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The Structure of the Atom
History, Structure, Properties and
Chapter 4
Atomic Structure
History, Structure and Properties of the Atom
Atomic Structure Objectives
1. I can illustrate and compare the different atomic
models proposed by scientists.
2. I can illustrate the organization of each
subparticle in a neutral atom.
3. I can distinguish between atoms of different
elements and of the same elements.
4. I can calculate the atomic mass for any element.
History of the Atom
•Great Thinkers (2,000yrs. Ago)
•Age of Reason and Thought
•Democritus vs. Aristotle view on the
make-up of matter.
Greek Philosopher
• Observed matter to be
made up of atoms.
• Atoms are the smallest
form of matter.
• Atoms cannot be broken
• The types of atoms in
matter determine its
Aristotle: Greek Philosopher (300b.c.)
Aristotle observed matter to be
composed of one or more of these
four elements:
air, earth, fire and water.
• Most people believed Artistole.
John Dalton’s Atomic Theory
• English school teacher
• Proved Democritus atoms
hypothesis using the
scientific method.
• His conclusions produced:
Dalton’s Atomic Theory
Dalton’s Atomic Theory
1. Atoms are the building blocks of all matter.
2. Atoms cannot be subdivided.
3. Each element has the same kind of atoms.
4. In a compound, the different atoms chemically
combine in the same way (fixed composition).
5. Atoms cannot be created or destroyed just
Thomson’s Cathode Ray Experiment
J.J. Thomson (pg. 105)
1897 discovered electrons in gas atoms using a cathode ray tube.
Determined electrons have a negative charge.
Electrons have the same charge in all atoms.
Thomson’s Plum Pudding Model
• If atoms are made of electrons how come most
matter does not shock us?
• Atoms must have positive particles, too.
• He proposed the Plum Pudding Atomic Model
An atom is
equally made up
of positive and
negative particles.
Goldstein’s Cathode Tube Experiment
•He discovered the positively charged particle, proton, using a cathode ray tube also.
Rutherford’s Gold Foil Experiment
•The laser beam consisted of positively charged alpha particles.
•Hypothesized that if the Plum Pudding model of the atom was correct then the
+ charged alpha particles should deflect or bend slightly when in contact with the gold foil
•His experimental results revealed something different. Most alpha particles paths were not
affected by any charge when in contact with gold foil. (Red straight lines on left diagram). Just
a few deflected back at large angles. Knew it had to be of the same charge, because repelled.
Rutherford’s Conclusion
•Most of the atom is empty space.
•Small dense region composed of
(+) charged particles.
Rutherford’s Nuclear Atom Model
Subatomic Particle: Neutron
•What keeps the protons within the nucleus ?
(Like particles repel each other)
•1932 - James Chadwick discovers the nucleus
also has neutral particles present. He called
them neutrons.
•Neutrons have a significant mass like protons.
Bell Ringer
1. Compare the different views Aristotle and
Democritus had about what matter was made of.
2. Which Greek philosopher was correct ?
3. Which later scientist proved his view?
Bell Ringer:
4. a. Identify and explain this Atomic Model?
b. Who concluded this atomic model?
Bell Ringer
5. Look at the following experiment below.
a. What was this experiment called?
b. Who developed this experiment?
c. Did his experiment prove or disprove the
Plum Pudding Model?
Kandium Lab Post Lab Question
mass (g)
mass (amu)
9.11 x 10 -28
so tiny, no mass
(10,000x smaller
than p+ or no
1.674 x10-24
1 atomic mass
unit for each
1.675 x10-24
1 atomic mass
unit for each
1. Abbreviate each isotope of Kandium using the new
symbol you assigned it and its mass number.
2. Using the table above convert the mass number of each
Kandium isotope from amu to grams.
Distinguishing Atoms
1) atomic number= The number of protons within
an atom’s nucleus.
• Atom’s I.D.
• Atoms of the same element always have the same
number of protons.
• Elements are arranged numerically on the periodic
table based on their atomic number.
Neutral Atoms
• Most matter in nature is neutral.
(Doesn’t shock us!)
