1 Chapter 4 Atoms and their structure 2 History of the atom n Not the history of atom, but really...

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Chapter 4Chapter 4

Atoms and their structureAtoms and their structure

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History of the atomHistory of the atom Not the history of atom, but really the idea of the atomNot the history of atom, but really the idea of the atom The original idea - Ancient Greece (400 B.C..)The original idea - Ancient Greece (400 B.C..) Democritus and Leucippus Greek philosophers who Democritus and Leucippus Greek philosophers who

were debating whether matter was continuous or were debating whether matter was continuous or discontinuous.discontinuous.

Continuous – divide matter forever and always have a Continuous – divide matter forever and always have a smaller piece of matter.smaller piece of matter.

Discontinuous – divide matter only so far and can go Discontinuous – divide matter only so far and can go no farther. no farther.

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History of AtomHistory of Atom Start with a box of marble divide in half Start with a box of marble divide in half

eventually you get down to one marble eventually you get down to one marble which if you divide again you no longer which if you divide again you no longer have a marble.have a marble.

The Greek word for “can not cut” is atomos – thus atom.

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Another GreekAnother GreekAristotle – another famous Greek philosopherAristotle – another famous Greek philosopherAll substances are made of 4 elementsAll substances are made of 4 elements

Fire - HotFire - HotAir - lightAir - lightEarth - cool, heavyEarth - cool, heavyWater - wetWater - wet

Aristotle and others believed in 4 elements of Aristotle and others believed in 4 elements of matter, combined in different proportions rather matter, combined in different proportions rather than indivisible particlesthan indivisible particles

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Who Was Right?Who Was Right? Greek society was slave based and so it was Greek society was slave based and so it was

beneath the famous to work with their hands.beneath the famous to work with their hands.

The Greeks did not experiment, they settled The Greeks did not experiment, they settled disagreements by argument (debates).disagreements by argument (debates).

Aristotle was more famous so his ideas carried Aristotle was more famous so his ideas carried through to the middle ages.through to the middle ages.

During the middle ages Alchemists tried to During the middle ages Alchemists tried to change lead to goldchange lead to gold

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Re-emergence of the Atomic TheoryRe-emergence of the Atomic Theory

Late 1700’s - John Dalton- Late 1700’s - John Dalton- EnglandEngland ((a major contributor a major contributor to today’s Atomic Theory)to today’s Atomic Theory)

A teacher who summarized results of his A teacher who summarized results of his experiments and those of other’sexperiments and those of other’s

In Dalton’s Atomic Theory he combined ideas of In Dalton’s Atomic Theory he combined ideas of elements with that of atomselements with that of atoms

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Dalton’s Atomic TheoryDalton’s Atomic Theory All All mattermatter is made of tiny is made of tiny indivisibleindivisible particles particles

called atoms.called atoms. Atoms of the same element are identical, those Atoms of the same element are identical, those

of different atoms are different.of different atoms are different. Atoms of different elements combine in whole Atoms of different elements combine in whole

number ratios to form compoundsnumber ratios to form compounds Chemical reactions involve the rearrangement Chemical reactions involve the rearrangement

of atoms. No new atoms are created or of atoms. No new atoms are created or destroyed.destroyed.

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Law of Definite ProportionsLaw of Definite Proportions• Each compound has a specific whole-number Each compound has a specific whole-number

ratio of elements; ratio is by mass [Definite ratio of elements; ratio is by mass [Definite Proportions]Proportions]

WaterWater HH22OO

Carbon dioxideCarbon dioxide COCO22

MethaneMethane CHCH44

8.0 g oxygen reacts with 1.0 g hydrogen (H2O)Ratio = 8:1 by mass

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Law of Multiple ProportionsLaw of Multiple Proportions if two elements form more that one compound, if two elements form more that one compound,

the ratio of the second element that combines the ratio of the second element that combines with 1 gram of the first element in each is a with 1 gram of the first element in each is a simple whole number.simple whole number.

•In hydrogen peroxide 16.0 g oxygen reacts with 1.0 g hydrogen (H2O2)

•Ratio of the masses of oxygen in hydrogen peroxide and water is 16:8 = 2:1 Therefore H2O2 contains twice as many oxygen atoms per hydrogen atom than H2O [Multiple Proportions]

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What?What?

