Lap 4 Chap 4 Atomic Structure Notes
Essential Question: How does our current model of the structure of the atom affect how atoms behave
in matter and chemical reactions?
4.1 Defining the Atom
A. Early Models of the Atom
Def: ________________________________________________________________________________
Democritus’s Atomic Philosophy
Greek philosopher
460 BC -370 BC
First to suggest existence of _________________
Believed atoms were indivisible and indestructable
_______________________ Greek word for atoms
Ideas proved to be true, but not based on scientific method
B. Dalton’s Atomic Theory
English chemist 1766-1844 (2,000 yrs. later)
Using experimental methods, he transformed Democritus’ ideas into a scientific theory on the real
nature of atoms
4 parts to his atomic theory
1. All __________________________ are composed of tiny, indivisible particles called atoms.
2. Atoms of the same element are identical. Atoms of any ___________ element are different
from those of any _______________ element.
3. Atoms of different elements can physically ____________ together or can chemically
combine in simple _________________________ ratios to form compounds.
4. Chemical reactions occur when atoms are ________________________________________
_____________________. Atoms of one element, however, are never
________________________ into atoms of another element as a result of a chemical reaction.
C. Sizing up the Atom
Atoms are very small
A penny contains 2.4 x 1022 atoms of Copper
24000000000000000000000
(over 1 sextillion!)
Only 7 x 109 people on Earth
Possible to see with a ________________________________________________________
4.1 Section Assessment
1. How did Democritus characterize atoms?
2. How did Dalton advance the atomic philosophy proposed by Democritus?
3. What instrument can be used to observe individual atoms?
4. In your own words, state the main ideas of Dalton's atomic theory.
5. According to Dalton's theory, is it possible to convert atoms of one element into atoms of
another? Explain.
6. With which of these statements would John Dalton have agreed in the early 1800s?
a. Atoms are the smallest particles of matter.
b. The mass of an iron atom is different from the mass of a copper atom.
c. Every atom of silver is identical to every other atom of silver.
d. A compound is composed of atoms of two or more different elements.
4.2 Structure of the atom
A. Subatomic Particles
Atom can be divided into 3 subatomic particles: ________________________________
Electron
Discovered in 1897 by J.J.Thomson
-passed electric current through gases sealed in glass tube fitted with metal plates called
electrodes at either end.
-when connected to source of electricity, cathode ray traveled from negative cathode towards
positive anode.
-determined stream of electrons were negatively charged particles, since attracted to a
positively charged plate
Mass found by Millikan to be 1/1840 the mass of a hydrogen atom
Protons
Positively charged particle
Mass 1840 times larger than electron
Neutrons
Particles with no charge
Mass __________________ as proton
(Protons and neutrons composed of even smaller particles called quarks -see scale of the
universe)
Next Question: How are the three subatomic particles arranged in an atom?
B. Atomic Nucleus
Early theory (Thomson) suggested that the electrons were spread out in the atom like “raisins in
plum pudding”
Actual structure was discovered by Rutherford in 1911
Gold Foil experiment
Shot a beam of massive, double charged positive alpha particles at a piece of gold foil
If plum pudding theory is true, particles should pass easily through with only slight
deflection when bullets of positive particles hit the spread out, weaker, positive protons
Did not happen as expected
Small number were greatly deflected and even bounced back, like "shooting a bullet at tissue
paper and having it come back and hit you."
Rutherford’s new theory of atomic structure
Atom is mostly empty space. This is why most particles passed easily through
Positive charge and almost all mass is located in central area of atom. This is why some particles
bounced off at huge angles
He called the center the nucleus
Def__________________________________________________________________
Rutherford's Atom = Nuclear Atom
Protons and neutrons are in the nucleus
Electrons are distributed around the nucleus and occupy almost all of the volume of the atom
If size of atom = football field, then nucleus = marble
4.2 Section Assessment
1. What are three types of subatomic particles?
2. How does the Rutherford model describe the structure of atoms?
3. What are the charges and relative masses of the three main subatomic particles?
4. Describe Thomson's and Millikan's contributions to atomic theory.
5. Compare Rutherford's expected outcome of the gold-foil experiment with the actual
outcome.
6. What led Rutherford to conclude that an atom is mostly empty space?
7. How did Rutherford's model of the atom differ from Thompson's?