The Structure of the Atom

History, Structure, Properties and Forces

Chapter 4

Atomic Structure

History, Structure and Properties of the Atom

History of the Atom

•Great Thinkers (2,000yrs. Ago)

•Age of Reason and Thought

•Democritus vs. Aristotle view on the make-up of matter.

Democritus(400b.c.)Greek Philosopher

• Observed matter to be made up of atoms.

• Atoms are the smallest form of matter.

• Atoms cannot be broken down.

• The types of atoms in matter determine its properties.

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.Why?

John Dalton’s Atomic Theory

• English school teacher• Proved Democritus atoms

hypothesis using the scientific method.

• His conclusions produced:

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 rearranged.

Dalton’s Atomic Theory

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

reich-chemistry.wikispaces.com

chemed.chem.purdue.edu

•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 atoms.•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.(Nucleus)

Rutherford’s Nuclear Atom Model

•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.

Subatomic Particle: Neutron

Properties of Subatomic Particles Atomic Subparticles

Charge mass (g) mass (amu) atom’s contribution

Electron(e-)

(+) 9.11 x 10 -28 so tiny, no mass(10,000x smallerthan p+ or no

Contributes:•(-) charge

Proton(p+)

(-) 1.674 x10-24 1 atomic mass unit for each proton.

Contributes:•(+) charge•mass

Neutron(no)

No charge 1.675 x10-24 1 atomic mass unit for each neutron

Contributes:•mass •minimizes repulsion force of p+ in nucleus

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 Ringer5. 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?

Modern Atomic Model

Properties of Subatomic Particles Atomic Subparticles

Charge mass (g) mass (amu) atom’s contribution

Electron(e-)

(+) 9.11 x 10 -28 so tiny, no mass(10,000x smallerthan p+ or no

Contributes:•(-) charge

Proton(p+)

(-) 1.674 x10-24 1 atomic mass unit for each proton.

Contributes:•(+) charge•mass

Neutron(no)

No charge 1.675 x10-24 1 atomic mass unit for each neutron

Contributes:•mass •minimizes repulsion force of p+ in nucleus

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 e-For 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 ParticlesProblem 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 = 78Problem 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

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

• Do you think an element’s isotopes have similar or different chemical properties?

C-12 C-14# 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 oxygen?

3. What is the number of p+, n0, and e- for each isotope of Oxygen ?

• Atomic mass= the weighted 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.

Atomic MassElement’s average mass

Atomic Mass(Element’s average mass)

• Carbon (C) has two isotopes: Carbon-12 has a natural abundance = 99%Carbon-13 has a natural abundance = 1%Carbon’s atomic mass=(12 x 0.99) + ( 13x0.01) = 11.88 + 0.13 = 12.01 amu (sig. figs.)

Atomic Mass1. What is the difference between an atom’s mass

number and an element’s atomic mass?

2. Calculate the atomic mass of the following magnesium isotopes.

Isotope Natural Abundance (%)I-127 80I-126 17I-128 3

Answer: 126.86 = 126.9 (sig. figs)