Home Up E1 E2 E3 E4 Topics - E1 Topics E2 Topics E3 Topics E4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 27 28 29 30 31


Atomic Theory

Elements - made up of small chemically indivisable particles - atoms.   Atoms are the building blocks of chemistry.

Dalton (1804)

1. elements composed of atoms, the smallest particle into which an element can be divided and still retain its identify as an element

2. atoms of the same element are identical in most ways

3. Atoms of different elements can combine only in certain ratios in forming compounds.  For example the ratio of hydrogen (H) atoms to oxygen (O) atoms in the compound water is always (2 to 1)  (essentially the law of constant composition Proust)

4. Chemical reactions involve the union separation or rearrangment of atoms.

5. Atoms of one element cannot be changed into atoms of another by chemical reactions.

Dalton, did not know what an atom looked like!

So what does an atom look like?


high mass density
neutrons - neutral
protons - positive charge

Outside of nucleus

Electrons - negative charge - surround the nucleus


What are the properties of the particles that make up an atom?

Name of Particle Symbol Position Relative Charge Absolute Mass (g) Approx relative mass (amu)*
Neutron n Inside Nucleus 0 1.625x10-24 1
Proton p Inside Nucleus +1 1.672x10-24 1
Electron e or e- Outside Nucleus -1 9.107x10-28 5.5x10-4 ~ 0

*1 amu = 1.66 x 10-24 g = 1/12 or the mass of the carbon atom that contains 6 protons, 6 neutrons, and 6 electrons.

How much do you weigh in atomic mass units (amu)? 




1 amu

= 4.1x1028 amu




1.66 x 10-24g


That's 41,000,000,000,000,000,000,000,000,000. g.

How big is your typical atom?

Approximately, 1x10-10m = 0.1nm

Put one atom in a row for every person on the earth and you get a line about 50cm long.
The size is mainly determined by the electrons, the nucleus is 100,000 time smaller (a walnut in a football stadium) - atoms are mostly empty space!

Building Up Atoms

Simplest atom - Hydrogen atom

1 proton, 0 neutrons, 1 electron

As you build atoms, you add protons, neutrons and electrons.

# electrons = # of protons
# neutrons ~ # protons for "light" elements
> # protons for "heavy"  elements

A= mass number = # of protons + # of neutrons (mass in amu, electron neglected)

Z= atomic number = # of protons

The atomic number (number of protons) defines an element.  It is the unique property that distinguished one element from another. 

Representing elements:

Note: The symbol X is from the periodic table.  The first letter of the symbol is always upper case, and for two letter symbols, the second letter is always lower case.

Isotopes - atoms that have the same number of protons (hence all the same element) but different numbers of neutrons.

Element # of protons # of electrons # of neutrons mass number (amu)   natrual abundance
Carbon 6 6 6 12 98.89%
Carbon 6 6 7` 13 1.11%
Carbon 6 6 8 14 trace
Hydrogen 1 1 0 1 99.98%
Hydrogen 1 1 1 2 (deuterium) 0.015%
Hydrogen 1 1 2 3 (tritium) trace

Let's turn it around:

  How many protons? How many electrons? How many neutrons? Natural Abundance
92 92 238-92 = 146 99.27%
92 92 235-92 = 143 0.72%
92 92 234-92 = 142 0.01%

What's important about the number of neutrons?  Can greatly affect stability.

(fission reactors), (breeder reactors)


Atomic mass (not mass number):  Look at periodic table:

atomic #


atomic mass

Atomic mass of hydrogen = 1.0079.  Why not 1?? 

Chemists work with collections of atoms that contain all isotopes of an element.  Small amount of deuterium and tritium result in the atomic mass of hydrogen being greater than one.

The atomic mass - average mass of the mixture of isotopes that reflects the masses and relative abundance of the elements as they occur in nature. 

How did we come to our current picture of the atom - brief history following that presented by G. Tyler Miller, in Chemistry: Principles and applications

Illustration Description Evidence

Greek philosopher Democritus - Greek atomic school of natural philosophy
- atomos = "indivisible"

Plato and Aristotle did not agree and their opinion that matter was continuous prevailed

No Evidence

Atomic theory (building blocks)
 - matter consists of tiny particles called atoms
- atoms cannot be created, destroyed, subdivided or converted from one type to another by chemcal means
- atoms of a particular element all have the same properties
- atoms combine in simple whole number ratios
- chemical change involves the joining separation and rearrangement of atoms

1. Law of conservation of mass (LaVoisier, 1789)

2. Law of constant composition (Proust, 1799) - mass percentage of a compound always the same.  Water is always 88.9% oxygen by mass

Atoms made up of smaller particles.


Thomson - observations of negative particles that would become known as electrons in cathode ray tube (your TV)

Goldstein - observation of positively charged particles (protons) by a similar experiment. 


Atoms mainly empty space with the majority of the mass concentrated in a small, positively charged region or nucleus "It was as though you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you"  referring to the results of his experiments on passing certain types of particles through atoms.
Electrons in an atom are "quantized" - allowed to travel only in certain circular orbits and varying radii. "Line spectra" - light is emitted by elements at only certain discrete energies rather than at all energies. 
Quantum mechanical model - electron location can only be spoken of in terms of a probability of being at a particular location. Lots of stuff:

Binnig and Rohrer - Nobel prize for seeing an atom - scanning tunneling microscopy