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Houston Community College System Chemistry 1405
Chapter 2Chapter 2
Atomic StructureAtomic Structure
By
Mounia Elamrani
Blei / Odians General, Organic, and Biochemistry
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Mounia Elamrani Chapter 2 The Atomic Structure
ObjectivesObjectives
Percent composition
Daltons atomic Theory
Constant composition of matter andconservation of mass
Atomic mass Atomic structure
Periodic table and properties of elements
Electron configuration and the valence shells Octet rule
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Mounia Elamrani Chapter 2 The Atomic Structure
The early atomic theory 2.1The early atomic theory 2.1
The concept of the atom had limited scientificusefulness until the discovery of twoimportant laws in 18th and 19th centuries: The Law of conservation of mass: No detectable
gain or loss of mass occurs in chemical reactions.
Mass is conserved.
The Law of constant composition: In a givenchemical compound, the elements are always
combined in the same proportions by mass.
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Mounia Elamrani Chapter 2 The Atomic Structure
Percent composition 2.1Percent composition 2.1
The usual form for describing the relativemasses of the elements in a compound is alist of percentages by mass, called thepercent composition
It is the number of grams of the element in100 g of the compound and can be calculatedusing:
100%element%samplewholeofmass
elementofmass=
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Mounia Elamrani Chapter 2 The Atomic Structure
ExampleExample
Analysis of 4.800g of niacin, one of the B-complex vitamins, yields 2.810g of C, 0.1954gof H, 0.5462g of N, and 1.249g of O. Calculatethe mass percent of each element in thecompound.
%26.02100%O%
%11.38100%N%
%4.071100%H%
%58.54100%C%
sampleg0.5462
Og1.249
sampleg4.800
Ng0.5462
sampleg4.800
Hg0.1954
sampleg4.800Cg2.810
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==
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Mounia Elamrani Chapter 2 The Atomic Structure
Dalton 2.2Dalton 2.2s Atomic Theorys Atomic Theory
Matter consists of tiny particles called atoms.
In chemical reactions, the atoms rearrange butthey do not themselves break apart.
In any sample of a pure element, all the atomsare identical in mass and other properties.
The atoms of different elements differ in massand other properties.
In a given compound the constituent atoms arealways present in the same fixed numerical ratio.
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Atomic Mass 2.3Atomic Mass 2.3
It follows from Daltons Atomic Theory thatatoms of an element have a constant,characteristic atomic mass or atomicweight
For example, for any sample of hydrogenfluoride: F-to-H atom ratio: 1 to 1
F-to-H mass ratio: 19.0 to 1.00 This is only possible if each fluorine atom is
19.0 times heavier than each hydrogen atom
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Mounia Elamrani Chapter 2 The Atomic Structure
)Atomic Mass Units )amu, u 2.3)Atomic Mass Units )amu, u 2.3
Atoms have extremely small masses. Themass of the heaviest known atom is about 4 x10-22 g.
We define the atomic mass unit, amu, usingCarbon-12 as the standard: 1 atom of carbon-12 = 12 u (exactly) 1 u = 1/12 of mass of 1 atom of carbon-12 (exactly)
This definition results in the assignment ofapproximately 1 u for the mass of hydrogen(the lightest atom)
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Mounia Elamrani Chapter 2 The Atomic Structure
The structure of atoms 2.4The structure of atoms 2.4
Experiments have been performed that showatoms are comprised of subatomic particles
There are three principal kinds of subatomicparticles:
Proton )p carries a positivecharge, found in thenucleus, mass1u
Electron )e carries a negativecharge, foundoutside the nucleus, about 1/1800 the mass of a
proton, mass0u
Neutron )n carries no charge, found in thenucleus, a bit heavier than a proton, mass1u
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Mounia Elamrani Chapter 2 The Atomic Structure
Atomic number and mass 2.4Atomic number and mass 2.4
numbernumber The number of protons in the nucleus is called
the atomic number(Z), and is unique foreach element.
Z = number of protons
The mass number(A) of an atom is the totalnumber of particles in the nucleus:
A = (number of protons) + (number of
neutrons)
The charge is given by:
Charge = number of proton - number ofelectrons
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Mounia Elamrani Chapter 2 The Atomic Structure
ExercisesExercises
What is the charge of an atom containing 9p,10n, and 9e?
What is the charge of an ion containing 12p,
13n, and 14e?
Which element has 22p and 26n, and what isits approximate mass?
