October 24, 2014

History of the Atom

• We know today that all matter is composed of atoms.• Atoms: basic unit of matter. Atoms are the smallest

particle of an element that retains its identity.

How did scientists figure out the structure of the atom? Can you just "look" at it?

October 24, 2014

Democritus and John Dalton

• Democritus: Greek philosopher, believed matter was made of indestructible particles.

• John Dalton: Used the scientific method to test Democritus's ideas and came up with the atomic theory.

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October 24, 2014

J. J. Thomson

• Cathode ray tube experiment

http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter02/Text_Images/FG02_03.JPG

October 24, 2014

J. J. Thomson

• Cathode rays are rays emitted when high voltage is applied between two electrodes in an evacuated glass tube.

• Observations:> The cathode ray is deflected by a magnetic field.> The cathode ray is repelled by a negative electric

field.> The cathode ray is the same kind of negative

particle no matter what metal is used.

October 24, 2014

JJ Thomson knew that an atom is neutrally charged and contains electrons.

What model of the atom explains J. J. Thomson's findings?

J. J. Thomson

• Conclusions:> Cathode rays are streams of negatively charged

particles. – Discovered first subatomic particle: electrons.

October 24, 2014

Plum pudding model

• Plum pudding model (or chocolate chip ice cream)> Uniform "pudding" of positive charge with

electrons scattered throughout.> Total charge is neutral.

http://en.wikipedia.org/wiki/Plum_pudding_modelhttp://en.wikipedia.org/wiki/Christmas_pudding

October 24, 2014

Ernest Rutherford

• Gold foil experiment

http://staff.norman.k12.ok.us/~cyohn/index_files/atom1notes.htm

October 24, 2014

Can this happen with the plum pudding model?

What is a better model for the atom?

Ernest Rutherford

• Observations:> Most of the alpha ( α) particles (positively charged

particles) passed straight through the foil.> Some were deflected at slight angles, a few even

backwards.

October 24, 2014

Ernest Rutherford

• Conclusions: > Atom is mostly empty space.> Atom's mass is concentrated at the center

(nucleus)> Nucleus has a positive charge-made of protons

http://myweb.usf.edu/~mhight/goldfoil.html

October 24, 2014

http://kids.britannica.com/comptons/art-156255/In-Niels-Bohrs-model-of-the-atom-electrons-can-circle

Parts of the Atom

October 24, 2014

Parts of the Atom

• Proton: positively charged particle found in the nucleus (charge opposite of electron)> Mass of 1.0073 amu (2000x bigger than electron)> Atomic number: (Z) # of protons in nucleus,

identifies elements

• Neutron: neutral particle found in the nucleus> Mass of 1.0087 amu (about the same as a proton)> Number of neutrons determines isotopes

http://chemistry.tutorcircle.com/inorganic-chemistry/atomic-structure.html

October 24, 2014

Parts of the Atom

• Electron: negatively charged particle that occupies a 3D region around the nucleus called orbitals> Mass of 5.5 x 10-4 amu (1/2000 the mass of a

proton)> Electrons are involved in chemical reactions.

http://chemistry.tutorcircle.com/inorganic-chemistry/atomic-structure.html

October 24, 2014

Parts of the Atom

• The nucleus makes up a very small part of the atom's volume and contains the protons and neutrons.

• The electrons occupy a 3D regions of space called orbitals that surround the nucleus.

http://kids.britannica.com/comptons/art-156255/In-Niels-Bohrs-model-of-the-atom-electrons-can-circle

October 24, 2014

Electron Orbitals and Energy Level

• Quantum Mechanical Model of the Atom> Describes electrons as waves> Electrons are found in 3-D atomic orbitals.

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October 24, 2014

Electron Orbitals and Energy Levels

• Principle energy level (shell) (n) indicates the relative size and energy of the various atomic orbitals.> n= integers: n=1, 2, 3...> As n gets bigger

– the orbitals (shell) gets larger– the electrons spends more time away from the

nucleus– the electrons are less tightly bound to nucleus

http://www.chem.ox.ac.uk/vrchemistry/Machinery/html/page02.htm

October 24, 2014

2818

32

Electron Orbitals and Energy Level

• Each shell can hold a maximum of 2n2 electrons.> n=1 can hold 2 electrons> n=2 can hold 8 electrons> n=3 can hold 18 electrons> n=4 can hold 32 electrons

• Valence electrons: electrons in outermost shell, involved in forming bonds.

middle school?

October 24, 2014

Parts of the Atom

October 24, 2014

Which element has 35 protons?

Which element is Z=12?

Elements

• Atomic number (Z): Number of protons in their nucleus. Defines elements.

http://chemistry.about.com/od/workedchemistryproblems/a/isotopes-nuclear-symbols-1.htm

October 24, 2014

Isotopes

• Isotopes: Atoms of the same element with different number of neutrons> Isotopes have the same # of protons, different #

of neutrons• Example: Hydrogen has 3 naturally occurring isotopes.

http://wikis.lawrence.edu/display/CHEM/5.+Isotopes+and+Ions+%28Brittany+Oleson%29

October 24, 2014

Isotopes

• Isotopes have different masses• Atomic mass: total mass of an atom.• Atomic mass unit (amu): unit to measure atomic

mass.> 1 amu = 1.66 x 10-24g> 1 amu = 1/12 the mass of a C-12 atom.

October 24, 2014

Standard Nuclear Notation

• Atomic number (Z) = # of protons• Mass number (A) = # of protons + # of neutrons

> Identifies isotopes

http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/nucnot.html

October 24, 2014

ExampleWrite the standard nuclear notation for the following:

• An atom with 8 protons and 9 neutrons.

• An atom with 15 protons and 15 neutrons.

• An atom with 34 protons and 38 neutrons.

October 24, 2014