Chapter 8 - Atoms and Periodic Properties
Will turn to a study of the properties of matter why materials have certain properties
chemistry - composition, structure and properties of substances and thetransformations they undergo
consider world - many objects with many propertiestrees bark, leaves, woodcar wheels, dash, hood
all substances made of combinations only
about 113 known elements
element - pure substance that cannot be decomposed into simpler substances by a chemical or physical process
well-defined properties
Water - H2O
Salt - NaCl
Periodic Table - lists all known elements
currently about 108 known elements
88 naturally occurring
others made in lab
Some substances known for long timehow to describe?
alchemists - “lead into gold”
antimony Sbconfusing
STANDARDIZATION - modern symbols
used world-wide now
how did we get these symbols?
+
HgAgAuNaFeSnPb
H HeC ClB BeO OsP PtS SeN Ni
Where do the names come from?
Pu Es AmU Fm FrHg Md EuNp Bh Cf
Cl - light greenTc - artificialNe - newHe - sunTe - Earth
Elements named after planets, people, places and descriptions!
Names passed by international council“Commission on the Nomenclature
of Inorganic Chemistry”
Names agreed upon worldwide standardized
Elements made up of large collection of atoms
Atom - smallest unit with the same chemical
identity as element (10-10 m, 10-24 g) chemical identity - physical and chemical
properties of a pure substance
structure of the atom: protons, neutrons, and electrons
+
-
+ ++
+
-
-
-
-
protonpositive chargestrong nuclear force
electron - negative charge (same as proton)-swarm around nucleus (electron cloud)-can be attracted away or added w/o chemical change -very light 1/1837 mass of proton (negligible)
neutronno chargesame mass as protondoes not affect chemical identity
NUCLEUS - fixed central part of atom
contains:
these are also called
nucleons-reside in nucleus
neutral atom-same number of p+ as e- (zero net charge)remember ion: atom with net charge
How to determine atomic structure
History: Ancient Greeks
Democritus-matter is discontinuous
cannot divide indefinitely
“atom” - Greek for uncuttable
Aristotle & Plato disagreed with this view
(wrongly) thought matter was continuous
John Dalton (1800’s) revisited idea of Atoms
1. All matter = indivisible atoms
2. An element is made up of identical atoms
3. Different elements have atoms with different masses
4. Chemical compounds are made of atoms in specific integer ratios
5. Atoms are neither created nor destroyed in chemical reactions
Dalton’s Atomic Theory
MODERN IDEAS – discoveries leading to atomic structure – indirect observations
J.J Thompson (late 1800’s) – discovery of electrons
cathode ray tubes – eject particles from plates
-cathode rays found to be negative
(opposites attract-not light)
-deflect in magnetic field (current-moving charge)
- measured charge-to-mass ratio
(crossed electric&magnetic fields)
Robert Millikan (1906) Oil drop experiment-charged oil drops in electric field-electric force opposed gravity – drop floats-droplet charge in multiples
of electron charge qe=1.6x10-19 C-found electron mass by using q/m from Millikan
me=9.11x10-31 kg very very small
Early model of the atom
Plum pudding modelElectrons embedded in blob of positively charged matter like “raisins in plum pudding”
But what is the positive charge that cancels tiny electrons?
Rutherford –alpha particle positive helium nucleus scattering- shoot alpha particle at gold sheet
Result : -most of the alpha particles passed through sheet -some alpha particles back-scattered
Conclusion: -atom contains small central part most of mass nucleus -electrons orbit at distance 100,000
times the size of the nucleus
the atom is mostly made up of EMPTY SPACE
Nucleus later found to be made of protons (Rutherford split nucleus) and neutrons (Chadwick-1932)
Describing the Modern Atom
atomic number – number of protons in nucleus-describes identity of element
-neutral atom number of e- = number of p+
mass number – number of protons and neutrons in nucleusindicates mass since the electrons are negligible
new mass scale – STANDARDIZEatomic mass units (amu, dalton) 1 amu is about mass of a proton
amu defined by mass of carbon-12:carbon 12: 6 protons and 6 neutrons
define to have mass of exactly 12 amuATOMIC MASS STANDARD
But mass number does not define elementcan have different numbers of neutrons
For example: Lithium
++ +
ATOMIC NUMBER: 3 3 3
MASS NUMBER: 3 amu 4 amu 5 amu
Li+
+ ++
+ +
Isotope: elements with the same number of protons, but different numbers of neutron – different mass numbers
How do we study isotopes?