Water is 8 grams of oxygen per gram of hydrogen. Water is 8 grams of oxygen per gram of hydrogen. (H(H22O)O)

Hydrogen Peroxide is 16 grams of oxygen per gram of hydrogen. Hydrogen Peroxide is 16 grams of oxygen per gram of hydrogen. (H(H22OO22))

16 to 8 is a 2 to 1 ratio 16 to 8 is a 2 to 1 ratio

Therefore HTherefore H22OO22 contains twice as many oxygen atoms per hydrogen atom than contains twice as many oxygen atoms per hydrogen atom than

HH22O [Multiple Proportions]O [Multiple Proportions]

True because you have to add a whole atom, you can’t add a True because you have to add a whole atom, you can’t add a

piece of an atom.piece of an atom.

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Parts of AtomsParts of Atoms

As scientists began to develop methods for more As scientists began to develop methods for more detailed probing of the nature of matter, the atom detailed probing of the nature of matter, the atom (supposedly indivisible) began to show signs of a (supposedly indivisible) began to show signs of a more complex structuremore complex structure

J. J. Thomson - English physicist. 1897J. J. Thomson - English physicist. 1897 Made a piece of equipment called a cathode Made a piece of equipment called a cathode

ray tube which was used to study the electrical ray tube which was used to study the electrical conductivity of gasses.conductivity of gasses.

The cathode ray tube is a vacuum tube - the air The cathode ray tube is a vacuum tube - the air has been pumped out.has been pumped out.

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Thomson’s ExperimentThomson’s Experiment

Voltage source

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Vacuum tube

Metal Disks

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Thomson’s ExperimentThomson’s Experiment

Voltage source

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Thomson’s ExperimentThomson’s Experiment

Voltage source

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Thomson’s ExperimentThomson’s Experiment

Voltage source

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Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

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Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

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Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

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Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

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Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field By adding an electric field

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Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field By adding an electric field

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-

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Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field By adding an electric field

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-

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Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field By adding an electric field

+

-

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Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field By adding an electric field

+

-

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Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field By adding an electric field

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-

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Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field he found that the By adding an electric field he found that the moving pieces were negative because the moving pieces were negative because the rays were attracted to the positive electrode rays were attracted to the positive electrode in the external field. in the external field.

+

-

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Cathode Ray TubeCathode Ray Tube

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• Thompson concluded that:Thompson concluded that:» Cathode rays consist of beams of particlesCathode rays consist of beams of particles

» The particles have a negative chargeThe particles have a negative charge

Based on his findings, a new fundamental Based on his findings, a new fundamental particle of matter was discovered – The particle of matter was discovered – The Electron!Electron!

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Thomsom’s ModelThomsom’s Model Couldn’t find positive (for a Couldn’t find positive (for a

while) while) Said the atom was like plum Said the atom was like plum

pudding or (blueberry muffin)pudding or (blueberry muffin) A bunch of positive stuff, with A bunch of positive stuff, with

the electrons embedded the electrons embedded (able to remove the (able to remove the embedded electrons) embedded electrons)

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Other piecesOther pieces Proton - positively charged pieces 1840 Proton - positively charged pieces 1840

times heavier than the electrontimes heavier than the electron Neutron - no charge but the same mass Neutron - no charge but the same mass

as a proton.as a proton. Where are the pieces?Where are the pieces?

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Rutherford’s experimentRutherford’s experiment Ernest Rutherford English physicist. (1910)Ernest Rutherford English physicist. (1910) Believed in the plum pudding model of the Believed in the plum pudding model of the

atom.atom. Used radioactivityUsed radioactivity Alpha particles - positively charged pieces Alpha particles - positively charged pieces

given off by uranium given off by uranium Shot them at gold foil which can be made a few Shot them at gold foil which can be made a few

atoms thick atoms thick

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Rutherford’s experimentRutherford’s experiment When the alpha particles hit a florescent When the alpha particles hit a florescent

screen, it glows.screen, it glows. Here’s what it looked likeHere’s what it looked like

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Lead block

Uranium

Gold Foil

Florescent Screen

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He ExpectedHe Expected Rutherford believed that the Rutherford believed that the particles particles

would pass through unhindered.would pass through unhindered. (The (The alpha particles to pass through without alpha particles to pass through without changing direction very much.)changing direction very much.)

