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Chapter 4
Atomic Structure
___________
• means indivisible, from Democritus (Greek,
400BC)
• smallest particle of an element that retains the
chemical properties of that element
____________: transformation of substances
into one or more new substances
Up to late 1700’s, info about reactions were
qualitative
Technology improved scales/balances so that
quantitative analysis would be more accurate.
Dalton’s Atomic Theory
In ______, John Dalton
transformed Democritus'
ideas into scientific theory
Dalton’s Atomic Theory
1) All ________ is composed of tiny indivisible
particles called atoms
2) Atoms of the same __________ are identical – size,
mass, etc…
Atoms of different elements are different.
3) Atoms of different elements combine in simple
whole-number ratios to form _________________
4) In chemical reactions, _______________________,
separated or rearranged. (but not created,
destroyed or transformed)
Modern Atomic Theory
Dalton thought atom was solid & indivisible
There are 3 major differences between
Dalton’s and the Modern theory
1) Atoms are made up of ___________________ –
protons, neutrons & electrons
2) Atoms can be changed from one element into
another – by __________ reaction
3) Atoms of the same element are not all exactly
the same - ___________
Structure of the Atom
Atom: smallest particle of an element that retains the
_____________________ of that element
Although teeny tiny, the atom can be seen using a
scanning tunneling microscope.
Once we can see it, we can manipulate it
There is an emerging branch of chemistry…
___________________ that is concerned with those
things that are very small… like microchips etc
Structure of an Atom
Atom consists of ___________________ – that
occupy the small nucleus and the larger
surrounding area
_________ at the center of the atom contains
________ (positively charged) and ________
_____________ (negatively charged) occupy a
region surrounding the nucleus
… but how did we discover these things?
Discovering the Electron
Late 1800’s, experiments
were carried out on
Cathode Ray tubes – basic
research to see what would
happen if electric current
passed through a gas at low
Pressure producing
______________________
J.J. Thomson (English guy) used this device to calculate
the ratio of charge of these cathode ray particles to
their mass, which he found to be negatively charged
and constant
These cathode ray particles are now known as
________________________
1909: Robert Millikan
(American physicist) showed
that mass of electron ~
1/2000 mass of Hydrogen
atom (its actually 1/1837th)
Millikan’s experiments
confirmed that
_______________________
_______________________
_______________________
_______________________
More Conclusions from Millikan
1) _______________, therefore,
there must be a positive
charge to balance the
negative electrons
2) Electrons are a small % of the
atom’s mass, therefore there
must be _______ ______that
make up most of the atom’s
mass
Protons
Since atoms are neutral, atoms must contain
positive charge as well.
Thompson believed that the atom was a cloud of
positive charge with negative electrons
embedded into it.
This is known as the ________________________.
The Nuclues
In 1911, Ernest Rutherford did an
experiment where he bombarded a
thin Gold foil with ______ _______–
small, positively charged particles
They believed the _______________________ evenly
distributed
Rutherford, along with Geiger & Marsden, expected
the alpha particles to simply go straight through, like a
small ball bearing through a thin veil, but some were
actually deflected back
Rutherford concluded that the atom consisted of a very
small, dense centre that is positively charged, which he
called the _____________
Also, that the nucleus was surrounded by a very large
empty region in which the electrons occupied, like planets
around the sun
Later, it was calculated that mass of a proton
is ___________________________of an
electron, even though they have equal but
opposite charge
The Neutron
The __________ is just slightly heavier that a
proton but is neutral
All atoms, except ____________, contain neutrons
An atom is neutral when it contains equal numbers
of protons and electrons
i.e. # protons = # electrons
It is solely the _____________that determine what
type of element the atom is
Protons, which are positively charged, can stay
close to each other in the nucleus because of
_______________
Some Facts…
Particle Symbol Charge Mass(kg) Relative Mass
__________ e-1
9.109 x 10-31
1
__________ p+
+1
1.673 x 10-27
1836
__________ no
0
1.675 x 10-27
1837
Size of atom is measured in picometres (pm) = 10-12m
Counting Atoms
_____________(Z): of an element is the number
of protons in the nucleus of each atom
e.g. every Carbon atom has 6 protons (and also 6 e-)
The Periodic Table is arranged in order of
____________________________
The Atomic Number identifies the element
(look at Periodic Table at back of book)
Isotopes: are atoms of the
_____________________________different masses.
Because all atoms of the same element must have the
same number of protons (and electrons), they can
only differ in the number of neutrons, which do not
change the atoms identity
EXAMPLE:
• Hydrogen consists of just 1 proton & 1 electron and
is also known as __________
• There is another kind of Hydrogen that consists of 1
proton, 1 electron & 1 neutron, called _________
which is only 0.015% of all Hydrogen
• There is still another type of Hydrogen which has 1
proton, 1 electron & 2 neutrons, called _________
_________________(A): is the total number of
protons & neutrons in the nucleus of an atom
So, for Hydrogen:
Name
Atomic #(Z)
Mass #(A)
# protons # neutrons protons+neutrons
Protium
1
0
1
Deuterium 1
1
2
Tritium
1
2
3
So,
# Neutrons = Mass #(A) – Atomic #(Z)
__________: general term for any isotope of
any element
Isotope/Nuclide
Protium
Deuterium
Tritium
Symbol
1 H
1
2 H
1
3 H
1
p+
1
1
1
e1
1
1
no
0
1
2
Atomic Masses
Mass (not weight) of atoms is very, very small, i.e. around
____________
Better to use a relative scale to compare mass of atoms to
each other, but you need a standard.
Choose ______, which has 6 protons & 6 neutrons, 126C, also
known as ______________
It has been assigned a mass of exactly
_________________units (amu)
So:
_________= 1/12 mass of Carbon-12 = 1.660540 x 10-27kg
All other atoms are determined using this relative scale.
______________________: is the weighted
average of the atomic masses of the
naturally occurring isotopes of an element
Example from book: Copper
-________ of all Copper is Copper-63 (6329Cu) which has a
mass of 62.929599 amu
-________ of all Copper is Copper-65 (6529Cu) which has a
mass of 64.927793 amu
Calculate Average Atomic Mass
(_______ x 62.929599amu) + (______ x 64.927793amu)=
63.545642 amu = 63.55 amu (2DP)
(Revise Z#, A#, Atomic Mass & Average Atomic Mass)
Suppose you have:
10 pens (20%) “weighing” ½ oz each, and
40 pens (80%) “weighing” ¾ oz each.
What is the average mass of each pen?
Or, another way:
(20% x ½ oz) + (80% x ¾ oz)
= (0.2 x ½ oz) + (0.8 x ¾ oz) = 0.7 oz