Chapter: Properties of Atoms and the Periodic Table

Table of Contents

Section 3: The Periodic Table

Section 1: Structure of the Atom

Section 2: Masses of Atoms

• Scientists havedeveloped theirown shorthand fordealing with long,complicated names.

• Chemical symbolsconsist of one capitalletter or a capitalletter plus one or twosmaller letters.

Scientific Shorthand1Structure of the Atom

• For some elements, the symbol is the firstletter of the element’s name.

• For other elements, the symbol is the firstletter of the name plus another letter from itsname.

Scientific ShorthandStructure of the Atom

• Because scientists worldwide use this system,everyone understands what the symbolsmean.

1

• An element is matter that is composed of onetype of atom, which is the smallest piece ofmatter that still retains the property of theelement.

• Atoms are composed ofparticles called protons,neutrons, and electrons.

Atomic Components Structure of the Atom

Click image to view movie.

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• Protons and neutrons are found in a smallpositively charged center of the atom calledthe nucleus that is surrounded by a cloudcontaining electrons.

• Protons areparticleswith anelectricalcharge of1+.

Atomic Components Structure of the Atom

1

• Electrons are particles with an electricalcharge of 1–.

Atomic Components Structure of the Atom

• Neutrons are neutral particles that do nothave an electrical charge.

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• Protons and neutrons are made up of smallerparticles called quarks.

Quarks—Even Smaller Particles Structure of the Atom

• So far, scientists have confirmed theexistence of six uniquely different quarks.

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• Scientists theorize that an arrangement ofthree quarks held together with strong nuclearforce produces a proton.

Structure of the Atom

• Another arrangement of three quarksproduces a neutron.

Quarks—Even Smaller Particles 1

• To study quarks, scientists accelerate chargedparticles to tremendous speeds and then forcethem to collide with—or smash into—protons. This collision causes the proton tobreak apart.

Finding Quarks Structure of the Atom

• The particles that result from the collisioncan be detected by various collection devises.

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• Scientists found five quarks and hypothesizedthat a sixth quark existed. However, it took ateam of nearly 450 scientists from around theworld several years to find the sixth quark.

The Sixth Quark Structure of the Atom

• The tracks of the sixth quark were hard todetect because only about one billionth of apercent of the proton collisions performedshows a presence of a sixth quark.

1

• Scientists and engineers use models torepresent things that are difficult tovisualize—or picture in your mind.

Models—Tools for Scientists Structure of the Atom

• Scaled-down models allow you to see eithersomething too large to see all at once, orsomething that has not been built yet.

• Scaled-up models are often used to visualizethings that are too small to see.

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• To study the atom, scientists have developedscaled-up models that they can use tovisualize how the atom is constructed.

Models—Tools for Scientists Structure of the Atom

• For the model to be useful, it must support allof the information that is known about matterand the behavior of atoms.

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• In the 1800s, John Dalton, an Englishscientist, was able to offer proof that atomsexist.

The Changing Atomic Model Structure of the Atom

• Another famous Greek philosopher, Aristotle,disputed Democritus’s theory and proposedthat matter was uniform throughout and wasnot composed of smaller particles.

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The Changing Atomic Model Structure of the Atom

• Dalton’s model of theatom, a solid spherewas an early model ofthe atom.

• The model haschanged somewhatover time.

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• By 1926, scientists had developed theelectron cloud model of the atom that isin use today.

The Electron Cloud Model Structure of the Atom

• An electroncloud is the areaaround thenucleus of anatom where itselectrons are mostlikely found.

1

• The electron cloud is 100,000 times largerthan the diameter of the nucleus.

The Electron Cloud Model Structure of the Atom

• In contrast, each electron in the cloud ismuch smaller than a single proton.

• Because an electron’s mass is small and theelectron is moving so quickly around thenucleus, it is impossible to describe its exactlocation in an atom.

1

Section Check

A. atomB. quarkC. neutronD. proton

Which is the smallest piece of matter thatstill retains the property of the element?

1Question 1

Section Check

The answer is A. An atom is the smallestpiece of matter that still retains the propertyof the element.

1Answer

Section Check

A. protons and electronsB. protons and neutronsC. neutrons and electronsD. quarks and electrons

What particles are found in the nucleus of anatom?

1Question 2

Section Check

The answer is B.Electrons arelocated in anelectron cloudsurrounding thenucleus of theatom.

1Answer

Section Check

What is the name of the small particles thatmake up protons and neutrons?

AnswerProtons and neutrons are made of smallerparticles called quarks.

1Question 3

Atomic Mass

Masses of Atoms2

Atomic Mass • The nucleus contains

most of the mass ofthe atom becauseprotons and neutronsare far more massivethan electrons.

