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Atomic Theory and what we understand

today about…Atoms, Molecules, and Ions

The Power of 10

Have you ever wondered your placement in the universe and the things that make it up?

Picture this!http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/

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A HISTORY OF THE STRUCTURE OF THE ATOM

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Let’s begin with… Greek Philosopher Democritus (460-370 B.C.):

all matter composed of small atoms

atomos = indivisible

What did Democritus conclude about cutting matter in half? There was a limit to how far you could divide matter. You would eventually end up with a piece of matter that could not be cut.

Why weren’t Democritus’s ideas accepted? Aristotle was a very famous Greek philosopher who believed that matter could be divided into smaller and smaller pieces forever. He held a very strong influence on popular belief and his views on this were accepted for two thousand years.

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1803, John Dalton: atoms are the fundamental building blocks of matter› He performed many experiments to

study how elements join together to form new substances

› He found that they combine in specific ratios and he supposed it was because the elements are made of atoms.

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Dalton's Postulates

Each element is composed of extremely small particles called atoms.› Atoms are indivisible and indestructible

particles.

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Dalton's Postulates

All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements.

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Dalton's Postulates

Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions.

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Dalton’s Postulates

Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.

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John Dalton’s Atomic Theory(ca 1803)

1. Each element is composed of extremely small particles called atoms.

2. All atoms of a given element are identical. The atoms of different elements are different and have different properties (including different masses).

3. Atoms of an element are not changed into different types of atoms by chemical reactions. Atoms are neither created nor destroyed in chemical reactions. This is the Law of Conservation of Mass.

4. Compounds are formed when atoms of more than one element combine. A given compound always has the same relative number and kind of atoms. This is the Law of Definite Composition.

John Dalton’s Atomic Theory

Almost right. A good start.

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Structure of the atom after Dalton (ca. 1810)

very small

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J.J. Thomson (1897): Cathode Rays

• Atoms subjected to high voltages give off cathode rays.

• I can demonstrate this for you!

JJ Thomson:

https://youtu.be/oddjdB0qfMg What did he discover?

› A particle that has MASS and has a NEGATIVE CHARGE

› The mass was 2000x smaller than the smallest known atom (Hydrogen)

› This particle must be FROM an atom!› Named it the electron

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J.J. Thomson: Summarizing what he saw

Electrons are in atoms.

Thomson concluded that the negative charges came from within the atom.

A particle smaller than an atom had to exist.

The atom was divisible!

Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom. But he could never find them.

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J.J. Thomson – The Electron

Structure of the atom after Thomson (ca. 1900)

“Plum pudding” model: Negative

electrons are embedded in a

positively charged mass.

Positively charged mass

Electrons (-)

Unlike electrical charges attract, and

that is what holds the atom together.

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Radioactivity

Radioactivity is the spontaneous emission of radiation by an atom.

First observed by Henri Becquerel (1852-1908).

Marie and Pierre Curie also studied it. Nobel Prize in 1903 (physics).

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Studies of Natural Radioactivity

Structure of the atom after Becquerel (early 1900s)

Positively charged mass

Electrons (-)

Some atoms naturally emit one or more of the following types of radiation:

alpha (α) radiation (later found to be He2+ - helium nucleus)

beta (β) radiation (later found to be electrons)

gamma (γ) radiation (high energy light)

γ

α

α

γ

Alpha particles

Somehow gamma radiation is in there, too.

Radioactivity Three types of radiation were

discovered by Ernest Rutherford:› particles (positive, charge 2+, mass 7400

times of e-)› particles (negative, charge 1-)› rays (high energy light)

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Ernest Rutherford (1910)

Scattering experiment:

Rutherford’s experiment Involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick)

The Nuclear Atom Most of the positively

charged “bullets” passed right through the gold atoms in the sheet of gold foil without changing course at all.

Some of the positively charged “bullets,” however, did bounce away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges.

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Rutherford’s ideas

This could only mean that the gold atoms in the sheet were mostly open space. Atoms were not a pudding filled with a positively charged material.

Rutherford concluded that an atom had a small, dense, positively charged center that repelled his positively charged “bullets.”

He called the center of the atom the “nucleus”

The nucleus is tiny compared to the atom as a whole.

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Ernest Rutherford The Nucleus and the Proton

Structure of the atom after Rutherford (1910)

The mass is not spread evenly throughout the atom, but is concentrated in the center, the nucleus.

Electrons (-) are now outside the nucleus.

The positively charged particles in

the nucleus are protons.

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James Chadwick – The Neutron

Structure of the atom after Chadwick (1932)

In the nucleus with the protons are particles of similar mass but no electrical charge called neutrons.

Electrons (-) are now outside the nucleus in quantized energy states called orbitals. (From Niels Bohr and quantum mechanics)

The positively charged particles in

the nucleus are protons.

nn+

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Structure of the Atom

proton (+)

neutron

nucleus - responsible for the mass of the atom, positively charged

electrons - responsible for the volume and size of the atom, negatively charged

10-14 m

10-10 m

If this stadium were the size of an atom’s electron cloud, the nucleus would be the size of a marble setting on the 50 yard line. Electrons occupy the VOLUME, protons and neutrons constitute the MASS of an atom.