• This means the atoms making up the matter is
neutral. 
• What are the two charged subatomic particles?
p+ and eFor an atom to be neutral the # of p+ = # of e-.
Atom’s Mass
• Mass number (amu):
What contributes mass to the atom?
Sum of protons and neutrons in nucleus of atom.
Problem A: Selenium(Se) has an atomic # of 34 and
a mass of 80 amu. How many p+, e-, and n0 are
there in selenium? p+ (proton) = 34 (atomic #)
e- (electron)= 34 (balance p+)
n0 (neutron)= 80 – 34 = 46
Atom’s Subatomic Particles
Problem B: A cesium (Cs) atom has a mass of
133 and an atomic number of 55 amu. How
many p+, e-, and no are there?
P+ = 55; e- = 55 ; no = 78
Problem C: An iron atom has an atomic number
of 26 and 30 neutrons.
a. How many p+ and e- are there?
P+ = 26; e- =26
a. What is iron’s mass number?
Mass number = 56 amu
Bell Ringer: Atomic Structure
Element Symbol
protons (p+)
neutrons (no)
Electrons (e-)
1. Complete the table above. Each atom is neutral.
2. Rank the atoms in increasing order based on atomic number.
3.Rank the atoms in decreasing order based on mass number.
4.Pick one of the atoms from the table to illustrate the number
of protons, electrons, and neutrons using Rutherford’s
model of the atom.
Atomic Structure Review
An Element’s Isotopes
• Isotopes= atoms with different number of
neutrons in their nucleus.
• Elements can have several isotopes.
Ex. Carbon C-12 and C-14 (mass #)
• Mass # changes. (varied # neutrons)
• The number of protons and electrons stay
the same.
An Element’s Isotopes
# of p+, n0, e-?
# of p+, n0, e-?
Element’s Isotopes
• Oxygen has three isotopes. They are O-16,
O-17, and O-18.
1. What is the mass number for each isotope of
oxygen in amu?
2. What is the atomic number for each isotope of
3. What is the number of p+, n0, and e- for each
isotope of Oxygen ?
Atomic Mass
Element’s average mass
• Atomic mass= the average mass of all the
element’s isotopes present within a naturally
occuring sample of matter.
• Calculate Atomic Mass of an Element
1. Multiply the mass # of each isotope by its
natural abundance ( common occurance) in
decimal form.
2. Add all the isotopes products together.
Carbon’s Atomic Mass
Calculate the atomic mass for Carbon:
Carbon-12 natural abundance is 98%.
Carbon-14 natural abundance is 2%.
Carbon’s atomic mass?
12 x 0.98 = 11.76 amu
14 x 0.02 = + 0.28 amu
= 12.04 amu
* Just remember to convert natural abundance from
percentage to decimal form.
Mass Number vs. Atomic Mass
Compare mass number of an atom with element’s atomic mass using the
Venn diagram below.
Atomic Mass
Atomic Mass
Calculate Iodine’s atomic mass. Watch significant figures.
Natural Abundance (%)
Answer: 126.86 = 126.9 (sig. figs)
Atomic Properties
If most of the atom is empty space, why doesn’t
matter pass through one another?
Atomic Properties
If most of the atom is empty space, why doesn’t
matter pass through one another?
Atomic Properties
What keeps the protons from leaving the nucleus
and being drawn towards the electrons?
Atomic Forces
• Electrostatic Forces :
• Nuclear Forces:
Bell Ringer: Atomic Forces
1. What are the two primary forces within an atom?
2. Distinguish between these forces.
3. a. Which one of the forces is stronger ?
b. Why is this important to the atom’s existence?
Atomic Forces
• Electrostatic Forces :
- Forces between charged
• Nuclear Forces:
Forces that holds
nucleus together
Charged Atoms
Ions = charged atoms.
How do ions form? Neutral atoms lose or gain
• Two types of atoms:
a. Cations = positively charged atoms
(more p+ than e-)
b. Anions = negatively charged atoms
(more e- than p+)
Distinguishing Between Atoms
Type of
# of p+
# of n0
# of e-
Mass Number Neutral or