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Mounia Elamrani Chapter 2 The Atomic Structure
Isotopes 2.5Isotopes 2.5
It turns out that most elements in nature areuniform mixtures of two or more kinds ofatoms with slightly different masses
Atoms of the same element with differentmasses are called isotopes
Isotopes have the same atomic number (Z)
but different mass (A)
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Mounia Elamrani Chapter 2 The Atomic Structure
Counting Particles in Isotopes 2.5Counting Particles in Isotopes 2.5
Naturally occurring carbon consists of threeisotopes, 12C, 13C, and 14C. State thenumber of protons, neutrons, and electrons ineach of these carbon atoms.
12C 13C 14C6 6 6
#P _______ _______ _______
#N _______ _______ _______#E _______ _______ _______
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Mounia Elamrani Chapter 2 The Atomic Structure
Calculating an element 2.5Calculating an element 2.5s averages average
atomic massatomic mass We average the masses of isotopes using
their masses and relative abundances to givethe average atomic mass of an element:
Naturally occurring chlorine is a mixture of
two isotopes:
ContributionMass(u)
Abundance(%)
Isotope
0.7577 * 34.9689 =26.50 u
34.968975.77Cl-35
0.2423 * 36.9659 =8.957 u
36.965924.23Cl-37
(Rounded) Total =35.46 u
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Mounia Elamrani Chapter 2 The Atomic Structure
The Periodic Table 2.6The Periodic Table 2.6
It Arranges elements by increasing atomicnumber
Elements are arranged in numbered rowscalled periods
The vertical columns are called groups orfamilies (group labels vary)
Elements with similar physical and chemicalproperties, are placed in vertical columns, or
groups.Li, Na, K: are soft, very reactive metals.
He, Ne, Ar:are non reactive gases.
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Mounia Elamrani Chapter 2 The Atomic Structure
2.6 Some important classifications2.6 Some important classifications
A groups = representative elements or maingroup elements I A= alkali metals II A = alkaline earth metals VI A = chalcogens VII A = halogens VIII = noble gases
B groups = transition elements Inner transition elements = elements 58-71
and 90-103 58 71 = lanthanide elements 90 103 = actinide elements
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Mounia Elamrani Chapter 2 The Atomic Structure
Modern Periodic Table 2.6Modern Periodic Table 2.6
Note: Placement of elements 58 71 and 90 103 saves space
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Mounia Elamrani Chapter 2 The Atomic Structure
Metals, Nonmetals, and 2.6Metals, Nonmetals, and 2.6
MetalloidsMetalloids Metals
Tend to shine (have metallic luster)
Can be hammered or rolled into thin sheets(malleable) and can be drawn into wire (ductile)
Are solids at room temperature (except Hg) and
conduct electricity
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Mounia Elamrani Chapter 2 The Atomic Structure
Metals, Nonmetals, and 2.6Metals, Nonmetals, and 2.6
MetalloidsMetalloids Nonmetals
Lack the properties ofmetals
At room T can be solid,liquid, or gas
React with metals toform (ionic) compounds
Metalloids Have properties between
metals and nonmetals(semiconductors)
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Mounia Elamrani Chapter 2 The Atomic Structure
2.6 Metals, Nonmetals, and2.6 Metals, Nonmetals, and
MetalloidsMetalloids
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Mounia Elamrani Chapter 2 The Atomic Structure
Electron Organization Within 2.7Electron Organization Within 2.7
The AtomThe Atom Some early scientists thought that electrons
were orbiting around the nucleus
Later scientist discovered, through theemission of light by the elements, that
electrons have energy states It all started with atomic emission spectra...
