The Life and Times of Atom A story of one atom’s coming of age

BIRTH » Dalton: pictured the atom as a tiny indestructible sphere Atom’s baby picture » Dalton: pictured the atom as a tiny indestructible sphere Atom’s baby picture

Early Childhood (The awkward years) 1897-Thomson: discovered very light weight negatively charged particles (electrons) Chemists determined that the negative charge must be balanced by a positive charge: the raisin bun model

Early Adolescence Rutherford (McGill University) - publishes the results from the famous gold-foil experiment

The Gold-Foil Experiment

Shocking Results!!!  Until this point, atoms were thought to be solid throughout  Most of the alpha particles went right through the foil!  Some alpha particles curved when they went through  Only a few alpha particles deflected back (This was the expected result - think of running into a solid wall)  Until this point, atoms were thought to be solid throughout  Most of the alpha particles went right through the foil!  Some alpha particles curved when they went through  Only a few alpha particles deflected back (This was the expected result - think of running into a solid wall)

Gold Foil Conclusions  The atom is made up of mostly empty space  Alpha particles are positive, they curved if they got too close to the small nucleus  Only alpha particles that hit the nucleus were deflected back, since this rarely happened, the nucleus must be very small!  The atom is made up of mostly empty space  Alpha particles are positive, they curved if they got too close to the small nucleus  Only alpha particles that hit the nucleus were deflected back, since this rarely happened, the nucleus must be very small!

Atom’s Troubled Teen-aged Years  An entirely positive nucleus would explode (+ charges repel)  The total mass of the atom couldn’t be accounted for  Atom gets a girlfriend! The neutron is discovered  An entirely positive nucleus would explode (+ charges repel)  The total mass of the atom couldn’t be accounted for  Atom gets a girlfriend! The neutron is discovered

Rutherford’s Model of the Atom  The nucleus is small and made up of protons and neutrons  The electrons circle around the nucleus  The nucleus is small and made up of protons and neutrons  The electrons circle around the nucleus

Problems in Paradise??? Rutherford’s model doesn’t quite work: ZElectrons should lose energy and crash into the nucleus (this clearly doesn’t happen) Z19th century physics dictates that a body in motion must continuously give off energy - seen as a continuous spectrum through a spectroscope - but we see a line spectrum

Bohr’s Addition to the Atom  Bohr explains why a line spectrum is seen instead of a continuous spectrum  Electrons are only giving off certain frequencies of light  Electrons travel in defined spaces called orbitals, which have a defined energy

How does a line spectrum tell us all that? When an electron is excited (given energy) it jumps from one orbital to a higher orbital The electron does not stay excited and eventually goes back to its ground state (original orbital) A wave of light is emitted (photon) from this process which can be seen as a line on a line spectrum

Problems with Bohr’s Theory  Bohr couldn’t explain why lines appeared in ones, threes, fives and sevens - more on this later!  Physicist Max Planck supported Bohr’s idea that atoms can absorb or emit only discrete quantities of energy called quantums  Einstein called these ‘packets’ of energy photons  Bohr couldn’t explain why lines appeared in ones, threes, fives and sevens - more on this later!  Physicist Max Planck supported Bohr’s idea that atoms can absorb or emit only discrete quantities of energy called quantums  Einstein called these ‘packets’ of energy photons

Adulthood  Schrodinger - derived the quantum mechanical model of the atom  Described electrons as having wave-like properties  Mathematically determined the shape of orbitals and the probability of an electron being in a certain place at a certain time - orbitals are not just spheres anymore!  Heisenburg - Heisenburg Uncertainty Principle: Although the shape of the orbital is predictable, the exact location of an e- can not be determined  Schrodinger - derived the quantum mechanical model of the atom  Described electrons as having wave-like properties  Mathematically determined the shape of orbitals and the probability of an electron being in a certain place at a certain time - orbitals are not just spheres anymore!  Heisenburg - Heisenburg Uncertainty Principle: Although the shape of the orbital is predictable, the exact location of an e- can not be determined

Atom’s Portrait 1927

The Four Quantum Numbers (which are actually letters)

Why Use Quantum Theory?  Quantum is the ‘new and improved’ Bohr-Rutherford diagram  This model shows e- placement which helps us determine valence e- and stability of an atom, this allows us to predict atom behaviour  Each orbital can hold a maximum of 2e-  Quantum is the ‘new and improved’ Bohr-Rutherford diagram  This model shows e- placement which helps us determine valence e- and stability of an atom, this allows us to predict atom behaviour  Each orbital can hold a maximum of 2e-

Orbital Shapes & Orientation s is a sphere shape - 1 orientation = 1 orbital = 2e- p is a figure eight - 3 orientations = 3 orbitals = 6e-

d orbitals have a ‘flower’ shape - 5 orientations = 5 orbitals = 10 e- f orbitals have many shapes - 7orientations in = 7 orbitals = 14 e-

Rules for Quantum 1.Aufbau Principle - each e- is added into the subshell with the lowest E orbital available 2.Hund’s Rule - Each orbital subshell gets a single electron first and then e- can pair. All e- are ‘up’ when single 3.Pauli Exclusion Principle - no e- can have the same 4 quantum #s in an atom - e- sharing an orbital have opposite spins 1.Aufbau Principle - each e- is added into the subshell with the lowest E orbital available 2.Hund’s Rule - Each orbital subshell gets a single electron first and then e- can pair. All e- are ‘up’ when single 3.Pauli Exclusion Principle - no e- can have the same 4 quantum #s in an atom - e- sharing an orbital have opposite spins