Download Unit 23 Inside Atoms

Unit 23 Inside Atoms
On completing this unit you should be able to:
explain why scientists use models of atoms
describe different models of the atom
list the properties of sub-atomic particles
explain what makes each chemical element unique
read information off the Periodic Table of the elements.
Model A: Space-filling
Scientists use models to describe the internal structure of atoms because they
cannot look inside atoms with existing microscopes. Only recently have we been
able to see images of whole atoms, such as the gold atoms sitting on a bed of
carbon atoms shown in the image above.
When scientists originally discovered that matter
was made of atoms, they thought atoms were solid
indivisible spheres, rather like billiard balls.
Space-filling Model
Coloured Balls
There is an international colour
code that is used when making or
drawing space-filling models of
atoms or molecules.
Colour Code for Atoms
Hydrogen atom
Although this model of the atom was found to be
incorrect, the idea is useful for understanding how
atoms are arranged in molecules (groups of bonded atoms) and in the lattices of
crystals.
Oxygen atom
In the space-filling model, different-coloured spheres are used to represent
individual atoms.
Carbon atom
The spheres give a good idea of the relative size of the atoms, and building a
model of a molecule gives you a picture of how the atoms are arranged.
Chlorine atom
Sulfur atom
Model B: Mini Solar System
As scientists investigated atoms more closely, they discovered that atoms were
actually made of smaller sub-atomic particles.
The three important sub-atomic particles that you need to know about are
protons, electrons and neutrons.
The protons and neutrons are packed into the nucleus (central core) of the atom.
The electrons travel at high
speed around the nucleus.
Atoms are largely empty space
except for the dense nucleus.
Solar System Model: Helium Atom
orbiting electrons
The New Zealand scientist
Ernest Rutherford thought that the
electrons orbited the nucleus rather
like the planets orbited the Sun. In
his model atoms were considered to
be like miniature solar systems.
But electrons are not found in
simple orbits like planets.
92 Atoms & Reactions
central
nucleus
stationary proton
stationary neutron
Helium has a central nucleus with two protons and
two neutrons. Two electrons orbit the nucleus.
Nitrogen atom
Sub-atomic Particles
Scientists have also compared the
mass and electrical charge of the
three sub-atomic particles.
Electrons are very light, negatively
charged particles travelling at high
speeds around the nucleus.
Protons and neutrons are heavier
particles that are closely packed in the
nucleus. Protons have a positive charge
but neutrons have no charge.
Atoms are largely empty space
except for the dense nucleus. The
attraction that the positively charged
protons have for the negatively
charged electrons keeps those
electrons flying around the nucleus.
Sub-atomic Particle
Location
Charge
Relative Mass
proton
nucleus
+1
1
neutron
nucleus
neutral
1
electron
in orbit
–1
0.0005
Atomic Number
An element consists of one type of atom only. There are
92 different naturally occurring elements, so there must be
92 different kinds of atoms. What makes them different?
The atoms forming a particular element all have the same
number of protons, e.g. oxygen atoms all have 8 protons.
Neutral Atoms
The number of protons in an atom is its atomic number.
The atomic number distinguishes different kinds of atoms.
Each proton has a positive electrical charge and each
electron has an equal size but negative electrical charge.
If an atom has equal numbers of protons and electrons,
then it is said to be neutral. The overall positive charge of
the protons is balanced by the overall negative charge of
the electrons.
Carbon Atom (neutrons not shown)
A carbon
atom has 6
six orbiting electrons
protons and
6 electrons.
The 6 positive
charges balance
the 6 negative
charges, giving
six
a neutral atom.
Model C: Energy Shells
As further discoveries were made, scientists refined their
model of the atom. Electrons are thought to be confined to
energy shells as they vibrate around the nucleus as waves.
