Atomic StructureAtomic Structure

I. AtomsI. Atoms

The atom is the The atom is the basicbasic unit of unit of matter.matter.

Smallest particle of an Smallest particle of an element that retains the element that retains the characteristicscharacteristics of that of that element.element.

Three Basic Subatomic Particles:Three Basic Subatomic Particles: proton neutron electronproton neutron electron Charge:Charge: +1 +1 0 0 -1 -1

Mass Mass Number:Number: 1 amu 1 amu 1 amu 0 amu 1 amu 0 amu

Location:Location: nucleusnucleus nucleus electron nucleus electron cloudcloud

What is an amu?What is an amu?

atomic mass unitatomic mass unit::

used to express very small used to express very small massesmasses

based on the mass of 1/12 of based on the mass of 1/12 of a carbon-12 atoma carbon-12 atom

1 amu = 1.66054 x 101 amu = 1.66054 x 10-24-24 grams grams

The atom is divided into two The atom is divided into two separate areas:separate areas:

NucleusNucleus Electron CloudElectron Cloud

1. Nucleus1. Nucleus Found in Found in centercenter of an atom of an atom SmallSmall and and densedense Contains Contains protonsprotons and and neutronsneutrons, so is , so is positivelypositively chargedcharged

Contains most of the Contains most of the massmass of of an atoman atom

The number of The number of protonsprotons is is ALWAYS theALWAYS the samesame . .

The number of The number of neutronsneutrons can can varyvary..

2. Electron Cloud2. Electron Cloud Located Located outsideoutside the nucleus the nucleus NOTNOT dense dense Contains Contains electronselectrons, so , so

negativelynegatively charged charged Mass of electrons is Mass of electrons is

negligiblenegligible Electrons determine reactivityElectrons determine reactivity

Periodic TablePeriodic Table

Each box on the periodic table Each box on the periodic table provides three important pieces provides three important pieces of information:of information:

1.1. SymbolSymbol2.2. Atomic NumberAtomic Number3.3. Atomic Mass (Mass #)Atomic Mass (Mass #)

Atomic numberAtomic number: :

The number of The number of protons protons in the in the nucleusnucleus

The number of protonsThe number of protons identifies identifies the elementthe element

Examples:Examples:Carbon: atomic # is 6, so 6 pCarbon: atomic # is 6, so 6 p++

Sulfur: atomic # is 16, so 16 pSulfur: atomic # is 16, so 16 p++

Because atoms are electrically Because atoms are electrically neutralneutral, , each atom contains the each atom contains the samesame number number of protons as electrons.of protons as electrons.

Examples: Examples:

Helium: 2 pHelium: 2 p++, so 2 e-, so 2 e-Magnesium: 12 pMagnesium: 12 p++, so 12 e-, so 12 e-Iron: 26 pIron: 26 p++, so 26 e-, so 26 e-

Atomic mass (mass #)Atomic mass (mass #): : The number of protons The number of protons plusplus the the number of neutrons in an atomnumber of neutrons in an atom

Examples:Examples: Carbon-12 = 6 pCarbon-12 = 6 p++ + 6 n + 6 n00

Mercury-201 = 80 pMercury-201 = 80 p++ + 121 n + 121 n00

Mass Mass NumberNumber

AtomicAtomicNumberNumber

X

Example: How many neutrons in C?

12

6C

Mass Number(# p+ + # no)Atomic

Number (# p+ )

12 -6 = 6 neutrons

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II. IonsII. Ions An ion is a An ion is a chargedcharged particle particle

# of p# of p++ does not equaldoes not equal the # of e the # of e--

The ONLY way to form an ion is to The ONLY way to form an ion is to changechange # of # of ELECTRONSELECTRONS

Positive ions (Positive ions (cationscations): # e): # e-- << # p # p++

Negative ions (Negative ions (anionsanions): # e): # e-- >> # p # p++

Example 1:Example 1:LithiumLithium Neutral atom: 3 pNeutral atom: 3 p++ and 3 e and 3 e--

Loses one eLoses one e- - to form Lito form Li+1 +1 ionion Ion: 3 pIon: 3 p++ and 2 e and 2 e--

Example 2:Example 2:OxygenOxygen Neutral atom: 8 pNeutral atom: 8 p++ and 8 e and 8 e--

Gains two eGains two e-- to form O to form O-2 -2 ionion Ion: 8 pIon: 8 p++ and 10 e and 10 e--

Remember:Remember:

Electrons control behavior in Electrons control behavior in ordinary chemical reactions!ordinary chemical reactions!

III. IsotopesIII. Isotopes

Atoms of the same element Atoms of the same element that differ from each other that differ from each other by the number ofby the number of neutrons neutrons they containthey contain

Most elements Most elements have at least have at least twotwo naturally occurring naturally occurring isotopes.isotopes.

