Atomic StructureChapter 4

4.1 Defining the Atom

• Democritus (460 BC – 370 BC)

• Had an atomic philosophy

• Matter was made of small invisible particles called atoms

• No scientific evidence, but it was logical

Dalton’s Atomic Theory

• John Dalton (1766-1844) was an English chemist and school teacher

• Dalton’s theory was the first scientific theory of the atom

John Dalton looks like another famous John

Dalton’s Atomic Theory

Dalton’s Atomic Theory

1. Matter is composed of tiny indivisible atoms

2. All atoms of a given element are exactly the same

3. Different elements are made of different atoms

4. Atoms of different elements can combine in simple whole number ratios to form compounds

5. Chemical reactions involve separating, combining, or rearranging atoms.

How big are atoms?

• Pretty small

• Just 4 grams of helium (He) contains 6.02 x 1023 atoms!

• Most atoms have a radius in the range of 5 x 10-11 m to 2 x 10-10 m

• Atomic sizes are in the pico- and nanometer range

Can we see atoms?

• Amazingly, yes!• STM• Scanning tunneling

microscope has made it possible to see atoms.

• These are gold atoms• They are only 2.3 Å

apart!

Here is a picture of a mixture of tin, lead, and silicon atoms!

Custance's team demonstrated their "fingerprinting" technique by using an atomic force microscope (AFM) to distinguish atoms of tin (blue) and lead (green) deposited on a silicon substrate (red). (Credit: Oscar Custance) physicsworld.com/cws/article/news/27208

The colors are added to help distinguish the atoms

4.2 Structure of the Nuclear Atom

When do I know I have learned this section? When…

• I can describe 3 types of subatomic particles• I can describe the structure of the nuclear atom• I can explain Thomson’s discovery

– experiment and model of the atom• I can explain Millikan’s discovery• I can explain Rutherford’s discovery

– experiment and model of the atom

Subatomic Particles• A cathode ray is deflected by electrically charged

plates.

4.2

Subatomic Particles

• In Dalton’s theory, atoms had no charge

• Thomson studied cathode rays (the cathode ray experiment)

• Resulted in discovery of the electron

• Electrons are negatively charged particles

• Discovered the charge to mass ratio of electron

JJ Thomson’s Plum Pudding Model

• Atom was a sphere

• Atoms were electrically neutral

• (-) charged electrons embedded in a (+) charged matrix

• Like a chocolate chip cookie

Millikan’s Oil Drop Experiment

• Discovered the actual quantity of charge for the electron

• Using Thompson’s charge/mass ratio, Millikan was also able to determine the mass of the electron

• Mass of electron is 1/1840 the mass of a hydrogen atom.

Protons

• If H atoms are neutral, and they give off negatively charged electrons, what must be left?

• Positive charge

• Proton discovered by Eugen Goldstein noticed a ray traveling in opposite direction of the electrons in the cathode ray tube

• Toward the negative end (cathode)

Neutrons

• Discovered by James Chadwick

Quarks

• Quarks are subnuclear particles

• Protons and neutrons are composed of quarks

The Atomic Nucleus• Ernest Rutherford discovered the nucleus• Gold foil experiment

Simulation of Rutherford’s Gold Foil Experiment

Interpretation of Rutherford’s Findings

Rutherford’s Conclusions

• All positive charge and nearly all the mass of the atom are concentrated in a very small nucleus

• Atom contains mostly empty space

• Rutherford’s model of the atom was like a mini solar system

Summary of Atomic Models

• DemocritusInvisible particles• Dalton Tiny homogeneous sphere

5 (4) statements

• Thomson Plum pudding• Rutherford Nuclear atom

Thomson’s Model of the Atom

• Plum Pudding• Negatively charged

electrons embedded in a positive matrix

Rutherford’s Model

• Nuclear Atom• Most of the volume of the

atom is empty space• All positive charge & most

mass in a small nucleus• Nucleus contains protons

and neutrons• Electrons occupy space

outside the nucleus

Distinguishing Among Atoms

• Atomic Number= number of protons in nucleus

Each element has unique atomic number

• Mass Number= number of protons and neutrons in nucleus

# neutrons = mass # - atomic #

Practice

Symbols of Elements

Elements are symbolized by one or two letters.

First letter is always capitalized.

Second letter, if there is one, is always lower case.

Atomic Number

All atoms of the same element have the same number of protons:

The atomic number (Z)

Mass Number

The mass of an atom in atomic mass units (amu) is the total number of protons and neutrons in the atom.

Isotopes

• Isotopes are atoms of the same element with different masses.

• Isotopes have different numbers of neutrons.

116C

126C

136C

146C

Different Ways of Indicating Isotopes

• Chemical symbols16

8O 188O

• Symbol and mass number

O-16 O-18

• Name and mass number

oxygen-16 oxygen-18

Isotopes of Neon

Atomic #

Mass #

Atomic Mass

• Measured in amu’s (atomic mass units) because their mass is so small

• Definition: one atom of carbon-12 equals exactly 12 amu’s

• 1 amu = exactly 1/12 the mass of a C-12 atom

• 1 amu = 1.6605 x 10-24g

Different Isotopes Have Different Abundance & Atomic Masses

Isotope AbundanceAtomic mass (amu)

Chlorine-35 75.77% 34.969

Chlorine-37 24.23% 36.966

What is the atomic mass of chlorine on the periodic table?

Average Atomic Mass

Isotope Atomic Mass

Abundance

R-125 125 ________ ________

R-120 120 ________ ________

124

•Determining abundance of isotopes when average atomic mass is known

•Multiply the mass of each isotope by its relative abundance (decimal equivalent) and add the products

Periodic Table - Preview

• Allows you to easily compare and predict properties of elements

• Groups = Columns– Numbered 1-18 or– 1A-8A (representative elements)– 1B-8B (transition metals)

• Periods = Rows– Numbered 1-7