The Atom


Dalton's Atomic Theory (1803)

  1. All matter is composed of extremely small particles called atoms.

  2. All atoms of a given element are identical in size, mass, and other properties. Atoms of different elements differ in size, mass, and other properties.

  3. Atoms cannot be subdivided, created, or destroyed.
  4. Atoms of different elements can combine in simple, whole-number ratios to form compounds.

  5. In chemical reactions, atoms are combined, separated, or rearranged.

Adjustments to Dalton's Atomic Theory

We now know that atoms of the same element can have different masses and are called isotopes. Isotopes have a different number of neutrons than the "average" atom of an element. 

We have also discovered the possibility of atomic fission; splitting the atom (Hiroshima & Nagasaki). 

Modern Atomic Theory

  1. Elements are composed of atoms, which are extremely small. 

  2. All atoms of a given element have the same chemical properties and contain the same number of protons.

  3. Compounds are formed by the chemical combination of two or more different kinds of atoms. (Whole numbers only)

  4. Atoms are the units of chemical change.

Def:   An atom is the smallest unit of a substance that retains the properties of  that substance.

Atoms are composed of three types of particles:







Positive charge

Found in the nucleus



Neutral (no) charge

Found in the nucleus



Negative charge

Found orbiting the nucleus


Nucleus – The positively charged central region of an atom, composed of protons and neutrons and containing almost all of the mass of the atom.  More simply the nucleus is the “middle part” of an atom defined by an imaginary boundary separating it from the orbiting electrons.

Rutherford's view of the atom (1914) 


Bohr models of various atoms. (1920)  


Quantum model of a sodium atom. (Current)

Atomic Number- Determined by the number of protons

Atomic Mass Unit- mass of one proton or one neutron (they are about the same)

Atomic Mass- Mass of an atom: determined by adding the protons and neutrons
            (Actually this number is an average; re. isotopes)


    Atomic Number
    Atomic Symbol
    Atomic Mass


The number of protons is equal to the number of electrons
    p+ =  e-


The mass number minus the atomic number equals the number of neutrons
    mass #  -  atomic # = N0

Orbital- three-dimensional region about the nucleus in which a particular electron can be found.


Quantum numbers- numbers that specify the properties of atomic orbitals and of their electrons.

Principal Quantum number- indicates the main energy levels surrounding a nucleus, sometimes referred to as shells or orbitals. (1,2,3,4,5,6,7) Larger numbers are farther away from the nucleus and electrons found here have more energy.

Orbital quantum number- indicates the shape of an orbital. Often referred to as sublevels or subshells. (s,p,d,f in order of increasing energy) 

s orbitals can hold 2 electrons
p orbitals can hold 6 electrons
d orbitals can hold 10 electrons
f  orbitals can hold 14 electrons

Magnetic quantum number- indicates the orientation of an orbital about the nucleus. (x,y.z coordinates)

Spin quantum number- indicates the spin of an electron (+1/2, or -1/2)

Pictures of Atomic Orbitals

Atoms are mostly empty space.  If the nucleus of an atom had the diameter of a dime, the outer edge of the atom would be about 50 yards away.

If a proton were enlarged to the size of the head of a pin, the outer edge of the atom would be about 100 meters away.


    This is a very distorted model of the atom.


If this picture were drawn to scale:
And if the protons and neutrons were a centimeter in diameter;

Then the electrons would be less than the diameter of a hair; and
The entire atom's diameter would be greater than the length of 30 football fields!

If an atom were as large as a football stadium, the nucleus would be the size of a small ladybug crawling across the 50-yard line. In spite of this size difference, virtually an of the mass of an atom is concentrated in its nucleus. One electron, which has a negative charge, weighs only 1/1836 as much as the lightest of all nuclei, that of the hydrogen atom (proton).

In addition, all the particles (protons, neutrons, and electrons) are constantly in motion.

Rutherford's Gold Foil Experiment 












1911- Ernest Rutherford publishes his atomic theory describing the atom as having a central positive nucleus surrounded by negative orbiting electrons. This model suggested that most of the mass of the atom was contained in the small nucleus, and that the rest of the atom was mostly empty space. Rutherford came to this conclusion following the results of his famous gold foil experiment. This experiment involved the firing of radioactive particles through minutely thin metal foils (notably gold) and detecting them using screens coated with zinc sulfide (a scintillator). Rutherford found that although the vast majority of particles passed straight through the foil approximately 1 in 8000 were deflected leading him to his theory that most of the atom was made up of 'empty space'.

What percentage of the volume of an atom is just empty space?


Take an online Element Quiz  


If you know of some information you think should be included on this page, please let me know!! 


Interactive Periodic Charts: 



go on to learn about the Interaction of Matter



Sources: A good PowerPoint presentation about atoms cool particle applet more info on orbitals & shapes A SIMPLE VIEW OF ATOMIC STRUCTURE how do atoms/molecules "stick" together an excellent tutorial with movies & nice graphics online elements quiz

More Sites about atoms:

The Atom - Succeed in Physical Science 

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