Mass Spectrometer
oven
N SIons accelerated in electric field
Curve of ions depends on the charge-to-mass ratio-isotopes have different masses
Each isotope will form a spotAt different places on the screen
Natural Abundance-what percentage of each isotope exists for each element
Mass number –refers a particular isotope - specific atoms
Atomic Weight (Mass) – weighted average of the masses for different isotopes in a sample of an element for the element in general (all isotopes)
Notation: describes atomic structure: for an isotopenumber of protons, neutrons and electrons
11278Pt
mass number
atomic number
Means atomic number=78 and mass number =112 amu
Atomic structure-protons: 78electrons: 78 neutralneutrons: 112-78=34Example:
3517Cl
An important isotope : H lightest element
H D Thydrogen deuterium tritium
11H 2
1H 31H
normal heavy radioactivehydrogen hydrogenhydrogen-1 hydrogen-2 hydrogen-3
Natural 99.98% 0.015% < 0.005%Abundance
atomic weight: 1.008 amu
Remember atomic model: SOLAR SYSTEM MODEL
massive nucleus surrounded by electronsproblem: electron circles atom -
centripetal acceleration classical charge radiates if acceleratedloses energy e- falls into nucleus
New Theory needed -- F=ma didn’t workPlanck & Einstein :
matter absorbs discrete amounts of energyQUANTA
BOHR MODEL: tried to match experiments involving
absorption and emmision of light from hot solids
and gases - line spectra
not derived but phenomenological
Bohr’s Theory:
1. Electrons orbit the nucleus at
specific distances from the nucleus
-allowed orbits
2. Electrons in allowed orbits
do not radiate energy
-contrary to classical theory
3. Electrons gain energy by “jumping” to
a higher energy (further) orbit
-lose energy by falling to a lower energy
-energy loss or gain in
the form of a photon- particle of light
“Qnantum Leap”
n=1n=2
n=3
Explained line spectra - electrons in matter gain (absorb) or lose (emit) photons to make
quantum leaps
Wave-Particle duality : light travels like particles and waves
de Broglie : matter also travels like waveselectrons travel like waves
-normal objects have very small wavelength-electron motion governed by wave properties
STANDING WAVESOLUTION
Only certain wavelengths (energies) will fit correctly around nucleus
ALLOWED ORBITS
Led to the development of QUANTUM MECHANIC THEORY
Schrodinger Equation-solve with linear algebra and differential equations
Solution: electron orbital - 3D region surrounding nucleus where there is the greatest probability of finding an electron
Consequences of quantum mechanics
PROBABILITY DENSITY - probality at a particular position
Cannot isolate position of an electronHEISENBERG UNCERTAINTY PRINCIPLE
cannot measure momentum (motion) and position of electron exactly
Solution gives energy levels of electrons surrounding nucleus -gives electron configuration
-the arrangement of electrons in orbitals and suborbitals about the nucleus of an atom -describes properties of atom
“fingers of the atom” interact through electrons
SOLUTION TO WAVE EQUATION GIVES QUANTUM NUMBERS
-describe energies of the electrons-determine properties of electrons in atom-gives framework to “build” atoms-similar electron configuration gives similar properties-restrictions on what numbers can be
QUANTUM NUMBERS - n,m,l,sdescribe energy levels of electrons
Principal Quantum Number ( n ) main energy level of electron
-describes orbit-how far electron is from nucleus-similar to allowed Bohr orbit-restriction: whole number
greater than 0n=1,2,3,4,…
closest furthest
n=1n=2
n=3
Angular Momentum Quantum Number ( l ) shape of electron orbit
-how spread out the orbital is-restriction: l= 0 to (n-1)
l = 0sphere
l = 1hourglass
Magnetic Quantum Number ( m ) orientation of electron orbital
-the way the electrons are oriented about nucleus-restriction: -l > m > +l
Example : n=2 electronsl=0 electrons have possible m=0
only one wayto orient sphere
l=1 electrons have possible m= -1, 0, +1
oriented in y-dir m=-1
oriented in x-dir m=+1
oriented in z-dir m=0
n, l, m describe spatial properties of electronhow the electron cloud looks
each quantum number describes electron with specific energy in nucleus!
Spin Quantun Number (s) magnetic properties of the electron
electron - electric chargespin clockwise spin counterclockwise
spin up spin down
electron magnets interact with magnetic fieldsplit into two beams in magnet
- restriction s = +1/.2, -1/2
ELECTRON CONFIGURATION determined by the values of quantum numbers n, l, m, s
“fingers of the atom” how the electrons interact with their environment
PHYSICAL AND CHEMICAL PROPERTIES(CHEMICAL IDENTITY)
-- -- -- -
-S
N
N
S
CAN NOW BUILD ATOMS
Need some rules before building electron configurations
Electrically neutral atoms - same number of e- as p+have no net charge
Ground state atom - electrons occupy only the lowest
energy levels in atom
as opposed to excited state - electrons occupy higher energy states
lower energies unoccupied
Will add electrons up to atomic number-but how do we add electrons?