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Mounia Elamrani Chapter 2 The Atomic Structure
Continuous SpectrumContinuous Spectrum vs.vs. LineLine
SpectrumSpectrum
Hydrogen Line Spectrum
Na KRb
Visible Light Spectrum
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Mounia Elamrani Chapter 2 The Atomic Structure
Electromagnetic Radiation and 2.7Electromagnetic Radiation and 2.7
EnergyEnergy Radiation carries energy through space
Light is a form ofelectromagneticradiation
Electromagnetic radiation is characterized by
its wave nature The frequency , (in Hz or s-1), of a radiation
is the number ofwaves, or completevibrations, in one second
Each color of the rainbow has a specificfrequency
Visible light range: 8x104 to 4x104 Hz
The higher the energy of the radiation, the
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Mounia Elamrani Chapter 2 The Atomic Structure
Electromagnetic Radiation and 2.7Electromagnetic Radiation and 2.7
EnergyEnergy
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Mounia Elamrani Chapter 2 The Atomic Structure
Atomic Energy States 2.7Atomic Energy States 2.7
Plancksaid that energy
can only be absorbed orreleased from atoms infixed amounts calledquanta
Atoms absorb energyfirst, then they release itin the form of a radiation(or color if in the visiblerange)
When energy isabsorbed, the e- jumpsto the excited state
When energy isreleased, or emitted, thee- drops to the ground
Excited State
Ground State
Absorption Emission
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Mounia Elamrani Chapter 2 The Atomic Structure
The Quantum Mechanical Atom 2.8The Quantum Mechanical Atom 2.8
Complex mathematical theory that was ableto predict a variety of atomic properties
The detailed structure of the atom canexplain the periodicity of the chemical
properties of elements Elements can be built by adding electrons
and protons to the nuclei
There are rules guiding the way the electrons
are organized
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Mounia Elamrani Chapter 2 The Atomic Structure
Bohr Model 2.8Bohr Model 2.8
First model of the electronstructure
Gives levels where anelectron is most likely to be
found Incorrect today, but a key
in understanding the atom
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Mounia Elamrani Chapter 2 The Atomic Structure
Shells, Subshells, and OrbitalsShells, Subshells, and Orbitals
Shells Contain electrons that are similar inenergy and distance from nucleus: Low energy electrons are closest to the nucleus
Identified by a whole number (1, 2, 3) called theprincipal quantum number (n)..
The first shell (n=1) is lowest in energy, the 2ndlevel is next in energy (n=2) and so on: 1
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Mounia Elamrani Chapter 2 The Atomic Structure
Number of ElectronsNumber of Electrons
Maximum number of electrons in any level =2n2
n =1 2(1)2 = 2n =2 2(2)2 = 8
n =3 2(3)2 = 18
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Mounia Elamrani Chapter 2 The Atomic Structure
Order of Electron FillingOrder of Electron Filling
All electrons in the same energy level havesimilar energy.
Shell 1 2 electrons
Shell 2 8 electrons
Shell 3 18 electrons (8 first,later 10)
Order of filling for the first 20 electrons
Shell 1 2 3 4
2e 8e 8e 2e
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Mounia Elamrani Chapter 2 The Atomic Structure
Electron ConfigurationElectron Configuration
Lists the shells containing electrons
Written in order of increasing energy
Element \ Shell 1 2 3
He 2
C 2 4F 2 7
Ne 2 8
Al 2 8 3Cl 2 8 7
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Mounia Elamrani Chapter 2 The Atomic Structure
ExercisesExercises
A. The electron configuration for sulfur
1) 2,6 2) 8,2,6 3) 2, 8, 6
B. The element in period 3 with two electrons
in the outermost energy level1) Mg 2) Ca 3) Na
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Mounia Elamrani Chapter 2 The Atomic Structure
Valence ElectronsValence Electrons
Valence or outer shell, or outermostelectrons, are the electrons filling the lastlevel of energy in a given atom
Indicate the number of valance electrons:A. O 1) 4 2) 6 3) 8
B. Al 1) 13 2) 3 3) 1
C. Cl 1) 2 2) 5 3) 7
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Mounia Elamrani Chapter 2 The Atomic Structure
Periodic LawPeriodic Law
All the elements in a group have the sameelectron configuration in their outermostshells
Example: Group 2A
Be 2, 2Mg 2, 8, 2
Ca 2, 8, 8, 2
Specify if each pair has chemical properties
that are similar (1) or different (2):
A. Cl and Br B. 2 - 5 and 2 - 8 -7
C. 2 - 4 and 2 - 8 4 D. P and S
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Mounia Elamrani Chapter 2 The Atomic Structure
9. Electron Configurations and the Periodic Table9. Electron Configurations and the Periodic Table
- Note that the 3dorbital fills after the 4s orbital.
- Similarly, the 4forbital fills after the 5dorbital.
SiV
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Mounia Elamrani Chapter 2 The Atomic Structure
Electron Configurations and the Periodic TableElectron Configurations and the Periodic Table
There is a shorthand way of writing electron configurations
Write the core electronscorresponding to the filled Noble gasin square brackets.
Write the valence electrons explicitly.
Example, P: 1s22s22p63s23p3
but Ne is 1s2
2s2
2p6
Therefore, P: [Ne]3s23p3.
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Electron ConfigurationsElectron Configurations
There are a coupleexceptions due to morestable electronconfiguration:
Cr group prefers to have ahalf filled nd orbital with 5electrons Cr: [Ar] 4s2 3d4 becomes
Cr: [Ar] 4s1 3d5
Cu group prefers to have acompletely filled nd orbitalwith 10 electrons
C [A ] 42
3d9
b