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SACE Stage 1 Chemistry - The Essentials
1.
Structure and Properties of the Atom
1.1
Atoms:
A simple definition of the atom is that it is the smallest particle that contains the properties of that element. The
idea of atoms was first suggested by Democritus, an ancient Greek who lived in the fourth century BC. John
Dalton (1766-1844) was an Englishman who improved on the atomic theory in order to explain chemical
behaviour. This theory included the following ideas:
1.
All elements are composed of tiny, indivisible particles called atoms.
2.
Atoms of the same element are identical. The atoms of any one element are different from those of any
other element.
3.
Atoms of different elements can combine with one another in simple (whole number) ratios to form
compounds.
4.
Chemical reactions occur when atoms are separated, joined or rearranged. However, atoms of one
element are not changed into atoms of another by a chemical reaction.
Some of Dalton's atomic theory is still accepted today. One revision concerns his idea that atoms are
indivisible. Using increasingly powerful atom smashers, physicists have found dozens of subatomic particles.
In fact, so many particles have been found that no single theory of atomic 'structure' can account for all of them.
In chemistry, we chemists concern ourselves with only three of these particles:
electrons, protons and neutrons.
1.2
Fundamental Atomic Particles:
We have seen that atoms are composed of even smaller particles. The differences between atoms (and therefore
elements) are caused mainly by the fact that they contain different numbers of these fundamental atomic
particles. The simplest model of the atom describes it as a very small positively charged nucleus (containing
most of the mass of the atom ie. protons and neutrons) surrounded by moving electrons. The nucleus of the
atom is very small compared with the size of the atom. The atom is mainly empty space.
Energy levels
(electrons orbit nucleus)
Nucleus
contains protons & neutrons
(nearly all of mass of atom)
Representation of an Atom.
•
•
•
an electron is a particle which carries a single negative charge. It has virtually no mass compared with the
neutron and proton.
the proton is a particle which carries a single positive charge. It has a much larger mass than an electron.
This mass can be defined as one atomic mass unit (1 a.m.u.).
the neutron carries no charge (ie. it is electrically neutral). It has a mass approximately equal to that of the
proton.
Characteristics of the Fundamental Atomic Particles
charge
mass relative to an electron
electron
-1
1
orbits the nucleus
proton
+1
1840
within the nucleus
neutron
0
1840
within the nucleus
1
position
Chapter 1: Structure & Properties of the Atom
QUESTIONS:
1.
Describe the composition of the nucleus of the atom.
2.
Compare the relative size and relative density of a nucleus to its atom.
3.
Complete the following table:
name of particle
where found in atom
charge
+1
proton
neutron
relative mass
in nucleus
electron
4.
Explain how atoms which have electrically charged particles still have no overall electrical charge.
5.
(a)
Explain what is meant by the term subatomic particles
(b)
Give two examples of a sub-atomic particle
(1)
(2)
2
SACE Stage 1 Chemistry - The Essentials
1.3
Atomic Number and Mass Number
Each type of atom contains a different number of protons in its nucleus. This means that a different number of
protons in a nucleus gives rise to a different type of atom eg. hydrogen (atomic number 1) contains one proton
in its nucleus. Fluorine (atomic number nine) contains nine protons in its nucleus. Therefore each of the
elements in the periodic table has a different atomic number (usually represented with a Z). All the atoms of the
different elements are electrically neutral ie. have no overall electric charge. Thus the number of protons in the
nucleus of an atom must equal the number of electrons around its nucleus. Most of the mass of an atom is
concentrated in its nucleus. Thus the total number of protons and neutrons in the nucleus is the mass number of
an element (usually written as A).
The full symbol for an atom X is written: A
ZX
The symbol identifies the element, while the atomic number and mass number allow determination of the
number of protons, electrons and neutrons in the atom.
Example:
A sodium atom (with atomic number of 11 and a mass number of 23)
number of protons + number of neutrons
mass number
23
Na
11
symbol of element
atomic number
number of protons
Table: Details Of The First Ten Elements Of The Periodic Table.
Name
Symbol
Atomic
Number
Composition of
Nucleus
Protons
Neutrons
Mass
Number
Number of
Electrons
Hydrogen
H
1
1
0
1
1
Helium
He
2
2
2
4
2
Lithium
Li
3
3
4
7
3
Beryllium
Be
4
4
5
9
4
Boron
B
5
5
6
11
5
Carbon
C
6
6
6
12
6
Nitrogen
N
7
7
7
14
7
Oxygen
O
8
8
8
16
8
Fluorine
F
9
9
10
19
9
Ne
10
10
10
20
10
Neon
3
Chapter 1: Structure & Properties of the Atom
QUESTIONS:
6.
Explain why the mass of an electron is ignored when the mass of an element (in atomic mass units) is
being calculated.
