The Structure of the Atom

History, Structure, Properties and Forces

Chapter 4

Atomic Structure

History, Structure and Properties of the Atom

Atomic Structure Objectives1. 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.

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

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•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

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?

Kandium Lab Post Lab Question Atomic Subparticles

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

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

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

Neutron 1.675 x10-24 1 atomic mass unit for each neutron

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 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

Bell Ringer: Atomic StructureElement Symbol protons (p+) neutrons (no) Electrons (e-)

N 8

F 9

C 8

O 9

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

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 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

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 amu14 x 0.02 = + 0.28 amu = 12.04 amu * Just remember to convert natural abundance from percentage to

decimal form.

Isotopes

Mass Number vs. Atomic MassCompare mass number of an atom with element’s atomic mass using theVenn diagram below.

Atomic Mass

Atomic MassCalculate Iodine’s atomic mass. Watch significant figures.

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

Answer: 126.86 = 126.9 (sig. figs)

Atomic PropertiesIf most of the atom is empty space, why doesn’t

matter pass through one another?

Atomic PropertiesIf most of the atom is empty space, why doesn’t

matter pass through one another?

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Atomic PropertiesWhat keeps the protons from leaving the nucleus

and being drawn towards the electrons?

Atomic Forces

• Electrostatic Forces :

• Nuclear Forces:

Bell Ringer: Atomic Forces1. 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 particles.

• Nuclear Forces:Forces that holds nucleus together

Charged AtomsIons = charged atoms.

• How do ions form? Neutral atoms lose or gain electrons.

• 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 Atom

# of p+ # of n0 # of e- Mass Number Neutral or Chargedatom

Atom’s Symbol

Calcium 20 41

28 18 48

7 10 15

36 28 65