• The mass of a protonis about the same asthat of a neutron—approximately

Masses of Atoms2

Atomic Mass • The mass of each is

approximately1,836 times greaterthan the mass of theelectron.

Masses of Atoms2

Atomic Mass

• The unit of measurement used for atomicparticles is the atomic mass unit (amu).

• The mass of a proton or a neutron is almostequal to 1 amu.

• The atomic mass unit is defined as one-twelfth the mass of a carbon atom containingsix protons and six neutrons.

Masses of Atoms2

Protons Identify the Element

• The number of protons tells you what type ofatom you have and vice versa. For example,every carbon atom has six protons. Also, allatoms with six protons are carbon atoms.

• The number of protons in an atom is equal toa number called the atomic number.

Masses of Atoms2

Mass Number • The mass number of an atom is the sum of

the number of protons and the number ofneutrons in the nucleus of an atom.

Masses of Atoms2

Mass Number • If you know the mass number and the atomic

number of an atom, you can calculate thenumber of neutrons.

Masses of Atoms

number of neutrons = mass number – atomic number

2

Isotopes

• Not all the atoms of an element have thesame number of neutrons.

• Atoms of the same element that havedifferent numbers of neutrons are calledisotopes.

Masses of Atoms2

Identifying Isotopes • Models of two isotopes of boron are shown.

Because the numbers of neutrons in theisotopes are different, the mass numbers arealso different.

• You use the name ofthe element followedby the mass number ofthe isotope to identifyeach isotope: boron-10 and boron-11.

Masses of Atoms2

Identifying Isotopes • The average atomic mass of an element is

the weighted-average mass of the mixture ofits isotopes.

• For example, four out of five atoms of boronare boron-11, and one out of five is boron-10.

Masses of Atoms

• To find the weighted-average or the averageatomic mass of boron, you would solve thefollowing equation:

2

Section Check

How is the atomic number of an elementdetermined?

AnswerThe atomic number of an element is equalto the number of protons in an atom of thatelement.

2Question 1

Section Check

The element helium has a mass number of 4and atomic number of 2. How manyneutrons are in the nucleus of a heliumatom?

2Question 2

Section Check

Recall that the atomic number is equal to thenumber of protons in the nucleus. Since themass number is 4 and the atomic number is2, there must be 2 neutrons in the nucleus ofa helium atom.

2Answer

Section Check

How much of the mass of an atom iscontained in an electron?

AnswerThe electron’s mass is so small that it isconsidered negligible when finding themass of an atom.

2Question 3

Organizing the Elements

• Periodic means “repeated in a pattern.”

• In the late 1800s, Dmitri Mendeleev, aRussian chemist, searched for a way toorganize the elements.

The Periodic Table

• When he arranged all the elements knownat that time in order of increasing atomicmasses, he discovered a pattern.

3

Organizing the Elements

• Because the pattern repeated, it wasconsidered to be periodic. Today, thisarrangement is called a periodic table ofelements.

• In the periodic table, the elements arearranged by increasing atomic number and bychanges in physical and chemical properties.

The Periodic Table3

Mendeleev’s Predictions

• Mendeleev had to leave blank spaces in hisperiodic table to keep the elements properlylined up according to their chemicalproperties.

• He looked at the properties and atomicmasses of the elements surrounding theseblank spaces.

The Periodic Table3

Mendeleev’s Predictions • From this

information, hewas able topredict theproperties andthe massnumbers of newelements thathad not yet beendiscovered.

The Periodic Table3

Mendeleev’s Predictions • This table shows

Mendeleev’spredictedproperties forgermanium,which he calledekasilicon. Hispredictionsproved to beaccurate.

The Periodic Table3

Improving the Periodic Table • On Mendeleev’s table, the atomic mass

gradually increased from left to right. Ifyou look at the modern periodic table, youwill see several examples, such as cobaltand nickel, where the mass decreases fromleft to right.

The Periodic Table3

Improving the Periodic Table

• In 1913, the work of Henry G.J. Moseley,a young English scientist, led to thearrangement of elements based on theirincreasing atomic numbers instead of anarrangement based on atomic masses.

The Periodic Table

• The current periodic table uses Moseley’sarrangement of the elements.

3

The Atom and the Periodic Table

• The vertical columns in the periodic table arecalled groups, or families, and are numbered1 through 18.

The Periodic Table

• Elements in each group have similarproperties.

3

Electron Cloud Structure

• In a neutral atom, the number of electronsis equal to the number of protons.

The Periodic Table

• Therefore, a carbon atom, with an atomicnumber of six, has six protons and sixelectrons.

3

Electron Cloud Structure • Scientists have found that electrons within

the electron cloud have different amountsof energy.

