Atoms and Subatomic Particles
Chapter 5
  Element: a pure substance that cannot be broken down
into simpler pure substances
  Compound: a pure substance that can be broken down to
two or more simpler substances
  Atom: the smallest particle of an element that still has the
properties of the element
  Molecule: a group of two or more atoms that functions as
a unit because the atoms are tightly bound together
O
H
John Dalton
O=C=O
H
1
19th century
Experimental observations:
-most natural materials are mixtures of
pure substances
-pure substances are either elements or
compounds
The Law of Definite Proportions
-a given compound always contains the
same proportion, by mass of the
elements
e.g.
water always contains 8 grams of
oxygen for every 1 gram of hydrogen
2
1
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Dalton s Atomic Theory (original early 1800 s)
(Atomic Theory of Matter)
  All matter is made up of small, neutral particles called atoms. There are 112
plus, different types of atoms. Each type corresponds to a different element.
  All atoms of a given element are identical to one another (original version).
All atoms of a given element are similar to one another (modern version).
  Atoms of a given element are different from those of any other element.
  Atoms of one element combine with atoms of other elements to form
compounds.
  A compound always has the same relative numbers and types of atoms.
  During a chemical reaction, changes occur only in the way atoms are grouped
together
ie: atoms are neither created nor destroyed in chemical reactions
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Prediction:
Atoms of a given pair of elements could combine in different
proportions and produce different compounds.
e.g:
N nitrogen and O oxygen
1:1
NO
nitric oxide, colourless gas
1:2
NO2 nitrogen dioxide, brown gas
2:1
N2O nitrous oxide, colourless gas, (laughing gas)
3 different pure substances (compounds) each with its own
distinct set of chemical and physical properties but made up
from the same two elements.
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2
2
Charge
  A piece of amber rubbed with cloth will attract some
objects to it and repel others.
  A glass rod rubbed with silk will attract some objects and
repel others.
  The glass rod will repel other glass rods that have been
rubbed with silk; the amber will repel other pieces of
amber that have been rubbed with cloth. The glass rod and
the amber will attract each other.
  The amber and the glass rod have become charged
  By convention the amber is assigned a NEGATIVE
charge.
  The glass rod has a POSITIVE charge
  Like charges repel, opposite charges attract
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Structure of the Atom -Subatomic Particles
Electrons:
1890 s
J. J. Thomson
observations:
-atoms of any element could be made to emit tiny negative
particles
-same (-)ve particle no matter which element
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3
3
J.J. Thomson (1856-1940):
  experimented with Cathode-ray tubes
  beam could be deflected with an applied electrical field or
applied magnetic field
  deflected in manner expected for negatively charged particles
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J.J. Thompson
  Conclusions:
  cathode rays are negatively charged fundamental
particles of matter found in all atoms
  all atoms contain these negative particles (electrons)
  measured charge-to-mass ratio of an electron
  electrons have almost negligible mass
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4
4
J.J. Thompson s Atomic Model
- atoms are neutral
-electrons distributed randomly in a diffuse positive
cloud.
- plum pudding model: raisins dispersed in pudding.
Plum Pudding model
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Subatomic Particles
1911 Ernest Rutherford
-atoms are neutral therefore there must be a positive
component
experiment:
-directed positive alpha particles (heavy particles with a
+2 charge) toward a thin metal foil
prediction based on plum pudding model :
-large, fast alpha particles would pass straight through with
no deflection
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5
5
Rutherford s gold foil experiment
02_24
Some alpha
particles are
scattered
Uranium source of
alpha particles (embedded
in a lead block to absorb
most of the radiation)
Beam of
alpha particles
Luminescent screen
to detect scattered
alpha particles
Most particles
pass straight
through foil
Thin
metal foil
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Subatomic Particles
  observations:
-most alpha particles passed straight through
-some were deflected with large angles
-some were reflected straight back
  conclusion:
-there must be a large centre of concentrated positive
charge in the atom
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6
6
Subatomic Particles: The Nucleus
  Nucleus:
-dense centre of positive charge surrounded by moving
(-)ve electrons.
-positive charge must balance the negative charge of the
total number of electrons
  Protons:
-particle of (+)ve charge, contained in the nucleus
-same magnitude of charge as the electron (e-)
-much greater mass than an electron (~1800 x mass of e-)
-number of protons must be equal to the number of
electrons
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Subatomic Particles
Neutrons: 1932 Chadwick
-most nuclei also contain neutral particles called neutrons
-slightly larger mass than a proton but no charge
-neutrons and protons collectively called nucleons
- some elements can have atoms with different numbers
of neutrons
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7
7
Revise Dalton s Atomic Theory
All atoms of the same element contain the same number of
protons and electrons.
but
Atoms of a given element may have different numbers of
neutrons.
All atoms of a given element are similar to one another
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Modern View of the Atom
The Nuclear Atom
  nucleus: -dense centre of positive charge
-contains (+)ve charged particles, protons,
and neutral particles, neutrons
  electrons: -particles of (-)ve charge, fill the space around
the nucleus
  mass: - protons and neutrons make up most of the mass of
an atom
  neutral: - same number of protons and electrons
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8
8
One model of the atom
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Electrons are important!
The chemistry of an atom
arises from its electrons
Why?
