Atomic Structure

How do you discover and study something you can’t see?

WHAT IS A THEORY?

A hypothesis is a proposed explanation made as a starting point for further investigation (It’s bright outside because the sun is probably out)

A theory is an explanation confirmed through repeated observation and experimentation (When the sun is out, it tends to be bright outside)

A law is a fact of nature that is observed so often it is accepted as truth.

Theories EXPLAIN laws.Theory vs. Law- Ted Ed

ATOMIC THEORYWHAT IS IT??

The idea that matter is made up of atoms, the smallest pieces of matter.

THE ATOM

The atom is the smallest particle of an element that retains the properties of that element.

A scanning tunneling microscope allows individual atoms to be seen.

Imagine you could increase the size of an atom to make it as big as an orange. At this new scale, an orange would be as big as Earth.

History of the Atom

As a class we will be creating a timeline of the discovery of the atom…

Your group will research the following information and present it to the class:

– Date – What was the time or year of the scientist’s work concerning the atom?

– Observations/Experiments/Evidence –Describe or explain what observation, evidence, or experiment the scientist used to devise his theory of the atom.

– Discoveries/Conclusions – Explain what discovery or conclusion the scientist came to about the atom. What was his atomic theory?

– Contribution – Explain how the scientist helped in the development of atomic theory. How did his work contribute to the theory? What did his experiment/discovery/conclusion do for the theory? Think about the larger picture and how the theory progressed as a result of his work.

– Be able to draw a diagram of the scientist’s model of the atom

Each group will be responsible for one of the following scientists:

Democritus

Rutherford

Bohr

JJ Thomson

Schrodinger/Heisenberg (quantum mechanics)

Chadwick

Dalton

Greek philosophers believed all matter was made up of four basic elements: fire, earth, water, and air.

History of Atomic Theory

And that matter could be endlessly divided into smaller and smaller pieces

History of Atomic Theory

Democritus: 460 – 370 B.C.

– Proposed the idea that matter was not infinitely divisible, but made of individual particles called “atomos”.

Aristotle (484-322 B.C.) disagreed because he did not believe empty space could exist. His viewswent unchallenged for 2000 years.

History of Atomic TheoryJohn Dalton revived the idea of the atom in the early 1800’s with his “Atomic Theory of Matter”

Which parts are still considered true? Which are not considered true?

LAW OF CONSERVATION OF MASS

John Dalton also easily explained the law of conservation of mass (which was proposed by Lavoisier) in a chemical reaction as the result of the combination, separation or rearrangement of atoms.

LAW OF CONSTANT COMPOSITION

Dalton’s theory also helps explain a law proposed by

Proust in the early 1800’s. The law of constant

composition states that the elemental composition

of a compound is always the same.

ex: water is always 11% hydrogen and 89%

oxygen (by mass)

Discovery of Atomic StructureJJ Thomson (1897)

Cathode Ray Tube experiment

_

+

Thomson released cathode rays

from one side of the tube and they

were attracted to the positive plate. Discovered first subatomic particle: the Electron.

What attracts to a positive charge?

JJ Thomson Video

Discovery of Atomic Structure

Rutherford (1895)Gold foil experiment

Most particles passthrough, but some arebounced back towards thesource.

Discovered a positively charged nucleus and that atoms are mostly empty space.

Alpha particles

Plum pudding atom Nuclear atom

Discovery of Atomic StructureLater Rutherford (1919) refined his model to include the positively charged particles in the nucleus called protons.

Chadwick (1932) noticed the dense nucleus had a particle that was not charged!

Discovered the neutron

Rutherford and Chadwick Video

Bohr (early 1900s)

proposed the “electron cloud” in which electrons orbit at a given distance from the nucleus.

Small orbits = low energy

Big orbits = high energy

Modern Atomic Theory (quantum mechanical model)

Nucleus: dense, central part of the atom.

– Protons and neutronsare found in the nucleus.

Electron cloud: large area outside of the nucleus.

– Electrons occupy the electron cloud in orbitals.