Because the positive charges were Because the positive charges were spread out evenly within the atom. The spread out evenly within the atom. The positive charge in the atom was not positive charge in the atom was not enough to stop the alpha particlesenough to stop the alpha particles

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What he expected

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Because

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Because, he thought the mass was evenly distributed in the atom

particles

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Because, he thought the mass was evenly distributed in the atom

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What he got

A small percentage of the particles were being reflected at unexpected angles, inconsistent with the “muffin model”

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How he explained it

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Atom is mostly emptyAtom is mostly empty Small dense,Small dense, positive piece at centerpositive piece at center Alpha particles are deflected by it if they Alpha particles are deflected by it if they

get close enoughget close enough

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+ particles

Rutherford explained his observations as follows:Rutherford explained his observations as follows:

•Atom is mostly empty space• Small, dense, and positive at the center• Alpha particles were deflected if they got close enough

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Modern ViewModern ViewThe atom is mostly empty spaceThe atom is mostly empty space

Two regionsTwo regions

Nucleus- protons and neutronsNucleus- protons and neutrons

Electron cloud- region where you might find an electronElectron cloud- region where you might find an electron

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Density and the AtomDensity and the Atom Since most of the alpha particles went through, Since most of the alpha particles went through,

the atom is mostly empty.the atom is mostly empty. Because the alpha particles turned so much, Because the alpha particles turned so much,

the positive region of the atom is heavy.the positive region of the atom is heavy. Small volume, big mass, big densitySmall volume, big mass, big density This small dense positive area is the This small dense positive area is the nucleusnucleus

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Subatomic particlesSubatomic particles

Electron

Proton

Neutron

Name Symbol ChargeRelative mass

Actual mass (g)

e-

p+

nº

-1

+1

0

1/1840

1

1

9.11 x 10-28

1.67 x 10-24

1.67 x 10-24

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Structure of the AtomStructure of the AtomThere are two regionsThere are two regions

1.1. The The nucleusnucleus – with protons and neutrons so – with protons and neutrons so that it has a Positive charge and almost all the that it has a Positive charge and almost all the massmass

2.2. Electron cloudElectron cloud- Most of the volume of an atom - Most of the volume of an atom and is the region where the electron can be and is the region where the electron can be found (extra nuclear)found (extra nuclear)

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Size of an atomSize of an atom Atoms are small and are measured in Atoms are small and are measured in

picometers, 10picometers, 10-12-12 meters meters Hydrogen atom, 32 pm radiusHydrogen atom, 32 pm radius Nucleus tiny compared to atom. If the atom was Nucleus tiny compared to atom. If the atom was

the size of a stadium, the nucleus would be the the size of a stadium, the nucleus would be the size of a marble.size of a marble.

Radius of the nucleus near 10Radius of the nucleus near 10-15-15m.m. Density near 10Density near 101414 g/cm g/cm33

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Counting the PiecesCounting the PiecesAtomic NumberAtomic Number = number of protons (p= number of protons (p++) ) The number of protons determines kind of atomThe number of protons determines kind of atom – –

2 protons in the nucleus means that this is a 2 protons in the nucleus means that this is a Helium atom. Helium atom.

Chemists use Chemists use ZZ as a symbol for atomic number.as a symbol for atomic number.In a neutral atom there is the same number of In a neutral atom there is the same number of

electrons (eelectrons (e--) and protons (atomic number)) and protons (atomic number)Mass NumberMass Number == number of number of protons + neutronsprotons + neutrons

[Sum of p[Sum of p++ and n and nº (º (pp++ + n + nº)] The symbol used º)] The symbol used for mass number is for mass number is AA..

The neucleons (pThe neucleons (p++ and n and nº)º) make up the mass of make up the mass of the atom.the atom.

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IsotopesIsotopes Dalton was wrong when he said that all atoms Dalton was wrong when he said that all atoms

of one element are the same.of one element are the same. Atoms of the same element can have different Atoms of the same element can have different

numbers of neutrons and therefore have numbers of neutrons and therefore have different mass numbers and different masses.different mass numbers and different masses.

The atoms of the same element that differ in The atoms of the same element that differ in the number of neutrons are called the number of neutrons are called isotopes of isotopes of that element.that element.

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Nuclear SymbolsNuclear Symbols Contain the symbol of the element, the Contain the symbol of the element, the

mass number and the atomic numbermass number and the atomic number

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Nuclear SymbolsNuclear Symbols

E A

Z

Contain: Contain:

the symbol of the symbol of the elementthe element

the mass numberthe mass number

the atomic numberthe atomic number

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SymbolsSymbols Find the Find the

– number of protonsnumber of protons

– number of neutronsnumber of neutrons

– number of electronsnumber of electrons

– Atomic numberAtomic number

– Mass NumberMass Number

F19 9

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SymbolsSymbols Find the Find the

–number of protonsnumber of protons

–number of neutronsnumber of neutrons

–number of electronsnumber of electrons

–Atomic numberAtomic number

–Mass NumberMass Number

Br80 35

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SymbolsSymbols if an element has an atomic if an element has an atomic

number of 34 and a mass number number of 34 and a mass number of 78 what is the of 78 what is the