Electrons are outside the nucleus in the outer regions
of the atom
Electrons will interact when atoms come close together
Atoms can gain or lose electrons
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9
9
Description of an Atom
  Atomic Number
-characteristic of the element
-equal to the number of protons in the nucleus
(therefore also equal to the number of electrons in the
neutral atom)
  Mass Number
-equal to the number of protons + the number of
neutrons in the nucleus of the atom
These are both counted numbers
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The Periodic Table of the Elements
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10
10
Representation of an Element
A
Z
X
Where X is the symbol of the element
A is the mass number (always a whole number)
Z is the atomic number
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6
C
14
7
16
8
N
O
so this carbon atom has 6 protons, 6 electrons and 6 neutrons
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Isotopes
-atoms of the same elements with the same number of
protons, but with different number of neutrons
12
6
C,
14
6
C
1
1
H,
2
1
H,
D, deuturium
3
1
H
T, tritium
Isobars
-atoms that have the same mass number but different
atomic number (so they are different elements)
14
6
C,
14
7
N
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Mass of an Atom
  Atomic Mass (weight)
eg: one atom of 12C weighs 1.99 x 10-23 grams
  Atomic Mass Unit – AMU
a unit of mass, set relative to a standard mass
definition: mass of 1 atom of
12C
= 12.00000….
amu
A defined number, therefore unlimited number of sig figs
12.0000 amu = 1.99 x 10-23 grams
1 amu = 1.66 x 10-24 grams
All other atomic masses are measured relative to mass of
12C
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Atomic Masses of the Elements
  atomic masses of the individual elements are generally
shown on the periodic table
  represents the average mass, in amu, of an atom of the
element, considering the relative abundance of the isotopes
Atomic mass of C given as 12.011 amu
isotope % abundance
Isotope mass
Contributing mass
12C
98.98%
12.000 amu
11.866
13C
1.11%
13.003 amu
0.144
14C
<0.01 %
14.003 amu
0.001
Weighted average, atomic mass of C:
12.011 amu
BUT: no atom of carbon actually weighs 12.011 amu!
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Other Atomic Species
  Ions
-obtained when electrons are added to, or removed from a
neutral species
  Cation
-a positively charged ion
-the result of removing 1 or more electrons from a neutral
species
Li → Li+ + e  Anion
-a negatively charged ion
-the result of a neutral species gaining one or more electron
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O + 2e- → O2-
  The formation of ions is only ever due to the addition or
loss of electrons.
  Protons cannot be removed or added to atomic or other
species under normal chemical circumstances
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Ions
Simple ions:
  charged species containing only one atom:
eg:
Cl- , Na+ , Ca2+, O2-
Polyatomic ions:
  charged species containing a group of atoms held
together by strong bonds:
NO2-, CN- , NH4+, SO42see Table 8.4 Chapter 8 (page 314) for a list
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Structural Units of Pure Substances
  the smallest units:
atoms
(neutral)
Add or remove electrons
ions
(charged species)
These small units combine in various ways to produce pure
substances
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Structural Units of Pure Substances
1)
Combinations of Neutral Atoms
atoms are the building blocks of matter
a)
Elements
-contain only one kind of atom, grouped together
-an extended array of individual atoms
Atomic solids
eg: gold, carbon , iron
Atomic gases
eg: argon , neon
Atomic liquid
eg: mercury
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Structural Units of Pure Substances
b) Molecules:
-group of two or more atoms that function as a unit
(molecule)
-atoms are tightly bound together in the unit
-each molecule behaves as an individual unit or particle
Elements
-all atoms are the same, atoms are grouped in
molecules
eg: O2 oxygen, N2 nitrogen, Cl2 chlorine
Compounds
-more than one type of atom in the molecule
H2O, CO2 ,
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15
Structural Units of Pure Substances
2)  Combination of ions:
Ionic Compounds
-from combinations of anions and cations to
form neutral species
eg: Na+ and
Cl-
give NaCl
Mg2+ and Cl- give MgCl2
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Natural States of Elements
  Most elements are very reactive and so tend to exist as
compounds
  Some are found in pure form in nature
  Noble Metals
-inert, that is unreactive, very stable
- eg: gold, silver, platinum, palladium
Noble (Inert)Gases
-eg: helium, neon, argon, krypton, xenon
Air
-oxygen, nitrogen, hydrogen
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Writing Chemical Formula
Chemical Formula
-chemical representation of a substance
-use chemical symbols of the atoms
-gives some indication of the relative numbers and types of
atoms or ions in that substance
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Chemical Formula
  Elements (extended array of atoms)
-represented by the element symbol
Ag, Au, Ne, Fe
  Molecular compounds
-element symbols plus numerical subscript giving the number
of atoms of each element present in one molecular unit
Elements: molecular units: H2 O2 N2 F2 Cl2 Br2 I2
(P4) (S8)
Compounds: eg: H2O, CCl4,, P2O4
-formula does not give any information about how the atoms
are arranged or joined together, or anything about the shape
of the molecule
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Chemical Formula
  Ionic Compounds
-represent the simplest whole number ratio of anions to
cations
charge must balance to give a neutral species
Combination of simple ions:
NaCl , MgCl2 ,
Combination of simple ions and polyatomic ions:
Na+ and SO42- Na2SO4 ,
Ca2+ and PO43- Ca3(PO4)2
Combination of polyatomic anions and cations
NH4+ and NO3NH4NO3
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Summary
Matter
physical techniques
pure substances
elements
Aggregate of
like atoms
Cu
Au
Ag
chemical means
Homoatomic
molecules
O2
F2
N2
compounds
Molecular
compounds
H2O
CO2
NO2
Ionic
compounds
NaCl
CaF2
(NH4)2CO3
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