Atom
The smallest particle of an element that retains the
characteristics of that element.
Democritus (460-370 BC)
believed all matter was made of atoms, which were indivisible
& indestructible
John Dalton (1766-1844)
English chemist, physicist, & meteorologist– also
an expert on color blindness (which he was)
Dalton’s Atomic Theory – 1808
1. All matter is made up of tiny particles called atoms.
2. All atoms of a given element are similar to one another
and different from atoms of other elements.
3. Atoms of two or more different elements combine to
form compounds: A particular compound is always made up of the same
kinds of atoms and always has the same number of each kind of atom.
4. A chemical reaction involves the rearrangement, separation
or combination of atoms. Atoms are never created or destroyed during
a chemical reaction.
Dalton initially conceived of atoms as solid, indivisible
spheres: "The Billiard Ball Model."
By the late 1800’s, scientists realized this was not
the case.
Electrical Charges in an Atom
Atoms are composed of smaller parts called
Subatomic particles.
Some subatomic particles have electrical charges.
Like charges repel.
Unlike charges attract.
Hair gets flyaway look when you take off a hat, leaving
hair with positive charge
Clothes from the dryer stick together and have static
electricity
Structure
of the Atom
1897 English Physicist J. J. Thompson
Discovered atoms contained both positive and negative
charges
cathode-ray tube experiment
electron e–
a negatively-charged subatomic particle having a very
small mass that orbits the nucleus in mostly empty space
proton p+
a positively-charged subatomic particle having a mass
of 1 a.m.u. (atomic mass unit) and found in the very small, dense nucleus
of an atom
Atoms are electrically neutral.
In a neutral atom,
the number of protons equals the number of electrons.
1911 New Zealand Physicist Ernest Rutherford
atoms mostly empty space, protons contained in the nucleus
gold foil experiment
Scientists
knew mass of individual protons
knew the mass of the nucleus
But the mass of the protons was not enough to account
for the mass of the nucleus.
There had to be another particle inside the nucleus.
1932, British Physicist James Chadwick
polonium-beryllium experiment
neutron nº
a neutral subatomic particle having a mass of 1 a.m.u.
and found in the nucleus of an atom
nucleus
the compact center of an atom, containing the protons
and neutrons of the atom
Mass of the Atom
One proton has mass of 1.7 x 10–24
g
One electron has a mass 1/1840 of a hydrogen atom.
atomic mass unit (amu)
a small mass unit used to describe the mass of very
small particles such as atoms and subatomic particles
1 amu = one-twelfth the mass of a C-12 atom (6 nº
+ 6 p+)
Relative Atomic Masses of the
Subatomic Particles
Neutron = 1 a.m.u
Proton = 0.99862349 a.m.u.
Electron = 0.00054386734 a.m.u
Notice that the sum of the masses of the proton &
electron is nearly 1 (0.999) and the sum of their charges is zero—same
as a neutron!
| Particle |
Charge |
Mass |
Location |
| proton, p+ |
positive, +1 |
0.999 a.m.u |
in the nucleus |
| neutron, nº |
none, 0 |
1.000 a.m.u. |
| electron, e– |
negative, –1 |
5.44 x 10–4
amu |
orbiting the nucleus |
|
What are the subatomic components
of the atom?
Practice Problems
Answer the following questions: |
|
| 1. Are all atoms of a given element
the same?
2. Are there any atoms of one element that are identical
to atoms of another element?
3. Do atoms have an electrical charge?
4. What are subatomic particles?
Name one.
5. Are there any subatomic particles with an electrical
charge?
Name one
6. Does the nucleus of an atom have an electrical
charge?
If so, what is the charge
and if not, why not?
7. Which have more mass-- protons or electrons?
8. Are protons electrically attracted to neutrons?
9. Because they have so little mass, do they have
electrical charge?
10. Because they have the same charge, are electrons
attracted to each other?
11. Are all the protons and neutrons of an atom
contained within the nucleus?
12. Because the atom is electrically neutral, are
there some electrons contained within
the nucleus?
13. How much of the volume of an atom is made up
of the nucleus?
How much of
the mass of an atom is made up by the nucleus?
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