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Atoms and Elements
Elementseach element has a unique number of
protons in its nucleusthe number of protons in the nucleus of an
atom is called the atomic numberthe elements are arranged on the Periodic
Table in order of their atomic numberseach element has a unique name and
symbolsymbol either one or two letters
one capital letter or one capital letter + one lowercase
Tro, Chemistry: A Molecular Approach 2
Structure of the Nucleus
Soddy discovered that the same element could have atoms with different masses, which he called isotopes
The observed mass is a weighted average of the weights of all the naturally occurring atomsthe percentage of an element that is 1 isotope is
called the isotope’s natural abundance
Tro, Chemistry: A Molecular Approach 3
Isotopesall isotopes of an element are chemically
identicalundergo the exact same chemical reactions
all isotopes of an element have the same number of protons
isotopes of an element have different massesisotopes of an element have different numbers of
neutronsisotopes are identified by their mass numbers
protons + neutrons
Tro, Chemistry: A Molecular Approach 4
Tro, Chemistry: A Molecular Approach 5
• Atomic Number Number of protons Z
• Mass Number Protons + Neutrons Whole number A
• Abundance = relative amount found in a sample
Tro, Chemistry: A Molecular Approach 6
9.25%221210Ne-22 or
0.27%211110Ne-21 or
90.48%201010Ne-20 or
Percent Natural Abundance
A, Mass Number
Number of Neutrons
Number of ProtonsSymbol
Ne2010
Ne2110
Ne2210
ExamplesWhat are the atomic number (Z), mass
number (A) and symbol for the carbon isotope with 7 neutrons
How many protons, electrons and neutron are present in an atom
Cr52
24
Reacting Atomswhen elements undergo chemical reactions,
the reacting elements do not turn into other elementsDalton’s Atomic Theory
since the number of protons determines the kind of element, the number of protons in the atom does not change in a chemical reaction
however, many reactions involve transferring electrons from one atom to another
Tro, Chemistry: A Molecular Approach 8
Charged Atomswhen atoms gain or lose electrons, they acquire
a chargecharged particles are called ionswhen atoms gain electrons, they become
negatively charged ions, called anions (Cl-)when atoms lose electrons, they become
positively charged ions, called cations (Na+)ions behave much differently than the neutral
atome.g., The metal sodium, made of neutral Na atoms, is
highly reactive and quite unstable. However, the sodium cations, Na+, found in table salt are very nonreactive and stable
Tro, Chemistry: A Molecular Approach 9
Atomic Structures of Ions
Nonmetals form anionsFor each negative charge, the ion has 1
more electron than the neutral atomF = 9 p+ and 9 e-, F─ = 9 p+ and 10 e-
Tro, Chemistry: A Molecular Approach 10
•Metals form cations•For each positive charge, the ion has 1 less electron than the neutral atom
Na atom = 11 p+ and 11 e-, Na+ ion = 11 p+ and 10 e-
Mendeleevorder elements by atomic masssaw a repeating pattern of properties Periodic Law – When the elements are
arranged in order of increasing atomic mass, certain sets of properties recur periodically
put elements with similar properties in the same column
used pattern to predict properties of undiscovered elements
where atomic mass order did not fit other properties, he re-ordered by other propertiesTe & I
Tro, Chemistry: A Molecular Approach 11
Mendeleev’s Predictions for Ekasilicon (Germanium)
12
Property Silicon’s Props
Tin’s Props
Predicted Value
Measured Value
Atomic Mass
28 118 72 72.6
Color Grey White metal
Grey Grey- White
Density 2.32 7.28 5.5 5.4
Reaction w/ Acid &
Base
Resists Acid, Reacts Base
Reacts Acid,
Resists Base
Resists Both
Resists Both
Oxide SiO2 SnO2 Eks1O2 GeO2
Tro, Chemistry: A Molecular Approach 13
Metalssolids at room temperature, except
Hgreflective surface
shinyconduct heatconduct electricitymalleable
can be shapedductile
drawn or pulled into wireslose electrons and form cations in
reactionsabout 75% of the elements are metalslower left on the tableTro, Chemistry: A Molecular Approach 14
Nonmetalsfound in all 3 statespoor conductors of heatpoor conductors of electricitysolids are brittlegain electrons in reactions to
become anionsupper right on the table
except H
Tro, Chemistry: A Molecular Approach 15
Sulfur, S(s)
Bromine, Br2(l)
Chlorine, Cl2(l)
Metalloidsshow some
properties of metals and some of nonmetals
also known as semiconductors
Tro, Chemistry: A Molecular Approach 16
Properties of Siliconshiny
conducts electricitydoes not conduct heat well
brittle
= Metal
= Metalloid
= Nonmetal
The Modern Periodic TableElements with similar chemical and
physical properties are in the same columncolumns are called Groups or Families
designated by a number and letter at toprows are called Periodseach period shows the pattern of
properties repeated in the next period
Tro, Chemistry: A Molecular Approach 18
Tro, Chemistry: A Molecular Approach 19
Tro, Chemistry: A Molecular Approach 20
= Alkali Metals
= Alkali Earth Metals
= Noble Gases
= Halogens
= Lanthanides
= Actinides
= Transition Metals
Important Groups - Hydrogennonmetalcolorless, diatomic gas
