Transcript atomic number

Elements
Elements are any single thing found in the
periodic table (often called the periodic table of
elements)
Examples of elements: Au, Gold; S, Sulfur; Pb,
Lead; Na, Sodium…
In 1803, Dalton proposed an atomic theory that
is still the basis for many of our theories about
the atom.
Dalton’s Atomic Theory
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Elements are composed of atoms
- tiny, hard, unbreakable, spheres. (we will
see that atoms can be broken up)
All atoms of an element are identical
- atoms of different elements are different
- every carbon atom is identical to every
other carbon atom because they have the
same chemical and physical properties
- but carbon atoms are different from sulfur
atoms because they have different chemical
and physical properties.
Atoms combine in simple, whole-number ratios to
form molecules or compounds because
- atoms are unbreakable
- each molecule of a compound contains the
exact same types and numbers of atoms
- Law of Constant Composition
John Dalton
(1766-1844)
Dalton’s Atomic Theory
4.
In chemical reactions, atoms are not broken or changed into another
type.
- all atoms present before a reaction occurs are present after
- atoms are not created or destroyed, just rearranged
- therefore the total mass will remain the same
» Law of Conservation of Mass
- atoms of one element do not change into atoms of another element
in a chemical reaction
-
cannot turn Lead into Gold by a chemical reaction
5. Using compositions of compounds and assumed formulas, Dalton
was able to determine the relative masses of the atoms
- Dalton based his scale on H = 1 amu
- It is now based on C-12 = 12 amu exactly
- 1 unit = atomic mass unit
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amu or dalton
Some Notes on Charge
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Two Kinds of Charge called positive (+)
and negative (–):
Opposite Charges Attract
+ attracted to –
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Like Charges Repel
+ repels +
– repels –
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To be Neutral, there must be no net
charge or there must be equal amounts
of opposite charges.
Maintaining and Restoring Charge Balance
Plum Pudding Atom
•JJ Thomson favored the
plum pudding model. He
believed that the atom is a
large mass of positive
charge, with tiny electrons
moving around in circles
on the surface.
• or (William Thomson)
Lord Kelvin’s Model
Thomson’s Plum
Pudding Model
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Work done by J.J. Thomson and others proved that the atom had pieces
called electrons
Thomson found that electrons are much smaller than atoms and carry a
negative charge
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the mass of the electron is 1/1836th the mass of a hydrogen atom
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the charge on the electron is the fundamental unit of charge which we
call –1 charge, e-1
The atom’s structure has electrons suspended in a positively charged
electric field
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must have positive charge to balance negative charge of electrons
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because there was no experimental evidence of positive matter,
Thomson assumed there must be positive energy
Ernest Rutherford:
• Ernest Rutherford, a student of Thomson,
was working with alpha particles, large
positively charged particles. He found that
the behavior of alpha particles was not
consistent with Thomson’s model.
Plum Pudding
Atom
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if atom was like
a plum pudding,
all the a particles
should go
straight through
Rutherford’s Experiment
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Over 98% of the a particles went straight through
About 2% of the a particles went through but were deflected by large angles
About 0.01% of the a particles bounced off the gold foil
– “...as if you fired a 15” canon shell at a piece of tissue paper and it came
back and hit you.”
Rutherford’s Conclusions
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Atoms are mostly empty space
– because almost all the particles went straight through
Atoms contain a dense particle that is small in volume compared to the total
atom but large in mass
– because of the few particles that bounced back
This dense particle is positively charged
– because of the deflections of some of the alpha particles
Rutherford’s Gold Foil Experiment
a few of the
a particles
do not go through
Nuclear Atom
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most a particles
go straight through
some a particles
go through, but are deflected
Rutherford’s Interpretation – the Nuclear Model
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7)
The atom contains a tiny dense center called the nucleus
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the amount of space taken up by the nucleus is only about 1/10 trillionth
the volume of an atom
The nucleus has essentially the entire mass of an atom
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the electrons weigh so little they give practically no mass to the atom
The nucleus is positively charged
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the amount of positive charge balances the negative charge of the
electrons
The electrons move around in the empty space of the atom surrounding the
nucleus
Rutherford proposed that the nucleus had a particle that had the same
amount of charge as an electron but opposite sign
- based on measurements of the nuclear charge of the elements
These particles are called protons
- protons have a charge of +1 and a mass of 1 amu
Since protons and electrons have the same amount of charge, to be
neutral, an atom must have equal numbers of protons and electrons.