Structure of the Atom

Parts of the Atom

Subatomic Particle

Location Charge Relative

Mass

Proton Nucleus 1++1.602 X 10-19 C

11.67x10-24 g

Neutron Nucleus 0 11.67x10-24 g

Electron e- cloud 1--1.602 X 10-19 C

1/18409.11x10-28 g

What do all the numbers mean on the PT?

Atomic Number- # of protons in

an atom. It is also equal to the number of electrons in a neutral atom.

Atomic Mass – the decimal number on the periodic table. The average mass of all isotopes of that element.

The atomic number determines an

element’s identity!

What about Neutrons?

The mass of the atom

comes from the nucleus.

Each proton has a mass

of 1 amu (atomic mass

unit)

Each neutron has a mass

of 1 amu.

Add the number of

protons and neutrons to

find the mass number of

the atom!

Isotopes

What are they? atoms of the same element that

have different mass numbers.

This means the number of ________

is the same, and the number of

_________ is different.

Carbon-12

Mass Number = 12

Atomic Number = 6

Neutrons = Mass – Protons

Carbon-14

Mass Number = 14

Atomic Number = 6

Neutrons = Mass - Protons

ISOTOPES OF CARBON

6 Neutrons 8 Neutrons

Protons

Neutrons

2 ways to depict isotopes

Hyphen Notation: C-14

Super/Sub Script:

Element Atomic

#

Mass # # p+ # n0 # e- Super/sub

method

Hyphen

method

Ruthenium

78 117

32 41

Scandium

Practice:1. What is the atomic number of zinc? _______2. How many electrons does silver have? _______3. How many protons does iodine have? _______4. How many neutrons does Fe-53 have? _______5. What is the mass number of Chlorine-37? _______6. How many electrons does Chlorine-37 have? _______7. What is the mass number of tritium? _______8. How many neutrons does neon-20 have? _______9. How many neutrons does deuterium have? _________

ATOMSDiffer by number of protons

ISOTOPESDiffer by number of neutrons

IONSDiffer by number of electrons

Differ by number of electrons

ISOTOPES

Differ by number of neutrons

How is the mass of atoms measured?

Atoms can’t easily be measured in

grams because they are so small.

Scientists devised “atomic mass

units” (a carbon-12 isotope is

12.000000 amu’s)

What is the mass, in amu’s, of

Hydrogen-1?

You would think it was

1.000000… right?!

It’s actually a little different.

Average atomic mass is a different kind of average – a “weighted” average. –

think about your six weeks average.

This means that we take into account the abundance of each isotope found in

nature.

Saying that Boron’s average mass is 10.5 would be misleading since there’s

so much more Boron-11.

Average Atomic Mass

The average of the masses of the naturally

occurring isotopes weighted for their abundance

in nature. (weighted average)

(mass)(abundance/100)=

(mass)(abundance/100)= +

(mass)(abundance/100)= +

Is the AAM

on the

Periodic

Table?

Using this formula will ensure

the correct number of sig

figs…

Calculating Average Atomic Mass1. Argon has three isotopes with the following percent

abundances: Ar-36 with a mass of 35.968 amu and an

abundance of 0.3337 %. Ar-38 with a mass of 37.963 amu

and an abundance of 0.063 %. Ar-40 with a mass of 39.962

amu and an abundance of 99.600 %. Calculate the

average atomic mass, but first write down what you expect

the average atomic mass to be close to!

3. The atomic weight of gallium is 69.72 amu. The masses

of naturally occurring isotopes are 68.92 amu for Ga-69

and 70.92 amu for Ga-71. Calculate the percent

abundance of each isotope.

The atomic weight of gallium is 69.72 amu. The masses of naturally

occurring isotopes are 68.92 amu for Ga-69 and 70.92 amu for Ga-71.

Calculate the percent abundance of each isotope.

[(68.92)(Ga-69 abundance) + (70.92)(Ga-71 abundance)]/100 =

69.72

We have 2 unknowns – we need another equation.

Can you think of a mathematical relationship between the two

abundances????

HINT – What should the abundances add up to?

Answer – Abundance of Ga-71 is 40% and abundance of

Ga-69 is 60%.