atomic structure and function: outline early history in 18 th and 19 th centuries discovery of...

Atomic Structure and Function:Outline

• Early history in 18th and 19th centuries• Discovery of elements and the Periodic Table (1869)• Discovery of the electron (1897)• Discovery of the nucleus (1911)• Atomic structure with consternation• Atomic structure: the Bohr atom (1913)• Atomic Structure: Theory of Quantum Mechanics

(1930’s)• Applications of our understanding

Atomic Structure and Function:Take Home Message

• Science as a evolutionary, incremental process over decades (e.g., exploring the structure of atom)

• Role of hypothesis development and testing in the sciences

• Demystification of physics and chemistry: structure of the atom is a simple concept

• Understanding of atomic structure underpins future course topics (e.g., geology, biochemistry, environmental sciences, binding of chemicals)

• Applications – some examples

Early History in 18th and 19th Century

• Atomic Theory of Dalton– Atoms– Elements

– Molecules/compounds (e.g., water or H2O)

• Atoms as basis for understanding chemistry but without electrons, protons and neutrons (analogy to a marble)

Early History in 18th/19th Century:Discovery of Elements

• 30 elements known in early 1800’s• Mendeleev and the Periodic Table (1869)

– 63 elements known w/o any organization– Ordering of elements

• Mass from low to high• Chemical property by column (all behave similarly in a

chemical sense); examples– Column I: react with chlorine (1:1 ratio)

» Hydrogen (H), Lithium (Li), Sodium (Na), Potassium (K)

– Column VII: react with chlorine in (2:1 ratio)

» Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr)

Draft of Mendeleev’s Table of Elements

Mendeleev’s Contribution

• Arrange elements from light to heavy

• Arranged elements by chemical behavior

• Spatial ordering via rows and columns

• Product: Periodic Table of Elements– Predictive tool– Immensely creative insight (informatics)– Example of missing elements and predictive

capability of Table


• Early history in 18th and 19th centuries• Discovery of the electron (1897)• Discovery of the nucleus (1911)• Atomic structure with consternation• Atomic structure: the Bohr atom (1913)• Atomic Structure: Theory of Quantum

Mechanics (1930’s)• Applications of our understanding

Thompson’s Experiment: Discovery of the Electron

• Idea that there may be some smaller components that comprise atoms

• Hypothesis: Atoms consist of some subcomponents, one of which is negatively charged

• Experiment to test hypothesis

Rutherford’s Experiment: Discovery of a Nucleus

• Idea that there may be some smaller components that comprise atoms, including negative and positive charges components

• Hypothesis: Atoms consist of multiple subcomponents, some negative and some positive

• Experiment to test hypothesis– Alpha particles (+), gold foil, “bullets”, and “tracks”

Rutherford’s Experiment:

Explanation

Observations, Hypothesis and Conclusion

• Observations:– Almost all alpha’s passed through foil unaffected– Very small number of alpha’s deflected @ small

angle– 1/1000 deflected at large angle (struck “head on”)

• Hypothesis: Atoms consist of some positive and negative subcomponents

• Accept or reject

New Model of Atom

• Small, dense center, positively charged – called the nucleus (later to be called the proton)

• Small, light (w/o mass) negative charged subcomponent circles the nucleus (electron)

• Later additions (1932)– Neutrons– Protons– Charge (neutrality calls for + and – charges)

Atomic Structure with Consternation

Logic of consternation with Rutherford Model– Electrons moving, so must give off energy;– 2nd Law of Thermodynamics– Electrons should collapse– Yet atoms are billions of years old

Conclusions: Model violated fundamental laws of physics

Atomic Structure: Bohr Atom

• Observations (1913)– Heated hydrogen gas and light is emitted as

a discrete wavelength (not continuous spectrum)

– Other gases behave the same in producing discrete wavelength, but each gas unique in wavelengths produced


• Hypothesis– Electrons circle the nucleus at specific

distances from the nucleus in defined orbits

• Diagram


• Explanation of light in discrete wavelengths– As “excited”/heated electrons drop back from

orbit, a packet of energy is lost as a function of distance from the nucleus

– Packet of energy is called photon (light)


• Key aspects of the Bohr atom– No space between the orbits for electrons to be so

“jump” has to be “all or none” – quantum levels of electrons

– Excited electrons in outer orbit has 2 options:• Drop down to next lower orbit and emit a packet of energy

(light) unique for that distance• Drop back to even lower level (ground state) and emit

packet of energy (light) unique for that that distance– Energy release is unique for each element and is a

“window” on atomic structure”• Ideas verified two decades later using quantum

mechanics

Theory of Quantum Mechanics

• Observations– Behavior of electrons in heated hydrogen gas were

consistent with Bohr Model (orbits, etc.)– Behavior of other heavier gases could not be

explained by Bohr Model• Investigators:

– Heisenberg, Shrodinger, etc.• Key: Integrated mechanisms of waves and

particles, focusing on “fuzzy electron clouds”• Product: new model of atomic structure and

function– “Quantum mechanics”

Quantum Mechanics Model

• Key Principles– Electron behaves as a wave not a particle– Energy of electron behaves as a “spread-out

wave” – not discrete bundle of energy

• Description is based on quantum numbers– Distance from nucleus– Energy level and sublevel– Orientation of electrons in space– Direction of electron spin

Quantum Mechanics Model: Principle Energy Level

• Main energy level of electrons

• Distance from nucleus– n = 1 (lowest energy level)– n = 2– n = 3– n = 4 (highest energy level)– etc.

Quantum Mechanics Model: Sub-energy Levels

• Electrons in “fuzzy” probability distribution called an “orbital”– e.g., n = 1 has I sub-energy level/orbital (2

electron orientations)

• Electrons in an orbital can only exist in pairs (2) and the pair spin in opposite directions – Pauli Exclusion Principle

Quantum Mechanics Model: Electron Configuration

• Electrons always adopt most energetically stable energy levels/orbitals consistent with Laws of Thermodynamics and previous conditions (above)– n = 1 with 2 electrons in 1 orbital (s orbital)– n = 2 with 8 electrons in 4 orbitals (p orbitals)

• pw has 2 electrons• px has 2 electrons• py has 2 electrons• pz has 2 electrons

Applications of Our Understanding of Atomic Structure

• Spectroscopy: quantum differences in light emitted or absorbed result in unique signature for each element (“fingerprint”)

• LASER (Light Amplification by Stimulated Emission of Radiation)

• Binding of elements to create compounds• Neurotransmitters in the brain• Allergy reactions• Etc……………..

Atomic Structure and Function:Take Home Message

• Science as a evolutionary, incremental process over decades (e.g., exploring the structure of atom)

• Role of hypothesis development and testing in the sciences

• Demystification of physics and chemistry: structure of the atom is a simple concept

• Understanding of atomic structure underpins future course topics (e.g., geology, biochemistry, environmental sciences, binding of chemicals)

• Applications – some examples

atomic structure and function: outline early history in 18 th and 19 th centuries discovery of...

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