Atomic Concepts

Review: Chemistry is the study of matter and the changes it undergoes.

Matter is anything that has mass and takes up space.

What’s the matter?

All matter is made of atoms.

Then… what’s an atom?

Everything!

Atomic Concepts

Where did the idea of the atom come from? Democritus(~450 BC) – an ancient Greek

– Coined the term atomos - a particle so small, it is

“uncuttable.”

– Only a philosophical idea, no scientific proof.

• Antoine Lavoisier(1789) - French scientist

– Developed the Law of Conservation of Matter:

– “Matter can neither be created nor destroyed in a chemical reaction.”

– Matter doesn't appear or disappear in a reaction, it only changes

from one form to another.

Atomic Models

John Dalton's Model (1803)John Dalton's Model (1803)All matter is made of atoms.Atom- the smallest particle and fundamental building block of

all matter.• Dalton imagined them as simple, hard spheres.• Atoms can not be created nor destroyed.

• The atoms of the same element are identical & different elements have different atoms.

• Molecules- simple combinations of different atoms.

Atomic Models

J.J. Thompson’s Model (1897) Discovered the electron from his Cathode Ray Tube Experiment

electron –a negatively charged particle found in an atom. Has a mass equal to roughly 1/2000 of a proton. Thompson did not know about protons or neutrons

Believed atoms were positive spheres with electrons embedded inside

Was called “Thompson’s pudding” (looks like a chocolate chip cookie to me)

Atomic Models

Ernest Rutherford’s Model (1909)• Gold Foil Experiment

•Alpha particles (positively charged helium atoms) from a radioactive source were aimed at super-thin gold foil•fluorescent screen detected where particles went• Most particles passed through, but a few were deflected

• This proved:1. the atom is mostly empty space. 2. the nucleus of the atom is very small & has a positive charge.

Atomic Models

Max Planck (1900)•Proved that energy could be absorbed and radiated like particles•Called them quanta (quantum) – the minimum amount of something

Neils Bohr (1913)•Used Planck's theory•Demonstrated electrons in an atom absorb and release energy at specific wavelengths•Believed electrons were located in areas of space called principal energy levels.•energy levels were depicted as orbits around the nucleus of an atom, like planets in the solar system

Atomic Models

Bohr Model Continued•Electrons could move between energy levels

• absorb energy to go to a higher level• Release energy to drop to a lower level• Ground state- electrons are all in the lowest possible energy level• Excited state- one or more electrons have absorbed energy and moves to a higher

energy level• Excited state electrons are unstable and will release energy to become stable

•Energy is released at specific wavelengths of light, some of it visible. This is called spectra.

Atomic Models

1. An electron drops from the 4th energy level(n=4) to the 1st energy level(n=1). At what wavelength is the energy released?Answer: 97 nanometers(nm)

2. Is the energy released from an electron dropping from the 6th energy level(n=6) to the 2nd(n=2) in the visible range of light?Answer: Yes, at 410 nm.

3. An electron drops from the 6th energy level to the 3rd energy level. What wavelength of light is released?Answer: 1094nm4. Can it be seen by the human eye?Answer: No, it is in the infrared range.

Bohr Model(Continued)

Atomic Models

Wave-Mechanical Model (1926 to present)Erwin Schrödinger's contributions:•Like light, electrons can move like waves•A standing wave around the nucleus•Reclassified Bohr’s principle energy levels as orbitals- a region of space around the nucleus where an electron is most likely to be found.

Werner Heisenberg’s contributions:•Heisenberg’s Uncertainty Principle - location and speed of an electron cannot be predicted at the same time•The result is an electron cloud, where electrons are likely to be (as opposed to having a definite orbit)•Other Names for the Wave-Mechanical Mdl:

•Electron Cloud Model•Modern Model