History of the Atom

Imagine if you were a scientist in the time of the Greek philosophers. How do you see the world? What are we made out of? If we could look closely, what would we see?

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.

Atomos = indestructible

(Democritus)

http://home.wlu.edu/~mahonj/Ancient_Philosophers/Atomists.htm

• 460-370 B.C.• He hypothesized that all

matter is composed of tiny indestructible units called atoms.

• Atoms and voids• Atoms are solid.• Different shapes =

different properties.• Not based on scientific

method.

John Dalton

• 1766-1844• English chemist and a

schoolteacher• Used the scientific method to

study Democritus's ideas> Studied the ratios in

which elements combine in chemical reactions.

> A compound always contains the same ratio of elements (by mass).

http://www.biography.com/people/john-dalton-9265201

Dalton's Atomic Theory (1808)

1. All matter is made of indestructible particles called atoms.

Dalton's Atomic Theory (1808)

1. All matter is made of indestructible particles called atoms.

2. All atoms of the same element are identical.

Dalton's Atomic Theory (1808)

1. All matter is made of indestructible particles called atoms.

2. All atoms of the same element are identical.

3. Atoms cannot be created or destroyed in chemical reactions: They just rearrange.

4. In a chemical reaction, atoms of one element combine with atoms of other elements to form compounds in whole ratios* Law of definite proportions

J. J. Thomson

• 1856-1940• English physicist• Cathode ray tube experiment

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

J. J. Thomson

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

Observations:

1. The cathode ray is deflected by a magnetic field.

2. The cathode ray is repelled by a negative electric field.

3. The cathode ray is the same kind of negative particle no matter what metal is used.

J. J. Thomson

Conclusions:

Cathode rays are streams of negatively charged particles.

Determined the charge to mass ratio e/me.

Discovered first subatomic particle = electrons.

J. J. Thomson

J. J. Thomson discovered the electron, which has a negative charge.

He knew that an atom is neither positively or negatively charged.

Please draw a diagram of an atom that explains J. J. Thomson's findings.

William Thomson (Lord Kelvin)

http://en.wikipedia.org/wiki/Plum_pudding_model

Plum pudding model:

Uniform "pudding" of positive charge with electrons scattered throughout.

Total charge is neutral.

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

Ernest Rutherford

• 1871-1937• New Zealand physicist• Gold foil experiment

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

Can this happen with the plum pudding model?

What is a better model for the atom?

*Please draw a model for the atom that incorporates Rutherford's observations.*

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.

Conclusions:

1. Atom is mostly empty space.

2. Atom's mass is concentrated at the center (nucleus)

3. Nucleus has a positive charge-made of protons

Ernest Rutherford

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

James ChadwickDiscovered neutrons--a particle in the nucleus with no charge (neutral)

Robert Milikan

• 1868-1953• Oil drop experiment• Suspended negatively charged oil droplets between

two charged plates.• Calculated how strong an applied electric field has to

be to stop the drop from falling due to gravity.

Robert Milikan

He noticed that the charge was always a multiple of -1.60 x 10-19 Coulombs.

Conclusions:

1. The charge of an electron is -1.60 x 10-19 Coulombs.

We know much more now than Dalton did when he first described his "Atomic theory". Some of the laws still hold true:

Law of Conservation of Mass-In a chemical reaction, atoms (and therefore mass) are never lost or gained, only rearranged.

Law of Definite Proportions (constant composition)-in a pure compound, the proportions of elements by mass are always the same.

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

Parts of the Atom

Parts of the Atom

Particle Location Relative  Charge

Mass  (amu’s) Symbol

Proton Nucleus +  1 1.0073 p+  or  11H

Neutron Nucleus 0 1.0087 10n  or  n0

ElectronOrbital -­‐1 .00055 0-­‐1e  or  e-

Parts of the Atom

The nucleus makes up a very small part of the atom's volume.

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

Elements

Elements are defined by the number of protons in their nucleus = atomic number (Z)

Each element has a symbol (look at periodic table). For Example, carbon is C and Neon is Ne.

Isotopes = Atoms of the same element with different number of neutrons

Example: Hydrogen had 3 naturally occurring isotopes.

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

Ions

Ions are atoms that have gained or lost electrons.

• # electrons ≠ # protons

• e- > p+ --> anion (negative charge) Xn-

• e- < p+ --> cation (positive charge) Xn+

http://www.gcsescience.com/a4-sodium-ion.htm

Practice: How do you write a Zinc isotope with 35 neutrons in standard nuclear notation?

Standard Nuclear Notation:

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

atomic number (Z) = # of protons

mass number (A) = # of protons + # of neutrons

-Identifies isotopes

http://intro.chem.okstate.edu/1215F99/Lecture/Lec101599.html

Atomic number (Z)

Average atomic mass

Atomic mass = total mass of an atom.

Average atomic mass = weighted average mass of all isotopes of an element that occurs in nature.

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.