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The Structure of the Atom

Chemistry

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Learning Objectives for this Chapter:

1. Describe changes in the atomic model over time and why those changes were necessitated by experimental evidence.

2. Describe structure of atoms (protons, neutrons, electrons) and differentiate among these particles in terms of mass, charges and location in the atom.

3. Compare the magnitude and range of the 4 fundamental forces.

4. Apply the mole concept and the Law of Conservation of Mass.

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It started a long time ago…Today460 – 370 BC

Democritus

Beginning of Atomism

You cannot divide something in half forever. The smallest piece of matter is called an atom.

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Today1808

DemocritusAtomism

460 – 370 BC

Dalton’s Postulates

4. Compounds are made from combining atoms in simple whole number ratios.

1. All elements are made of tiny indivisible particles called atoms.

2. All atoms of the same element are the same, but different from atoms of every

other element.

3. Chemical reactions rearrange atoms but do not create, destroy, or convert atoms from one element to another.

Over 2,000 years later John Dalton comes up with the first “modern” atomic theory.

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Another kind of light?Today1808 1870

DemocritusAtomism

460 – 370 BC

Dalton“Modern”

atomic theory

William Crookes invents a tube in which virtually all the gas has been removed.

Under high voltage, a ray was emitted from the cathode end of the tube.

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It started a long time ago…Today1808 1870 1897

DemocritusAtomism

460 – 370 BC

Dalton“Modern”

atomic theory

CrookesCathode rays

Cathode rays must be negative.

J.J. Thomson discovers the electron

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Today1808 1870 1897 1910

DemocritusAtomism

460 – 370 BC

Dalton“Modern”

atomic theory

CrookesCathode rays

ThomsonDiscovery

of the electron Ernest Rutherford discovers the nucleus

It started a long time ago…

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The size of the atom comes mostly from the space occupied

by the electrons

The mass of the atom comes mostly from the nucleus

Size and mass

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electronsprotons

neutrons

What happens when you change the number of protons?

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You obtain a different element!

6 protons in carbon

7 protons in nitrogen

8 protons in oxygen

The number of protons is also called the atomic number for that element.

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electronsprotons

neutrons

What happens when you change the number of electrons?

You get an ion – a charged particle.

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A neutral sodium atom

A positive sodium ion

Na

Na1+

The protons andelectrons cancel each other out

One proton is not neutralized by an electron, making this a +1 charged atom

One electron short

Balanced charges

11 protons

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A negative oxygen ion

O-2Two electrons are not neutralized by protons, making this a –2 charged atom

Two extra electrons

8 protons

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The electron cloud

Except for mass, virtually every property of atoms is determined by electrons, including size and chemical bonding

Electrons are very light and fast. They are not organized along orbits around the nucleus.

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electronsprotons

neutrons

What happens when you change the number of neutrons?

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Atomic number

Neutrons act as “glue.” They hold protons together in the nucleus.

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mass number: total number of protons and neutrons in a nucleus.

Mass number = 6 p + 6 n = 1212C

“carbon-12”

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Mass number

12 13 14

Name Carbon-12 Carbon-13Carbon-14

# protons 6 6 6# neutrons 6 7 8

isotopes: atoms or elements that have the same number of protons in the nucleus but different number of neutrons

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Atomic particle determinationa. Neutrons = mass number of nucleotide –

atomic number of elementb. Protons = atomic number (Z) of elementc. Electrons = atomic number of elementd. Atomic Mass – average mass of isotopes of

an elemente. Mass number – the total protons and

neutrons in the nucleus of an atom

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V Atomic Mass Formulae1. Mole – the amount of a substance that contains the

same number of units as the number of atoms in exactly 12 g of Carbon – 16. This is Avogadro’s number of 6.022 x 1023 particles.

2. Mass to number of atoms = given mass in g of element 6.022 x 1023 atoms Molar mass in g of element

3. Number of atoms to mole: # atoms mole

6.022 x 1023 atoms4. Mass to moles:

given mass in g moles Molar mass in g

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Number of neutrons for each of 100 lithium atoms randomly sampled from nature

It’s an AVERAGE mass!

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Isotope periodic table (first 4 rows)

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Forces found in Nature:1. Gravitational – relatively very

weak but active over long distances

2. Electromagnetic – intermediate strength

3. Strong Nuclear – strongest force that binds protons and neutrons together in the nucleus

4. Weak Nuclear – involved in the decay of many elementary particles

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Examples of the Weak Nuclear force.

Some atoms are unstable and, to become stable, decay by emitting particles or energy or both.

Alpha particles, a

Beta, b

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Nuclear ParticlesProton

Mass: 1 amu Charge: +1

NeutronMass: 1 amu

Charge: 0

(1 amu = 1/12 mass of Carbon 12 atom)

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Electrons

Negative 0

e -1

Positive 0

e positron +1

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Uranium Isotopes

238 235

U U 92 92

Low Radioactive Fission

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Nuclear Reactions

• Nuclear Decay - a spontaneous process in which an unstable nucleus ejects a particle and changes to another nucleus.– Alpha decay– Beta decay

• Beta Minus• Positron

• Fission - a nucleus splits into two fragments of roughly equal size

• Fusion - Two nuclei combine to form a heavier nucleus.

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Alpha Decay

• This occurs when a helium nucleus is released.• This occurs only with very heavy elements.

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Beta (b-) Decay• A beta particle (negative electron) is released

when a nucleus has too many neutrons for the protons present. A neutron converts to a proton and electron leaving a greater number of protons.

• Neutron decay:

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Positron (b+) Decay

• Positron decay occurs when a nucleus has too many protons for the neutrons present. A proton converts to a neutron. A neutrino is also released.

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Fission

Fusion

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Learning Objectives for this Chapter: Atomic Structure1. Describe changes in the atomic model over time

and why those changes were necessitated by experimental evidence.

2. Describe structure of atoms (protons, neutrons, electrons) and differentiate among these particles in terms of mass, charges and location in the atom.

3. Compare the magnitude and range of the 4 fundamental forces.

4. Apply the mole concept and the Law of Conservation of Mass to make calculations.