Isotopes of the same element:Isotopes of the same element:

1.1. Have the Have the samesame atomic number atomic number

2.2. Have Have differentdifferent mass mass numbers numbers

3.3. Have Have similarsimilar behavior behavior

When you are working with a When you are working with a specific isotope, the mass specific isotope, the mass number on the periodic table number on the periodic table will not be exactly correctwill not be exactly correct

You need a way to write the You need a way to write the symbol so that it shows the symbol so that it shows the atomic number and the correct atomic number and the correct atomic mass of that particular atomic mass of that particular isotope. isotope.

1.1. To write an Isotope (Nuclear) SymbolTo write an Isotope (Nuclear) Symbol

Mass Mass NumberNumber

AtomicAtomicNumberNumber

X

Examples: How many neutrons in C-12?

12

6C

Mass Number(# p+ + # no)Atomic

Number (# p+ )

12-6 = 6 neutrons

How many neutrons in C-13?

13

6C

Mass Number(# p+ + # no)Atomic

Number (# p+ )

13-6 = 7 neutrons

2. 2. To name an isotope using a To name an isotope using a Hyphen Notation SymbolHyphen Notation Symbol

X – X – atomic mass of specific isotopeatomic mass of specific isotope

C-12 is carbon with a mass of 12 amu C-12 is carbon with a mass of 12 amu (6 protons + 6 neutrons)(6 protons + 6 neutrons)

C-13 is carbon with a mass of 13 amu C-13 is carbon with a mass of 13 amu (6 protons + 7 neutrons)(6 protons + 7 neutrons)

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IV. Relative Abundance andIV. Relative Abundance and Average Atomic Mass Average Atomic Mass

The naturally occurring The naturally occurring isotopes of each element isotopes of each element are present in specific are present in specific amounts known as amounts known as relative abundancerelative abundance

The The higherhigher the the % Abundance, % Abundance, the more the more commoncommon the isotope.the isotope.

Average Atomic MassAverage Atomic Mass--

Mass # on periodic tableMass # on periodic table

Weighted Weighted average of the mass average of the mass and abundance of the naturally and abundance of the naturally occurring isotopes of an occurring isotopes of an elementelement

To Calculate Average Atomic To Calculate Average Atomic Mass:Mass:

1.1. Multiply mass by % abundance for each Multiply mass by % abundance for each isotopeisotope

2.2. Add all answers from step #1 to determine Add all answers from step #1 to determine average atomic massaverage atomic mass

Hint: when calculated, Hint: when calculated, average atomic mass average atomic mass should equal the Mass should equal the Mass Number from the Periodic Number from the Periodic Table.Table.

Example 1: Example 1:

Calculate the average atomic Calculate the average atomic mass for the element Boron.mass for the element Boron.

Boron – 10 Boron – 10 19.8%19.8%

Boron – 11Boron – 11 80.2%80.2%

Step 1: Change % to decimals.Step 1: Change % to decimals.19.8% = .19819.8% = .19880.2% = .80280.2% = .802

Step 2: Multiply mass of each Step 2: Multiply mass of each isotope by its abundance.isotope by its abundance.

10 x .198 = x10 x .198 = x11 x .802 = y11 x .802 = y

Step 3: Add. x + y = Step 3: Add. x + y =

To make life easier, we will always round atomic masses to the nearest tenth. 10.8 amu

Answer: Answer:

10 x .198 = 1.9810 x .198 = 1.9811 x .802 = 11 x .802 = 8.82 +8.82 + 10.802 10.802

Example 2: Example 2:

Two isotopes of copper occur Two isotopes of copper occur in nature. in nature.

69.17% of copper atoms have 69.17% of copper atoms have a mass of 62.94 amua mass of 62.94 amu

30.83% have a mass of 30.83% have a mass of 64.93amu. 64.93amu.

What is the average atomic What is the average atomic mass?mass?

AnswerAnswer

62.94 x .6917 = 43.5462.94 x .6917 = 43.5464.93 x .3083 = 64.93 x .3083 = 20.02 +20.02 +

63.56 amu63.56 amu

63.6 amu63.6 amu

Example 3:Example 3: Uranium has 3 isotopes with the following Uranium has 3 isotopes with the following

masses and % abundance. Calculate its masses and % abundance. Calculate its average atomic mass.average atomic mass.

    234 U 234 U 0.0058%0.0058% 9292  235 U235 U 0.71%0.71% 9292

238 U238 U 99.23%99.23%9292

(234 x .000058) + (235 x .0071) + (234 x .000058) + (235 x .0071) + (238 x .9923) = 237.84 (238 x .9923) = 237.84

237.8 amu237.8 amu

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