Some Problems
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How could beryllium have 4 protons stuck together in the nucleus?
– shouldn’t they repel each other?
If a beryllium atom has 4 protons, then it should weigh 4 amu; but it
actually weighs 9.01 amu! Where is the extra mass coming from?
– each proton weighs 1 amu
– remember, the electron’s mass is only about 0.00055 amu and Be has
only 4 electrons – it can’t account for the extra 5 amu of mass!
to answer these questions, Rutherford proposed that there was another
particle in the nucleus – called a neutron
neutrons have no charge and a mass of 1 amu
– the masses of the proton and neutron are both approximately 1 amu
This lead to the modern model of the atom.
The Modern Model of the Atom
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Atoms are composed of three subatomic
particles: protons, neutrons and electrons.
The nucleus contains protons and neutrons.
The nucleus is only about 10-13 cm in diameter.
The electrons move outside the nucleus with an
average distance of about 10-8 cm
– therefore the radius of the atom is about
100,000 times larger than the radius of the
nucleus.
Subatomic
Mass
Mass
Location
Charge Symbol
Particle
g
amu
in atom
Proton
1.67
1
nucleus
+1
p, p+, H+
~0
empty space
-1
e, e-
1
nucleus
0
n, n0
x 10-24
Electron
0.0009
x 10-24
Neutron
1.67
x 10-24
Elements
• The identity of an element is determined by the number of
protons in the nucleus. the number of protons in the
nucleus of an atom is called the atomic number. For
example, any atom with 6 protons in the nucleus is a
Carbon atom.
• Elements are arranged in the periodic table by their atomic
number.
• In a neutral atom, # electrons = #protons.
• The symbol for an element is simply its 1, 2, or 3 letter
abbreviation from the periodic table. One capital letter or
one capital letter + one or two lower case letters.
• Two atoms that have the same number of protons, but a
different number of neutrons are called isotopes.
Isotopes
• Isotopes are atoms of the same element with
different numbers of neutrons.
• Most elements have atoms of more than one
isotope.
• Isotopes of an element are very similar and have
nearly identical properties. The number of protons
and electrons has much more to do with the
properties of an element.
• However, the number of neutrons may vary for
atoms of the same element.
Mass Number
• The mass number is the sum of the number of
protons and the number of neutrons in the nucleus
of an atom.
mass number = # protons + # neutrons
• The symbol for an isotope is its element symbol
along with its mass number (A) and atomic
number (Z).
• The name for an isotope of an element is the
element name followed by the mass number.
Mass Number (cont.)
• Consider the symbol:
31
15
P
31 (mass number = the sum of protons and
neutrons)
15 (atomic number = protons )
• The name of the isotope is Phosphorus-31.
• In order to determine how many neutrons are in
the nucleus of an atom simply subtract:
• # neutrons
= mass number – atomic number
= 31 – 15 = 16 neutrons
Nuclear Symbol
Mass number (# p + # n)
Cl
Atomic number (# p)
35
37
Cl
Cl
17
17
How many electrons (-), protons(+), and
neutrons(o) does each atom have? What would
be the names of these isotopes
Practice with Nuclear
Symbols
How many electrons, protons, and neutrons
are in each of the following?
14
C
6
#p
#e
#n
16
O
8
138
Ba
206
Pb
The Periodic Table of Elements
Dmitri Mendeleev
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Ordered the elements by atomic mass.
