Chapter 3Chapter 3

Atoms:Atoms:

The Building BlocksThe Building Blocks

Of MatterOf Matter

Section 3-1Section 3-1

The Atom:The Atom:

From Philosophical IdeaFrom Philosophical Idea

To Scientific TheoryTo Scientific Theory

Two Theories on the Nature of Two Theories on the Nature of MatterMatter

It is continuous and can be It is continuous and can be infinitely broken down into infinitely broken down into smaller and smaller parts.smaller and smaller parts.

It has some fundamental particle It has some fundamental particle that can be reached that cannot that can be reached that cannot be broken down any further.be broken down any further.

The idea of the fundamental The idea of the fundamental particle is the one accepted particle is the one accepted today.today.

The study of matter lead to two The study of matter lead to two basic laws of chemistrybasic laws of chemistry

The Law of Conservation of The Law of Conservation of Matter – During ordinary Matter – During ordinary chemical and physical reactions, chemical and physical reactions, matter is neither created nor matter is neither created nor destroyed, it is conserved.destroyed, it is conserved.

The study of matter lead to two The study of matter lead to two basic laws of chemistrybasic laws of chemistry

The Law of Definite Proportions The Law of Definite Proportions – A chemical compound – A chemical compound contains the same elements in contains the same elements in exactly the same proportions by exactly the same proportions by mass, regardless of the size of mass, regardless of the size of the sample or the source of the the sample or the source of the compound.compound.

John Dalton – (1766-1844)John Dalton – (1766-1844)

English English SchoolteacherSchoolteacher

Proposed an Proposed an explanation for the explanation for the Law of Conservation Law of Conservation of Mass and the of Mass and the Law of Definite Law of Definite Proportions in 1808.Proportions in 1808.

This was called This was called Dalton’s Atomic Dalton’s Atomic Theory. It has five Theory. It has five statements.statements.

Dalton’s Atomic TheoryDalton’s Atomic Theory

All matter is All matter is composed of composed of submicroscopic, submicroscopic, indivisible particles indivisible particles called atoms.called atoms.

Atoms of the same Atoms of the same element are the element are the same, atoms of same, atoms of different elements different elements are different.are different.

Dalton’s Atomic TheoryDalton’s Atomic Theory

Atoms cannot be Atoms cannot be broken down, broken down, created, created, destroyed, or destroyed, or changed into changed into other atoms.other atoms.

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

Dalton’s Atomic TheoryDalton’s Atomic Theory

In chemical In chemical reactions, atoms reactions, atoms are combined, are combined, separated, or separated, or rearranged.rearranged.

Dalton’s Atomic Theory is not Dalton’s Atomic Theory is not Completely CorrectCompletely Correct

Atoms are Atoms are divisible.divisible.

Atoms of the Atoms of the same element are same element are not always the not always the same.same.

Atoms are not Atoms are not submicroscopic.submicroscopic.

Atoms can be Atoms can be changed into changed into other atoms.other atoms.

Section 3-2Section 3-2

The Structure of the AtomThe Structure of the Atom

Structure of the AtomStructure of the Atom

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

It contains two It contains two regions, the regions, the nucleus and the nucleus and the electron cloud.electron cloud.

electron

neutron

proton

The Regions of the AtomThe Regions of the Atom

The NucleusThe Nucleus Small, Dense Small, Dense

region in the region in the center of an atom.center of an atom.

Holds at least one Holds at least one proton, most hold proton, most hold at least one at least one neutron.neutron.

The nucleus is The nucleus is positively positively charged.charged.

The Electron The Electron CloudCloud

Region around Region around the nucleus that the nucleus that contains contains electrons.electrons.

The Three Subatomic The Three Subatomic ParticlesParticles

Electrons – Negatively charged, Electrons – Negatively charged, found outside of the nucleus. found outside of the nucleus. Smallest subatomic particle.Smallest subatomic particle.

Protons – Positively charged, Protons – Positively charged, found in the nucleus. Middle found in the nucleus. Middle sized subatomic particle.sized subatomic particle.

Neutron – No charge, found in Neutron – No charge, found in the nucleus. Largest subatomic the nucleus. Largest subatomic particle.particle.

Discovery of the ElectronDiscovery of the Electron

In the late 1800’s many experiments In the late 1800’s many experiments were done by passing current were done by passing current through gases at low pressure.through gases at low pressure.

These were carried out in sealed These were carried out in sealed glass tubes called cathode ray tubes.glass tubes called cathode ray tubes.

Discovery of the ElectronDiscovery of the Electron

When a current was passed through the When a current was passed through the tube, the anodes (+) glowed.tube, the anodes (+) glowed.

The theory was that the particles that The theory was that the particles that made up the cathode ray were negatively made up the cathode ray were negatively charged, and moved from cathode to charged, and moved from cathode to anode.anode.

Joseph John Thompson (1856 – Joseph John Thompson (1856 – 1940)1940)

Conducted Conducted experiments to experiments to support this idea in support this idea in 1897.1897.

Called these particles Called these particles corpuscles.corpuscles.

Determined it had a Determined it had a very large negative very large negative charge with a very charge with a very small mass.small mass.

Conclusion was that Conclusion was that all atoms contained all atoms contained electrons.electrons.

The Thompson Model of the AtomThe Thompson Model of the Atom

Thompson’s model of Thompson’s model of the atom is different the atom is different from Dalton’s from Dalton’s because it shows because it shows negative electrons negative electrons stuck in positive ball stuck in positive ball of matter.of matter.