–number of protonsnumber of protons

–number of neutronsnumber of neutrons

–number of electronsnumber of electrons

–Complete nuclear symbolComplete nuclear symbol

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SymbolsSymbols if an element has 91 protons and if an element has 91 protons and

140 neutrons what is the 140 neutrons what is the

–Atomic numberAtomic number

–Mass numberMass number

–number of electronsnumber of electrons

–Complete nuclearComplete nuclear symbolsymbol

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SymbolsSymbols if an element has 78 electrons and if an element has 78 electrons and

117 neutrons what is the 117 neutrons what is the

–Atomic numberAtomic number

–Mass numberMass number

–number of protonsnumber of protons

–Complete nuclearComplete nuclear symbolsymbol

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Naming IsotopesNaming Isotopes Put the mass number after the name of Put the mass number after the name of

the elementthe element carbon- 12carbon- 12 carbon -14carbon -14 uranium-235uranium-235

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Atomic MassAtomic Mass How heavy is an atom of oxygen?How heavy is an atom of oxygen? There are different kinds of oxygen atoms.There are different kinds of oxygen atoms. More concerned with More concerned with average average atomic mass.atomic mass. Based on abundance of each isotope in nature.Based on abundance of each isotope in nature. Don’t use grams because the numbers would Don’t use grams because the numbers would

be too smallbe too small

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Measuring Atomic MassMeasuring Atomic Mass Unit is the Unit is the Atomic Mass Unit Atomic Mass Unit (amu)(amu) One twelfth the mass of a carbon-12 atom. One twelfth the mass of a carbon-12 atom. Each isotope has its own atomic mass we Each isotope has its own atomic mass we

need the average from percent abundance.need the average from percent abundance.

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Calculating averagesCalculating averages You have five rocks, four with a mass of 50 You have five rocks, four with a mass of 50

g, and one with a mass of 60 g. What is the g, and one with a mass of 60 g. What is the average mass of the rocks?average mass of the rocks?

Total mass = 4 x 50 + 1 x 60 = 260 gTotal mass = 4 x 50 + 1 x 60 = 260 g Average mass = 4 x 50 + 1 x 60 = 260 gAverage mass = 4 x 50 + 1 x 60 = 260 g

5 5 5 5 Average mass = 4 x 50 + 1 x 60 = 260 gAverage mass = 4 x 50 + 1 x 60 = 260 g

5 5 55 5 5

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Calculating averagesCalculating averages Average mass = 4 x 50g + 1 x 60g = 260 g = 52gAverage mass = 4 x 50g + 1 x 60g = 260 g = 52g

5 5 5 5 5 5 Average mass = .8 x 50g + .2 x 60g = 52gAverage mass = .8 x 50g + .2 x 60g = 52g 80% of the rocks were 50 grams80% of the rocks were 50 grams 20% of the rocks were 60 grams20% of the rocks were 60 grams

AverageAverage == %% asas decimaldecimal xx massmass11++ %% asas decimaldecimal xx massmass22 ++ %% asas decimaldecimal xx massmass33 ++……

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Atomic MassAtomic Mass Calculate the atomic mass of copper if Calculate the atomic mass of copper if

copper has two isotopes. 69.1% has a mass copper has two isotopes. 69.1% has a mass of 62.93 amu and the rest (30.9%) has a of 62.93 amu and the rest (30.9%) has a mass of 64.93 amu.mass of 64.93 amu.

0.6910.691 x 62.93amu 62.93amu 43.48463amu43.48463amu

+ 0.309 x 64.93amu = + 0.309 x 64.93amu = + 20.06337amu = 63.548amu + 20.06337amu = 63.548amu

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Atomic MassAtomic Mass Magnesium has three isotopes. 78.99% Magnesium has three isotopes. 78.99%

magnesium 24 with a mass of 23.9850 magnesium 24 with a mass of 23.9850 amu, 10.00% magnesium 25 with a mass of amu, 10.00% magnesium 25 with a mass of 24.9858 amu, and the rest magnesium 26 24.9858 amu, and the rest magnesium 26 with a mass of 25.9826 amu. What is the with a mass of 25.9826 amu. What is the atomic mass of magnesium?atomic mass of magnesium?

If not told otherwise, the mass of the If not told otherwise, the mass of the isotope is the mass number in amu isotope is the mass number in amu

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Atomic MassAtomic Mass is not a whole number because it is an average is not a whole number because it is an average

(are the decimal numbers on the periodic table).(are the decimal numbers on the periodic table).

Isotopes - atoms of the same element can have different numbers of neutrons and therefore have different mass numbers

When naming, write the mass number after the name of When naming, write the mass number after the name of the elementthe element

H11

Protium

H21Deuterium

H31

Tritium