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He saw a repeating pattern of properties and
developed the Periodic Law.
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Periodic Law – When the elements are arranged
in order of increasing relative mass, certain sets of
properties periodically are repeated.
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He used patterns to predict the properties of
undiscovered elements.
The Modern Periodic Table
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Elements with similar chemical and physical properties are in the same
column.
- Columns are called Groups or Families and are designated by
a number and letter at the top of the column.
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Rows are called Periods.
- each period shows a pattern of properties repeated in the next
period (row) below.
Most of the Elements on the Periodic Table are Metals
75% of the Elements are metals!
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Metals are solids at room temperature, except Hg,
mercury.
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They have a reflective surface:
– Shiny
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They conduct
- heat
- electricity
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They are malleable:
– can be shaped or molded.
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They are ductile
– drawn or pulled into wires
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They can lose electrons to form cations in chemical
reactions
Nonmetals
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found in all 3 physical states: solids, liquids,
gasses
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poor conductors of heat
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poor conductors of electricity
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solids tend to be brittle
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gain electrons in reactions to become anions
Metalloids
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Metalloids have some properties
of metals and some of
nonmetals.
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They are also known as
semiconductors: Silicon Valley
Properties of Silicon
shiny
conducts electricity
does not conduct heat well
brittle
Important Groups - Hydrogen
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Nonmetal
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colorless, diatomic gas
– very low melting point & density
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reacts with nonmetals to form molecular compounds
– HCl is acidic gas
– H2O is a liquid
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reacts with metals to form hydrides
– metal hydrides react with water to form H2
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HX dissolves in water to form acids
Specific Families and Blocks
• “Main Group” – Representative
elements
– Group 1A – Alkali metals.
– Group 2A – Alkaline earth metals.
– Group 7A – Halogens.
– Group 8A – Noble Gases.
• Transition Metals
Group IA (1), Alkali Metals
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hydrogen usually placed here, though it
doesn’t belong
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soft, low melting points, low density
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flame tests  Li = red, Na = yellow, K =
violet
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very reactive, never find them uncombined
in nature
lithium
sodium
potassium
tend to form water soluble compounds that
are colorless solutions
react with water to form basic (alkaline)
solutions and H2 gas
2 Na + 2 H2O  2 NaOH + H2
• releases a lot of heat
rubidium
cesium
Group IIA (2), Alkaline Earth Metals
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harder, higher melting, and denser than
alkali metals
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flame tests  Ca = red, Sr = red, Ba =
yellow-green
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reactive, but less than corresponding alkali
metal
beryllium
magnesium
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form stable, insoluble oxides from which
they are normally extracted
oxides are basic = alkaline earth
reactivity with water to form H2,  Be =
none; Mg = steam; Ca, Sr, Ba = cold water
calcium
strontium
barium
Groups VIIA (17), Halogens
Nonmetals
F2 & Cl2 gases; Br2 liquid; I2 solid
all diatomic
very reactive
Cl2, Br2 react slowly with water
Br2 + H2O→ HBr + HOBr
react with metals to form ionic compounds
fluorine
chlorine
bromine
iodine
HX: all acids
HF weak < HCl < HBr < HI
Groups VIIIA (18), Noble Gases
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all gases at room temperature,
– very low melting and boiling points
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very unreactive, practically inert
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very hard to remove an electron from or
give an electron to
Selected Element Properties
• Diatomic molecules: The following elements are
diatomic in their standard elemental form:
H2, N2,
O2, F2, Cl2, Br2, I2 (Horses Need Oats For Clear
Brown Is)
• Some elements may have more than one elemental
form (Allotropes). Consider carbon:
– Diamond
– Graphite
– Buckminsterfullerene (from soot)
More Selected Elements
• Noble Metals: Pt, Au, and Ag are found in their pure
form in nature – uncombined with other elements.