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Subatomic Particles Protons and electrons are the only particles

that have a charge. Protons and neutrons have essentially the

same mass. The mass of an electron is so small (2000

time smaller than the proton), we ignore it.

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0.0005486 is the way to write this number!

Atomic Facts

Feature Size Mass

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1 amu = 1 atomic mass unit = 1.66054 x 10-24 g

Electrons are outside the nucleus in quantized energy states called orbitals.

proton (+)

10-15 m 1.0073 amu

neutron (0)

10-15 m 1.0087 amu

electron (-)

10-18 m ???

0.0006 amu + n

n

Symbols of Elements

Elements are symbolized by one or two letters.

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Atomic Number

All atoms of the same element have the same number of protons: The atomic number (Z)

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Atomic Mass

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

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The number of protons in the nucleus is called the atomic number Z.

Z determines the identity of an element.

Saying “the atomic number of an element is 6” is the same as saying “carbon.”

The number of electrons in the atom is also Z (because atoms have no net electric charge).

How many neutrons are in C?

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Atomic NumberCarbon atom

- proton

- neutron

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12C

- proton

- neutron

Isotopes

• The number of protons and neutrons in an element is called the mass number A.

How many neutrons are in C? The answer is “it depends on the isotope.”

• An element may have different numbers of neutrons but NOT different numbers of protons.

• Atoms of an element with different numbers of neutrons are called isotopes of that element.

A = Z + number of neutrons.

A

6Z

Isotopes

Isotopes are atoms of the same element with different masses.

Isotopes have different numbers of neutrons.

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116C 12

6C13

6C14

6C

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Isotopesnumber of protons (Z)

number of neutrons

6 6

6 8

8 8

92 146

mass number (A)

number of electrons

symbol

12 12C or C-12

14C or C-14

16O or O-16

238U or U-238

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14 6

16 8

238 92

6

6

8

92

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Atomic Masses

Atomic masses are based on 12C.

The mass of 12C (or C-12) is defined to be exactly 12 amu.

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Atomic MassesThe mass (weight)

shown in the periodic table is the mass of the element as its occurs naturally.

If the element has more than one isotope, the mass shown is the weighted average of the masses of the isotopes.

Mg has 3 isotopes.

24Mg 78.99% 23.985 amu25Mg 10.00% 24.986 amu26Mg 11.01% 25.983 amu

weighted average of Mg: 0.7899x23.985 18.9460.1000x24.986 2.499 0.1101x25.983 +2.861 24.31 amu

atomic weight of Mg based on natural abundance: 24.31 amu

Ions

Atoms can gain or lose electrons to become charged particles called ions.› A chemical particle that contains a positive or

negative charge

Cations are positively charged ions.› Formed when an atom loses electrons

Anions are negatively charged ions.› Formed when an atom gains electrons

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Ions

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Net charge = 0 Net charge = +1

Formation of a cation

1p e- 1p e-+

Hydrogen atom1p, 0 n, 1 e-

Hydrogen ion (cation)1p, 0 n, 0 e-

1 H 1 H+

Ions

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8p8n 8e- 8p

8n

Oxygen atom8p, 8 n, 8e-

Oxygen ion (anion)8p, 8n, 10e-

16 O 16 O2-

10e-2e-+

Net charge = -2Net charge = 0

Formation of an anion

Nuclear Symbols

Mass Number Charge

AtomicNumber

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XCharge = # p - # e-

Nuclear Symbols

Using nuclear symbols to determine the number of p, n, e, and total charge

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O16

8Mass Number =

Atomic Number =

16

8

# protons = atomic number = 8

# neutrons = Mass # - Atomic # = 16 - 8 = 8

# electrons = # protons = 8

Nuclear Symbols

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Mass Number =

Atomic Number =

# protons = atomic number = 8

# neutrons = Mass # - Atomic # = 16 - 8 = 8

# electrons = # protons - charge = 8 - (-2) = 10

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8O

16

8

2-

Nuclear Symbols

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Ba137

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2+ Mass Number =

Atomic Number =

137

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# protons = atomic number = 56

# neutrons = Mass # - Atomic # = 137 - 56 = 81

# electrons = # protons - charge = 56 - (+2) = 54

Nuclear Symbols - Atoms

Example: Write the nuclear symbol for the following atoms:

1) 50 p, 70 n

2) 17 e-, 20 n

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120Sn50

37Cl17

Nuclear Symbols - IonsPractice writing nuclear symbols from

information given:

1) 53 p, 74 n, 54 e-

53 proton (= atomic number) I74 neutrons + 53 proton mass number = 12754 electrons (one more than protons) 1-

127I1-

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53

2) 23 e-, 30 n, net charge = +3 # protons?23 electrons, but charge of 3+ie 3 more protons than electrons p= 26 Atomic number = 26 element = Fe

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Fe56

26

3+

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Our classmates are going to present information about electrons/energy levels and more and that is how we will learn more on this timeline of the atomic theory.