very low melting point and densityreacts with nonmetals to form molecular
compoundsHCl is acidic gasH2O is a liquid
reacts with metals to form hydridesmetal hydrides react with water to form H2
HX dissolves in water to form acidsTro, Chemistry: A Molecular Approach 21
Important Groups - Alkali Metals
Group IA = Alkali Metalshydrogen usually placed here,
though it doesn’t belongsoft, low melting points, low
densityflame tests Li = red, Na =
yellow, K = violetvery reactive, never find
uncombined in naturetend to form water-soluble
compounds, therefore crystallized from seawater then molten salt electrolyzed
colorless solutionsreact with water to form basic
(alkaline) solutions and H22 Na + 2 H2O 2 NaOH + H2 releases a lot of heat
22
lithium
sodium
potassium
rubidium
cesium
Important Groups - Alkali Earth MetalsGroup IIA = Alkali Earth Metalsharder, higher melting, and
denser than alkali metals Mg alloys used as structural
materialsflame tests Ca = red, Sr = red,
Ba = yellow-greenreactive, but less than
corresponding alkali metalform stable, insoluble oxides
from which they are normally extracted
oxides are basic = alkaline earthreactivity with water to form H2
Be = none; Mg = steam; Ca, Sr, Ba = cold water
Tro, Chemistry: A Molecular Approach 23
magnesium
calcium
beryllium
strontium
barium
Important Groups - Halogens
Group VIIA = HalogensnonmetalsF2 and Cl2 gases; Br2 liquid; I2
solidall diatomicvery reactiveCl2, Br2 react slowly with water
Br2 + H2O HBr + HOBrreact with metals to form ionic
compoundsHX all acids
HF weak < HCl < HBr < HI
Tro, Chemistry: A Molecular Approach 24
bromine
iodine
chlorine
fluorine
astatine
Important Groups - Noble Gases
Group VIIIA = Noble Gases
all gases at room temperature very low melting and boiling
pointsvery unreactive, practically
inertvery hard to remove
electron from or give an electron to
Tro, Chemistry: A Molecular Approach 25
helium
neon
argon
krypton
xenon
Atomic Masswe previously learned that not all atoms of
an element have the same massisotopes
we generally use the average mass of all an element’s atoms found in a sample in calculations
we call the average mass the atomic mass
Tro, Chemistry: A Molecular Approach 26
nn isotope of massisotope of abundance fractional MassAtomic
Mass Spectrometrymasses and abundances of isotopes are
measured with a mass spectrometeratoms or molecules are ionized, then
accelerated down a tubesome molecules into fragments are broken
during the ionization processthese fragments can be used to help determine
the structure of the moleculetheir path is bent by a magnetic field,
separating them by masssimilar to Thomson’s Cathode Ray Experiment
Tro, Chemistry: A Molecular Approach 27
Mass Spectruma mass spectrum is a
graph that gives the relative mass and relative abundance of each particle
relative mass of the particle is plotted in the x-axis
relative abundance of the particle is plotted in the y-axis
Tro, Chemistry: A Molecular Approach 28
ExampleIf copper is 69.17% Cu-63 with a mass of
62.9396 amu and the rest Cu-65 with a mass of 64.9278 amu, find copper’s atomic mass
Magnesium has three naturally occurring isotopes with masses of 23.99 amu, 24.99 amu, and 35.98 amu and natural abundances of 78.99%, 10.00% and 11.01% respectively. Calculate the atomic mass of magnesium
Counting Atoms by Moles
If we can find the mass of a particular number of atoms, we can use this information to convert the mass of an element sample into the number of atoms in the sample.
The number of atoms we will use is 6.022 x 1023 and we call this a mole
1 mole = 6.022 x 1023 things Like 1 dozen = 12 things
Tro, Chemistry: A Molecular Approach 30
Chemical Packages - Moles
mole = number of particles equal to the number of atoms in 12 g of C-12
1 atom of C-12 weighs exactly 12 amu 1 mole of C-12 weighs exactly 12 g
The number of particles in 1 mole is called Avogadro’s Number = 6.0221421 x 1023
1 mole of C atoms weighs 12.01 g and has 6.022 x 1023 atoms
the average mass of a C atom is 12.01 amu
Tro, Chemistry: A Molecular Approach 31
ExamplesCalculate the number of atoms in 2.45 mol
of copperA pure silver ring contains 2.80 x 1022
silver atoms. How many moles of silver atoms does it contain?
Relationship Between Moles and Mass
The mass of one mole of atoms is called the molar mass
The molar mass of an element, in grams, is numerically equal to the element’s atomic mass, in amu
The lighter the atom, the less a mole weighs The lighter the atom, the more atoms there
are in 1 g
Tro, Chemistry: A Molecular Approach 33
Tro, Chemistry: A Molecular Approach 34
Substance Weight of
1 atom Pieces in 1 mole
Weight of 1 mole
hydrogen 1.008 amu 6.022 x 1023 atoms 1.008 g
carbon 12.01 amu 6.022 x 1023 atoms 12.01 g
oxygen 16.00 amu 6.022 x 1023 atoms 16.00 g
sulfur 32.06 amu 6.022 x 1023 atoms 32.06 g
calcium 40.08 amu 6.022 x 1023 atoms 40.08 g
chlorine 35.45 amu 6.022 x 1023 atoms 35.45 g
copper 63.55 amu 6.022 x 1023 atoms 63.55 g
1 molesulfur
32.06 g
1 molecarbon12.01 g
Converting between mass, moles and atoms
g C mol C
mol C g C
g C mol C atoms
Calculate the moles of carbon in 0.0265 g of pencil lead
Calculate the amount of copper (in moles in a 35.8 g pure copper sheetHow many atoms are there?
Calculate the mass (in grams) of 0.473 moles of titanium