Energy Shell Model
Electrons in a shell all have
the same amount of energy
and only a certain number of
electrons can occupy a shell.
third
shell
second
shell
The innermost shell can
accommodate up to two
electrons, the next shell can
accommodate up to eight
electrons.
stationary
Atoms can
central nucleus
protons
gain or lose
electrons. This makes them charged atoms or ions. The ions
will be either positively or negatively charged.
inner
shell
Whether an atom has a
filled or partly filled outer
energy shell affects how it reacts with other atoms.
Reading a Periodic Table
Science Skill
• Each element is different because it consists of a unique
kind of atom. Sulfur is a unique element because it
consists of sulfur atoms only.
Reading information off a Periodic Table:
• As there are 92 different kinds of atoms which have
been found in nature, there are 92 naturally occurring
types of elements.
2. In the box find the Atomic Number (16 – a sulfur atom
has 16 protons and therefore 16 electrons).
• Each element is given a unique chemical symbol.
Sometimes it is a single-letter code, but more often a
two-letter code (first letter a capital,
Atomic Number
second letter lower case, e.g. Al).
13
• Atoms of each element are unique,
Al
Aluminium
because of the number of protons they
possess. This number is called the Atomic Number.
• The elements are arranged on a chart called the
Periodic Table. They are arranged in order from the
lowest Atomic Number across rows. Each row relates to
electron energy shells – the first row has two elements,
the second row has eight elements, the third row has
eight elements, etc.
• A column of elements is called a group and elements in
a group have similar properties.
1. Locate the name of the element (e.g. Sulfur) – the box
will give you the chemical symbol (S).
3. From its position along a row you can tell whether it
has a full or partly full shell (sulfur has six electrons in
its outer shell – two short).
4. From the colour code on the chart identify whether it
is a metal or a non-metal (sulfur is a non-metal)
Periodic Table of the
First 20 Elements
1
H
2
He
Hydrogen
3
4
Li
Be
Na
Mg
19
20
K
Ca
Lithium Beryllium
11
12
5
B
Boron
6
C
Carbon
7
8
9
N
O
F
Nitrogen Oxygen Fluorine
Helium
10
Ne
Neon
13
14
15
16
17
18
Al
Si
P
S
Cl
Ar
Sodium Magnesium Aluminium
Silicon
Phosphorus
Sulfur
Chlorine
Argon
Metal Elements
Non-Metal Elements
Potassium Calcium
Atoms & Reactions
93
?
4 Decide whether the following
statements are true or false.
a) Atoms cannot be broken down into
smaller particles.
Assessment Activities
1 Match up the words with their meanings.
model
atoms
molecule
space-filling model
sub-atomic particle
protons
electrons
neutrons
nucleus
solar system model
electrical charge
neutral atom
ion
element
Atomic Number
electron shell
energy shell model
chemical symbol
Periodic Table
a) table of all the elements arranged to show trends
b) model in which atoms are viewed as solid balls
c) negatively charged particles orbiting the nucleus
d) central core of an atom
e) can be either negative, positive or neutral
f ) substance made of only one type of atom
g) basic building blocks of matter
h) neutral particles found in the nucleus
i) neither negatively nor positively charged
j) can be filled by a fixed number of electrons
k) positively charged particles in the nucleus of an atom
l) model where electrons occupy shells
m) code name for an element
n) group of atoms bonded together
o) equal to the number of protons in an atom
p) particle such as an electron, proton or neutron
q) model where electrons orbit the nucleus like planets
r) a way of visualising objects we cannot see
s) atom which has lost or gained electrons
2 Identify the molecules using the names in the box.
b) In a space-filling model, atoms are
represented by solid balls.
c) Three important sub-atomic
particles are the electron, proton
and nucleus.
d) The nucleus of an atom contains
electrons and protons.
e) Electrons and protons have
opposite charges.
f ) Attraction between neutrons and
electrons is what holds atoms
together.
g) Neutrons and protons are much
heavier than electrons.
h) A neutral atom has equal numbers
of protons and electrons.
i) The number of neutrons an atom
has determines which element it
belongs to.
j) An element contains only one type
of atom.
a)
b)
c)
d)
5 The Atomic Number of a neutral atom gives the
number of protons or electrons. The Mass Number gives
the total number of protons and neutrons in the nucleus.