The Periodic Table3

Electron Cloud Structure

The Periodic Table

• Scientists model the energy differences of theelectrons by placing the electrons in energylevels.

3

Electron Cloud Structure

• Energy levels nearer the nucleus have lowerenergy than those levels that are farther away.

The Periodic Table

• Electrons fill these energy levels from theinner levels (closer to the nucleus) to theouter levels (farther from the nucleus).

3

Electron Cloud Structure

• Elements that are in the same group havethe same number of electrons in their outerenergy level.

The Periodic Table

• It is the number of electrons in the outerenergy level that determines the chemicalproperties of the element.

3

Energy Levels • The maximum number of electrons that can

be contained in each of the first four levels isshown.

The Periodic Table3

Energy Levels

The Periodic Table

• For example, energy level one can contain amaximum of two electrons.

• A complete and stable outer energy level willcontain eight electrons.

3

Rows on the Table • Remember that the atomic number found on

the periodic table is equal to the number ofelectrons in an atom.

The Periodic Table3

Rows on the Table

The Periodic Table

• The first row has hydrogen with one electronand helium with two electrons both in energylevel one.

• Energy level one can hold only two electrons.Therefore, helium has a full or completeouter energy level.

3

Rows on the Table • The second row begins with lithium, which

has three electrons—two in energy level oneand one in energy level two.

The Periodic Table

• Lithium is followed by beryllium with twoouter electrons, boron with three, and so onuntil you reach neon with eight outer electrons.

3

Rows on the Table • Do you notice how the row in the periodic

table ends when an outer level is filled?

The Periodic Table

• In the third row of elements, the electronsbegin filling energy level three.

• The row ends with argon, which has a fullouter energy level of eight electrons.

3

Electron Dot Diagrams

• Elements that are in the same group havethe same number of electrons in their outerenergy level.

The Periodic Table

• These outer electrons are so important indetermining the chemical properties of anelement that a special way to represent themhas been developed.

3

Electron Dot Diagrams • An electron dot diagram uses

the symbol of the element anddots to represent the electronsin the outer energy level.

The Periodic Table

• Electron dot diagrams are usedalso to show how the electrons inthe outer energy level are bondedwhen elements combine to formcompounds.

3

Same Group—Similar Properties • The elements in

Group 17, thehalogens, haveelectron dotdiagrams similarto chlorine.

The Periodic Table

• All halogens haveseven electrons intheir outer energylevels.

3

Same Group—Similar Properties • A common property of the halogens is the

ability to form compounds readily withelements in Group 1.

The Periodic Table

• The Group 1 element, sodium, reacts easilywith the Group 17 element, chlorine.

• The result is thecompound sodiumchloride, orNaCl—ordinarytable salt.

3

Same Group—Similar Properties • Not all elements will combine readily with

other elements.

The Periodic Table

• The elements inGroup 18 havecomplete outerenergy levels.

• This specialconfiguration makesGroup 18 elementsrelatively unreactive.

3

Regions on the Periodic Table

• The periodic table has several regions withspecific names.

The Periodic Table

• The horizontal rows of elements on theperiodic table are called periods.

• The elements increase by one proton and oneelectron as you go from left to right in aperiod.

3

Regions on the Periodic Table • All of the elements in the blue squares are

metals.

The Periodic Table3

Regions on the Periodic Table

The Periodic Table

• Those elements on the right side of the periodictable, in yellow, are classified as nonmetals.

3

Regions on the Periodic Table

The Periodic Table

• The elements in green are metalloids orsemimetals.

3

A Growing Family • In 1994, scientists at the Heavy-Ion Research

Laboratory in Darmstadt, Germany,discovered element 111.

The Periodic Table

• Element 112 was discovered at the samelaboratory.

• Both of these elements are produced in thelaboratory by joining smaller atoms into asingle atom.

3

Elements in the Universe

• Using the technology that is available today,scientists are finding the same elementsthroughout the universe.

The Periodic Table

• Many scientists believe that hydrogen andhelium are the building blocks of otherelements.

3

Elements in the Universe • Exploding stars, or supernovas, give

scientists evidence to support this theory.

The Periodic Table

• Many scientists believe that supernovas havespread the elements that are found throughoutthe universe.

3

Section Check

How are the elements arranged in theperiodic table?

3Question 1

Section Check

The elements are arranged by increasingatomic number and by changes in physicaland chemical properties.

3Answer

Section Check

What do the elements in a vertical columnof the periodic table have in common?

3Question 2

Section Check

The vertical columns in the periodic tableare called groups; elements in the samegroup have similar properties, such aselectrical conductivity.

3Answer

Section Check

What do the dots in this electron dot diagramrepresent?

3Question 3

Section Check

The dots represent the electrons in the outerenergy level.

3Answer

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