TWO WAYS TO ADD PARTICLES:1. Put all electrons in lowest energy level
{n=1, l=0, m=0, s=1/2} cannot do for e- only for BOSONS (photons)
2. Pauli Exclusion Principle: no two electrons in the same atom can have the same set of
quantum numbers n, l, m, s
Law of nature for fermions (spin=1/2)
BUILD TABLE OF ELEMENTSFirst note: for the spatial orbital
n, l, m describe position in spacetwo values of s for each nlm combination
Start building - add electrons successively to each lowest energy orbital
H : atomic number = 1 one electronput electron in lowest energy
Electron configuration:n=1, l=0, m=0, s=1/2
He : atomic number = 2 two electrons Electron configuration:
n=1, l=0, m=0, s=1/2 still lowestn=1, l=0, m=0, s=-1/2 -different atom
just change to s=-1/2 (next energy)
LI : atomic number = 3 three electrons Electron configuration:
n=1, l=0, m=0, s=1/2n=1, l=0, m=0, s=-1/2 n=1 full, next n=2 n=2, l=0, m=0, s=1/2 electron capacity
Be to Ne are filled by adding two e- to each n, l, m
Electron capacity - maximum number of electrons that can be added to each orbital
Connections to Periodic TableNote: row (period) determined by highest
principal quantum number
electron capacity met at end of row: NOBLE GASES
outer shell not full - chemically reactive outer shell full - no electrons to interact
with other elements chemically inert NOBLE GAS
INERT GAS
get to next element by adding electron to next level in orbital up to electron capacity
Electron properties determined by principle (n) and angular momentum (l) Q.N
Electron orbital notation:specify n, l, and number of electrons
in the orbital (superscript)electron capacity
s orbital l =0 2p orbital l =1 6 electronsd orbital l =2 10 allowed in thef orbital l =3 14 orbital
Example : 3d2 n=3, l =2, 2 e- in orbital
Rewrite electron configuration in new notation
H Li Na KBe Mg CaB AlC SiN PO SF Cl
He Ne Ar
Sc next: things are different, but first
Periodic (Moseley’s) Law - electron configurations repeatproperties of elements repeat when ordered by increasing atomic number
periodic function of atomic number similar outer shell, similar properties
First column : Alkali metals s1 orbitalvery reactive - single electron
Second column: Alkaline Earth Metals s2 orbital
Last column : (Inert) Noble Gases s2p6 (s2 for He) very stable - octet (eight outer e- except He)
Electron configuration repeats, chemical properties repeat
Led to Periodic Table
Mendelev thought to be the father of the Periodic Table
Periodic Table – table of all know elements listed in order of atomic number
-periodicity in properties along rows (density, melting/boiling, hardness, etc)
DIVIDED INTO:Families (Groups) – vertical column of elements
these elements exhibit similar properties have same outer electron configuration
Eight main groups : Group IA to Group VIIIA MAIN GROUP or REPRESENTATIVE GROUPS
show similarities in outer e- shell ( want octet )
Group IA – Alkali Metals (react violently w/H2O) s1
never uncombined in nature
Group IIA – Alkaline Earth Metals (also reactive) s2
Group IIIA s2p1
Group IVA s2p2
Group VA s2p3
Group VIA s2p4
Group VIIA –Halogens (salt former w/metal) s2p5
Group VIIIA - Noble (Rare)Gasses s2p6
never bond with others
explains Periodic Law{
Periods – row groupings of the Periodic Tableproperties repeat as you go from one period to the next -periods begin reactive (IA) and end stable (VIIIA)
Transition Metals – B groups
Group IB to Group VIIB
fill inner electron orbitals
like Sc: 1s22s22p63s23p64s23d1 skips energy
change in orbital energies
higher orbitals have lower energy
i.e., 4s is lower than 3d
shows gap in periodic table
electron energy level order:
1s2s2p3s3p4s3d4p5s4d5p6s4f5d6p7s5f6d7p6f7dincreasing energy
can write any electron configuration
Example 1. Write configuration for:
Zr
V
Example 2. Identify element electron configuration
1s22s22p63s23p64s23d7
1s22s22p63s23p4
Note: Period and Group of element identified propertiesHistorically: some elements undiscovered-
chemists knew properties before it even existed
How to read Periodic Table
Magnesium12Mg
24.31
Name Group II (family): 2 e-Atomic number group: # of outer e-SymbolAtomic weight Period 4
Electron dot notationKERNEL – nucleus and inner electrons
-dots represent the outermost electrons-shows what’s available for the other
atoms to interact withC group IV 4 outer electrons
Metals non-metals and semiconductors (semimetals)
METALS- conducts heat and electricity- metallic luster (shiny)- maleable pond into sheets- ductile draw into wires (extrusion) -form positive ions by losing electronsLi 1s22s1 Li+ stable [He] config
Mg 1s22s22p63s2 Mg+2 stable [Ne] octet
NONMETALS- insulators - dull appearance - brittle - form negative ions to complete octet
Cl 1s22s22p63s23p5+1 Cl- stable [Ar] config
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