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The non-metal Bromine and the metal Mercury are the
only two liquid elements in their elemental forms at 25
0C
• Gases found in the periodic table include the following:
Hydrogen, Oxygen, Nitrogen, Fluorine, Chlorine and the
Noble Gases.
Charged Atoms
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The number of protons determines the element!
– all sodium atoms have 11 protons in the nucleus
In a chemical change, the number of protons in the nucleus of the atom doesn’t
change!
– no transmutation during a chemical change!!
– during radioactive and nuclear changes, atoms do transmute
Atoms in a compound are often electrically charged, these are called ions
Atoms acquire a charge by gaining or losing electrons
– not protons!!
Ion Charge = # protons – # electrons
ions with a + charge are called cations
– more protons than electrons
– form by losing electrons
ions with a – charge are called anions
– more electrons than protons
– form by gaining electrons
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Atomic Structures of Ions
Metals form cations, positively charged ions
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– Ca atom = 20 p+ and 20 e-, Ca2+ ion = 20 p+ and 18 eCations are named the same as the metal
sodium Na  Na+ + 1esodium ion
calcium Ca  Ca2+ + 2ecalcium ion
The charge on a cation can be determined from the Group number on the
Periodic Table
– Group 1A  +1, Group 2A  +2, Group 3A (Al, Ga, In)  +3
Nonmetals form anions
For each negative charge the ion has 1 more electron than the neutral atom
– F = 9 e-, F- = 10 eAnions are named by changing the ending of the name to -ide
fluorine F + 1e-  Ffluoride ion
oxygen O + 2e-  O2oxide ion
The charge on an anion can be determined from the Group number on the
Periodic Table
– Group 7A  -1, Group 6A  -2
Ions
Loss of electron
Gain of electron
Atoms can gain or lose one or more electrons to form ions
Na
Na+ + 1 e-
Mg
Mg2+ + 2 e-
Cl + 1 e-
Cl -
S + 2 e-
S2-
Positive ions are
called cations
(size ↓ )
Negative ions are
called anions
(size ↑)
Naming Ions
Metals - just name the metal
Nonmetals - change to end with ‘-ide’
Na
Na+ + 1 e-
sodium ion
Mg
Mg2+ + 2 e-
magnesium ion
Cl + 1 e-
Cl-
chloride ion
S + 2 e-
S2-
sulfide ion
Compounds that Contain Ions
• Compounds of metals with nonmetals are made of
ions: metal atoms form cations, nonmetal
atoms for anions
• Compounds must have no total charge, therefore
we must balance the numbers of cations and
anions in a compound to get 0 charge
• If Na+ is combined with S2-, you will need 2 Na+
ions for every S2- ion to balance the charges,
therefore the formula must be Na2S
Writing Formulas for Ionic Compounds
1.
Write the symbol for the metal cation and its charge.
2.
Write the symbol for the nonmetal anion and its charge.
3.
Charge (without sign) becomes the subscript for the other
ion.
4.
Reduce the subscripts to the smallest whole number ratio.
5.
Check that the sum of the charges of the cation cancels the
sum of the anions.
Write the formula of a compound made from
aluminum ions and oxide ions
1.
Write the symbol for the metal cation
and its charge
2.
Write the symbol for the nonmetal
anion and its charge
3.
Charge (without sign) becomes
subscript for other ion
4.
Reduce subscripts to smallest whole
number ratio
5.
Check that the total charge of the
cations cancels the total charge of the
anions
Al+3 column IIIA
O2- column VIA
Al+3 O2Al2 O3
Al = (2)∙(+3) = +6
O = (3)∙(-2) = -6
Practice - What are the formulas for
compounds made from the following ions?
• potassium ion with a nitride ion
• calcium ion with a bromide ion
• aluminum ion with a sulfide ion
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K+ with N3-
K3N
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Ca+2 with Br-
CaBr2
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Al+3 with S2-
Al2S3