Half of the vertical distance from the top to the bottom of a wave. Frequency, Frequency, The number of cycles that pass a point each second.
Longer wavelength = lower frequency = lower energy Shorter wavelength = higher frequency = higher energy
The SI unit of frequency ( ) is the hertz, Hz 1 Hz = 1 s -1 Wavelength and frequency are related c = c = c is the speed of light, 2.998 x10 8 m/s
The wavelength of an argon laser's output is 488.0 nm. Calculate the frequency of this wavelength of electromagnetic radiation. c = c = Convert nm to m 488 nm x (1 m / 10 9 nm) = 4.88 x 10 -7 m Then, substitute into c = λν (4.88 x 10 -7 m) (v) = 3.00 x 10 8 m s -1 v = 6.15 x 10 14 s -1 = 6.15 x 10 14 Hz
Electromagnetic Radiation Energy in the form of transverse magnetic and electric waves. Electromagnetic Spectrum Contains all forms of electromagnetic radiation Visible spectrum Portion of electromagnetic spectrum that we can see (colors)
‘White’ light is actually a blend of all visible wavelengths. They can separated using a prism.
Neils Bohr studied the spectra produced when atoms were excited in a gas discharge tube.
Each element produces its own set of characteristic lines
Bohr proposed a model of how electrons moved around the nucleus. He wanted to explain why electrons did not fall in to the nucleus. He also wanted to account for spectral lines being observed. He proposed that the energy of the electron was quantized - only occurred as specific energy levels.
In the Bohr model, electrons can only exist at specific energy levels (orbit). Each energy level was assigned a principal quantum number, n. Energy
The Bohr model is a ‘planetary’ type model. Each principal quantum represents a new ‘orbit’ or layer. The nucleus is at the center of the model.
Absorption Absorption – Electromagnetic radiation is absorbed by an atom causing electrons to jump to a higher energy state (excited state). Emission Emission – Energy is released by an atom as particle of light (photon) as electrons fall back to the lower energy state (ground state). Depending on frequency of photon, different colored light may be seen
Although electromagnetic radiation has definite wave properties, it also exhibits particle properties. Photoelectric effect. First observed by Hertz and then later explained by Einstein. When light falls on Group IA metals, electrons are emitted (photoelectrons). As the light gets brighter, more electrons are emitted. The energy of the emitted electrons depends on the frequency of the light.
The energy of a photon is proportional to the frequency. (Photon energy) E= h (Photon energy) E= h The energy is inversely proportional to the wavelength (remember c = λν so v = c/λ ). E = hc / E = hc / h is Plank’s constant, 6.626 x 10 -34 J. S c is the speed of light, 2.998 x10 8 m/s
Determine the energy, in kJ/mol of a photon of blue-green light with a wavelength of 486 nm. E = = = 4.09 x 10 -19 J h c (6.626 x 10 -34 J. s)(2.998 x 10 8 m. s -1 ) (4.86 x 10 -7 m)
=wavelength, meters h=Plank’s constant m=mass, kg v=frequency, m/s = hmvhmv
Using De Broglie’s equation, we can calculate the wavelength of an electron. = 6.6 x 10 -34 kg m 2 s -1 (9.1 x 10 -31 kg)(2.2 x 10 6 m s -1 ) The speed of an electron had already been reported by Bohr as 2.2 x 10 6 m s -1. = 3.3 x 10 -10 m = hmvhmv
In order to observe an electron, one would need to hit it with photons having a very short wavelength. Short wavelength photons would have a high frequency and a great deal of energy. If one were to hit an electron, it would cause the motion and the speed of the electron to change. According to Heisenberg, it is impossible to know both the position and the speed of an object precisely.
Schrödinger developed an equation to describe the behavior and energies of electrons in atoms. His equation is similar to one used to describe electromagnetic waves. Each electron can be described in terms of its quantum numbers.
Each electron in an atom has a unique set of 4 “numbers” which describe it Energy level Orbital shape Orientation Spin
Principal quantum number, n Tells the size of an orbital and largely determines its energy. n = 1, 2, 3, ……
Angular momentum The number of subshells that a principal level contains. It tells the shape of the orbitals. s p d f Orbitals An orbital is a region within an energy level where there is a probability of finding an electron Orbital shapes are defined as the surface that contains 90% of the total electron probability.
Magnetic quantum number, m l Describes the direction that the orbital projects in space. Think in terms of axes “x, y, z”
Pauli added one additional quantum number that would allow only two electrons to be in an orbital. Spin quantum number, m s. An electron can spin clockwise or counterclockwise
Pauli exclusion principle Pauli proposed that no two electrons in an atom can have the same set of four quantum numbers Unless you live together, none of you have the exact same address
Aufbau Principle Electrons are placed into orbitals, subshells, and shells in order of increasing energy
Hund’s Rule The most stable arrangement of electrons in a subshell is the one in which electrons have the most number of parallel spins possible.
Graphical representation of an electron configuration One arrow represents one electron Shows spin and which orbital within a sublevel Follow all rules(Aufbau principle, two electrons in each orbital, etc. etc.)
Use atomic number as number of electrons in an atom He Be Mg Si Ne
Diamagnetism Elements have all of their electrons spin paired All of an element’s subshells are completed Not affected by magnetic fields Paramagnetism Not all electrons are spin paired in an element Most elements are this Affected by magnetic fields
A list of all the electrons in an atom (or ion) Must go in order (Aufbau principle) 2 electrons per orbital, maximum We need electron configurations so that we can determine the number of electrons in the outermost energy level. These are called valence electrons. 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 6 6s 2 4f 14 … etc.
2p 4 Energy Level Sublevel Number of electrons in the sublevel
Orbitals grouped in s, p, d, and f orbitals (sharp, proximal, diffuse, and fundamental) s orbitals p orbitals d orbitals f orbitals
d and f orbitals require LARGE amounts of energy It’s better (lower in energy) to skip a sublevel that requires a large amount of energy (d and f orbtials) for one in a higher level but lower energy
A way of abbreviating long electron configurations Since we are only concerned about the outermost electrons, we can skip to places we know are completely full (noble gases), and then finish the configuration Find the closest noble gas to the atom (or ion), WITHOUT GOING OVER the number of electrons in the atom (or ion). Write the noble gas in brackets [ ]. Step 2: Find where to resume by finding the next energy level. Step 3: Resume the configuration until it’s finished. Example: [Ne] 3s 2 3p 5
Remember d and f orbitals require LARGE amounts of energy If we can’t fill these sublevels, then the next best thing is to be HALF full (one electron in each orbital in the sublevel) There are many exceptions, but the most common ones are For the purposes of this class, we are going to assume that ALL atoms (or ions) that end in d 4 or d 9 are exceptions to the rule. This may or may not be true, it just depends on the atom.
d 4 is one electron short of being HALF full In order to become more stable (require less energy), one of the closest s electrons will actually go into the d, making it d 5 instead of d 4. For example: Cr = [Ar] 4s 2 3d 4 Since this ends exactly with a d 4 it is an exception to the rule. Thus, Cr = [Ar] 4s 1 3d 5 Remember, half full is good… and when an s loses 1, it too becomes half full! d 9 works the same way