There are two There are two problems. First, it problems. First, it does not show how does not show how easy it is to remove easy it is to remove electrons, and electrons, and second, it does not second, it does not show numbers of show numbers of electrons.electrons.

Assumptions from Thompson’s Assumptions from Thompson’s ExperimentsExperiments

Atoms must be divisible. This Atoms must be divisible. This proves part of Dalton’s theory proves part of Dalton’s theory wrong.wrong.

Since atoms are neutral, there must Since atoms are neutral, there must be a positively charged particle to be a positively charged particle to counteract the negatively charged counteract the negatively charged electron.electron.

Because electrons are so small, Because electrons are so small, there must be another particle that there must be another particle that gives the atom its mass.gives the atom its mass.

Lord Ernest Rutherford (1871 – Lord Ernest Rutherford (1871 – 1937)1937)

Thompson’s graduate Thompson’s graduate student.student.

He, Ernest Marsden He, Ernest Marsden and Hans Geiger and Hans Geiger worked together firing worked together firing alpha particles at a alpha particles at a sheet of gold foil.sheet of gold foil.

Observed that most of Observed that most of the particles passed the particles passed through the foil, but through the foil, but some were deflected some were deflected back at them.back at them.

Discovery of the NucleusDiscovery of the Nucleus

To the scientists, To the scientists, this was like firing a this was like firing a gun at a piece of gun at a piece of tissue paper and tissue paper and having it bounce off having it bounce off back at them.back at them.

Rutherford theorized Rutherford theorized that the particles that the particles were bouncing off a were bouncing off a small, dense, small, dense, positively charged positively charged region in the atom.region in the atom.

He called this the He called this the nucleus.nucleus.

The Rutherford Model of the AtomThe Rutherford Model of the Atom

Rutherford Rutherford developed a new developed a new atomic model in atomic model in 1913. 1913.

In it, the electrons In it, the electrons move in space move in space around the positively around the positively charged nucleus.charged nucleus.

The problem with his The problem with his model is that the model is that the nature of electrons is nature of electrons is not accurately not accurately described by this described by this model.model.

The Atomic NucleusThe Atomic Nucleus

The nucleus contains protons and The nucleus contains protons and neutrons.neutrons.

It is the number of protons that It is the number of protons that determines the identity of an element.determines the identity of an element.

Atoms with more than one proton should Atoms with more than one proton should not exist.not exist.

Nuclear forces hold the nucleus together. Nuclear forces hold the nucleus together. These are strong attractions between These are strong attractions between protons and protons, neutrons and protons and protons, neutrons and neutrons, and protons and neutrons.neutrons, and protons and neutrons.

Atoms are very small and Atoms are very small and Numerous.Numerous.

Neon for example is a gas that Neon for example is a gas that makes up only 0.002% of our air.makes up only 0.002% of our air.

In each breath you take, you In each breath you take, you breath in 5 * 10breath in 5 * 101717 atoms of neon. atoms of neon.

If the nucleus of an atom were If the nucleus of an atom were increased in size to 1cmincreased in size to 1cm33, it , it would have a mass of 2 * 10would have a mass of 2 * 1088 tons.tons.

Section 3-3Section 3-3

Counting AtomsCounting Atoms

There are three numbers that There are three numbers that identify an atomidentify an atom

Atomic NumberAtomic Number Mass NumberMass Number Atomic MassAtomic Mass

Atomic NumberAtomic Number

The number of protons in the The number of protons in the nucleus.nucleus.

This is the number that identifies This is the number that identifies an element.an element.

This is the whole number on the This is the whole number on the periodic table.periodic table.

Mass NumberMass Number

Most elements have isotopes.Most elements have isotopes. Isotopes – Atoms of the same elements Isotopes – Atoms of the same elements

with different numbers of neutrons.with different numbers of neutrons. Mass Number – The number of protons Mass Number – The number of protons

plus the number of neutrons.plus the number of neutrons. There are two ways to write the isotope of There are two ways to write the isotope of

an element.an element. Write the name, then a hyphen, then the Write the name, then a hyphen, then the

mass number – Carbon – 14mass number – Carbon – 14 Write the symbol, mass number as a Write the symbol, mass number as a

superscript, atomic number as a subscript.superscript, atomic number as a subscript. An isotope is also called a nuclide.An isotope is also called a nuclide.

Atomic MassAtomic Mass

Do not use grams to measure Do not use grams to measure mass of an atom, instead we use mass of an atom, instead we use atomic mass units (amu).atomic mass units (amu).

Average atomic mass is the Average atomic mass is the weighted average of the masses weighted average of the masses of all of the isotopes of an of all of the isotopes of an element.element.

This is the decimal number on This is the decimal number on the periodic table.the periodic table.

There are three ways to compare There are three ways to compare numbers of atoms.numbers of atoms.

MolesMoles Avogadro’s NumberAvogadro’s Number Molar MassMolar Mass

The MoleThe Mole

The amount of The amount of substance that substance that contains as contains as many particles many particles as there are in as there are in 12 grams of 12 grams of carbon – 12.carbon – 12.

Avogadro’s NumberAvogadro’s Number

The number of The number of particles in one particles in one mole of a pure mole of a pure substance.substance.

Named after Named after Italian scientist Italian scientist Amadeo Amadeo Avogadro (1776 – Avogadro (1776 – 1856).1856).

6.02 * 106.02 * 102323 particles.particles.

Molar MassMolar Mass

The mass of one mole of a pure The mass of one mole of a pure substance.substance.

Expressed in grams.Expressed in grams. It is equal to the atomic mass It is equal to the atomic mass

(decimal number on the periodic (decimal number on the periodic table) in grams.table) in grams.