Use these definitions to complete the following table.
e)
f)
g)
h)
Atom Atomic Mass Number of …
Number Number electrons protons neutrons
j)
i)
l)
k)
• ammonia • nitrogen • oxygen • hydrogen chloride •
• carbon monoxide • water • sulfur trioxide •
• nitrogen dioxide • sulfur dioxide • hydrogen •
• carbon dioxide • chlorine •
Note: mono = one, di = two and tri = three
3 Copy and complete the table below.
Sub-atomic Particle
Location
Charge
proton
nucleus
94 Atoms & Reactions
-1
0
Relative Mass
Carbon
6
12
Oxygen
8
16
Lithium
3
7
6 6
Fluorine
9
Chlorine
17
35
Aluminium 13
6 Complete
the diagram of
a sodium atom.
g) How many
protons will
it have?
a)
b)
h) How many
electrons are
in each shell?
i) What’s its
Atomic Number?
c)
6
10
14
d)
}
e)
f)
7 Complete the diagrams to show the number and
location of the electrons in these electrically neutral
atoms. Name the atoms involved.
Periodic Table of the
First 20 Elements
1
b)
a)
1 0 Copy and complete this Periodic Table of the first 20
elements. Then answer the questions below.
H
3
4
Beryllium
11
12
Magnesium
19
K
d)
c)
20
5
B
13
Al
6
7
Carbon
Nitrogen
14
15
Silicon
P
Phosphorus
8
O
16
Sulfur
2
9
Helium
10
Fluorine
Neon
17
18
Cl
Metal Elements
Non-Metal Elements
Calcium
a) How many non-metals are shown on this Periodic Table?
b) Where are the non-metals found?
c) How many metals are shown on this Periodic Table?
d) Where are the metals located?
e) Which elements have chemical symbols of one letter only?
f ) Which elements have completely filled electron shells?
g) Which elements are one electron short of a complete
outer shell?
8 Name the different models of the atom illustrated
below.
a)
b)
c)
h) Which elements have only one electron in their outer shell?
i) Which elements are found in the first group on the
Periodic Table of elements?
j) Which elements are found in the last group of the table?
k) Why are elements in the far right group unreactive?
l) What happens to the number of protons as you go along
a row of the Periodic Table from left to right?
m)How many elements are not shown on this table?
n) Is hydrogen a metal or a non-metal?
9 Read the scientific article opposite,
then answer the questions below.
a) Why have scientists had to rely on
indirect evidence for the existence
of atoms in the past?
b) How many nanometres are there in a
metre?
c) How big are the iodine atoms (the
pink and blue bumps) in the image?
d) How far apart are the iodine atoms?
e) What does a scanning tunnelling
microscope use to observe atoms?
f ) How does the STM scan the
surface contours?
g) Why is the image not an actual
photograph?
h) Can a scanning tunnelling microscope observe inside atoms?
Seeing and Feeling Atoms
• Atoms are extremely small objects. The
iodine atoms shown in the image are
less than 1 nanometre in size. A nanometre is one billionth of a metre.
• Scientists believed we
would never see atoms.
But in 1988 physicists
Binnig and Rohrer published images of atoms
made by a small device
called a scanning
tunnelling electron
microscope (STM).
• In the STM an extremely fine needle moves back and
forward over a surface of atoms. More
current flows through the needle when
it is close to an atom and less when it is
further away.
• The height of the needle tip above the
surface of the atoms is adjusted so that
the current flow is regular.
• The vertical (up and down) movements
of the needle are used to produce a
contour map of the surface of the substance.
The ‘hills’ represent
atoms.
• Current variations are
also used to produce
artificially coloured
images of the atoms,
which gives a threedimensional appearance. The atoms look solid.
• Scientists are working on a robotic
manipulator, which would allow your
hand to feel the bumps of atoms as the
needle moves across a surface.
Atoms & Reactions
95