Bohr's Model of the Atom

The gateway to our current ideas about atoms

SCH 3U0/4C0

[physicsoftheuniverse.com]

Back to Thomson...

JJ Thomson [wikimedia]Thomson's cookie model. Mmmm, cookies

[dipity.com]

...And Rutherford

Rutherford [nobelprize.org]Tossing cookies? Ewww.

[reich-chemistry.wikispaces.com]

[blogspot.com]

Problems with Rutherford's Model

● Electrons in orbits radiate.● When they give off radiation, they lose

energy.● When they lose energy, they slow down.● When they slow down, they fall into the

nucleus.● When they fall into the nucleus, the atom

ceases to exist.

Rutherford's Problems (cont'd)

● So Rutherford's model says all atoms should collapse.

● Problems with this? Look around you!

Atomic collapse looks like this.[salishsea.typepad.com]

Bohr's Solution

● Electrons aren't particles in orbits (like planets round the sun): they're waves!

● These electron waves can't have any old orbit: they only orbit at certain allowed distances. The simplest electron wave around

a nucleus.

Bohr's Solution (cont'd)

The second simplest electron wave The third simplest electron wave

Bohr's Solution (cont'd cont'd)

● Electron waves are only allowed at distances where their wave joins up “nicely”.

Nice on the left. Not nice on the right. What dowe mean when we say nicely? [wikimedia]

[astro.queensu.ca]

Proof of Bohr's Model

● Electrons can only have certain orbits.● Because only certain orbits are allowed,

they can only give off certain colours: i.e., spectroscopy!

● No other model could predict that atoms should have discrete spectral lines.

Spectroscopy

Is used to identify the compositions of stars, for example [astro.queensu.ca]

Names For Where Electrons Wave Around Atoms

● The places where electrons wave around an atom is called an orbital.

● These orbitals occur in energy levels.● There are four kinds of orbitals.● Each orbital can be made up of

suborbitals.

Details About spdf Orbitals

● s holds up to 2 e-; 1 suborbital.

● p holds up to 6 e-; 3 suborbitals.

● d holds up to 10 e-; 5 suborbitals.

● f holds up to 14 e-; 7 suborbitals.

There are other orbitals, but we won'tneed or see them in this course. How manyelectrons does each suborbital hold up to?

Filling Orbitals with Electrons

The filling order diagram. Being a littleconfused at this point is normal.

● The number in front of each orbital is called the energy level.

● Follow the arrows, one at a time, from the top down, to put electrons around any atom.

How Many Electrons are in an Atom?

JIC: The atomic number is the one in red.

● Look at the atomic number.

● How many electrons are in an atom of H? He? C? U? F?

● Then we use the filling order diagram to place the electrons.

Electron Configurations (ECs)

● Use the filling order diagram.

● How are electrons arranged around an atom of: H? Li? C? Fe? Cu? Lr?

● These arrangements are called electron configurations (ECs)

[chemtutor.com]

[jamesannjo.wordpress.com]

Lies I Was Told?

● What happened to “the first shell holds two, the second shell holds eight, the third shell holds [depends], and don't go past element 20?”

● How many does the third “shell" hold? The fourth?

● For those not afraid of math: number of electrons in a “shell” is 2n2, where n is the energy level (“shell” number).

Truths to Still Hang on To?

● Remember valences? Valences are how many electrons an atom wants to gain or lose to be stable.

● What does “stable” mean? In grade 9/10, full or empty.

You can tell the number of valenceelectrons for non-transition metals

directly from the PT [kgortney.pbworks.com]

Predicting Valences for Non-Transition Metals with ECs

● Use the ECs we've already constructed to make sense of the valences for those elements.

Go one more column, and predict valences [sparknotes.com]

[chemwiki.ucdavis.edu]

Transition Metals: Multivalent Atoms (MVAs)

● Cu is one or two;

Fe is two or three;

Hg is two or four;

Some MVAs; there are many more.● Use their ECs to make sense of their

valences.

And – as Always – There's a Pattern to Things

What is it about this pattern that allows the transition metals to bemultivalent? [chemwiki.ucdavis.edu]