7.
Complete the following table.
Name of Element
Symbol
Atomic
Number
Mass
Number
Number of
Protons
Number of
Electrons
No. of
Neutrons
12
12
23
11 Na
19
__
F
Magnesium
1
1H
14
Calcium
14
40
195
78
Pt
79

   Au
Nitrogen
8.
9.
20
7
118
14
State the number of electrons, protons and neutrons in an atom of each of the following elements.
(a)
112
48
Cd
(b)
210
84
Po
(c)
53
24
Cr
Elements are sometimes referred to as Uranium - 238 or Uranium - 235. With the aid of a Periodic
Table determine the number of electrons, protons, and neutrons in these isotopes of uranium.
(a)
Uranium- 238
(b)
Uranium- 235
4
SACE Stage 1 Chemistry - The Essentials
1.4
Isotopes:
The nuclei of the atoms of a given element must all contain the same number of protons, but the number of
neutrons in such atoms may vary. Atoms that have the same number of protons but different numbers of
neutrons are called isotopes ie. they have the same atomic number but a different mass number. Despite these
differences isotopes are chemically alike. This is because the chemical properties of an element are largely due
to the number of electrons in the outer shell. As an example let us consider the oxygen atom.
Subatomic Particles Present In The Three Isotopes Of Oxygen
16
8
O
17
8
O
18
8
O
protons
8
8
8
electrons
8
8
8
neutrons
8
9
10
Since each isotope has the same number of electrons (8) and since the electrons around the nucleus determine
the chemical properties it can be seen that each isotope of oxygen has the same chemical properties. Since each
isotope has a different number of neutrons they have slightly differing masses and hence their physical
properties differ slightly (for example density, m. pt and b. pt). Almost all of the elements have isotopes but the
relative abundance of each varies. For example the element chlorine has two isotopes:
35
37
17 Cl at 75.77% natural abundance, and 17 Cl 24.23% natural abundance.
While tin has 10 naturally occurring isotopes, more than any other element and none of which are radioactive:
120
117
124
122
118
116
119
50 Sn (32.5%); 50 Sn (24%); 50 Sn (14.5%); 50 Sn (8.5%); 50 Sn (7.5%); 50 Sn (5%); 50 Sn (4.5%);
111
50 Sn
115
(1%); 114
50 Sn (0.5%); 50 Sn (0.5%).
Uranium consists mainly of three radioactive isotopes:
238
235
234
92 U (99.3%); 92 U (0.7%); 92 U (0.005%)
Uranium-235 and Uranium-238 both undergo radioactive decay via a sequence of other elements (all
radioactive), until they arrive at lead 206 and lead 207 respectively, which are stable atoms. Uranium-234 is
produced as part of the decay series of uranium-235. Uranium -238 has a half-life of 4.5 billion years;
uranium -235 has a half-life of 700 million years; uranium -234 has a half-life of 245 000 years. Half-life is
defined as the time taken for the concentration of a substance to fall to half its initial value.
QUESTIONS:
10.
Complete the following table:
Atom
A
Name of
Element
Atomic
Number
Mass
Number
18
36
B
C
Number of
Protons
34
Number of
Neutrons
No. of
Electrons
18
20
18
D
19
E
20
20
5
16
Chapter 1: Structure & Properties of the Atom
11.
12.
13.
(a)
Explain why isotopes of the same element have the same atomic number, but differing mass
number.
(b)
Write the symbols representing the isotopes of carbon having mass numbers 12, 13, and 14
respectively.
Using the periodic table, determine the number of neutrons in these atoms.
(a)
32
(c)
108
Ag
(b)
80
(d)
207
Br
Pb
If X is a general symbol representing various elements, state, with reasons, which ones of the following
are isotopes of the same element.
32
16
14.
S
X,
16
30
8 X, 16
15
18
32
8 X, 9 X, 15
X,
X.
State how isotopes of the same element are:
(a)
identical to and
(b)
different from one another
(a)
(b)
15.
Write
A
Z X symbols
(a)
neon-22.
(b)
an oxygen atom with equal numbers of protons and neutrons.
(c)
platinum-195.
(d)
a calcium atom with 24 neutrons.
for the following isotopes (hint; you may need to refer to a periodic table)
6
SACE Stage 1 Chemistry - The Essentials
1.5 Electronic Configuration:
Because the chemical properties of an element are determined by the number of electrons in its atom, it is
important to understand the arrangement of the electrons - ie. electronic configuration. One common picture
of an atom is to represent it as a miniature solar system with the nucleus at the centre and the electrons moving
like planets in orbits around it. We replace the orbits with shells which are numbered n = 1, 2, 3,....... 7 or given
labels K, L, M, ..............Q. The number of electrons that can fit in these shells is governed by the rule 2n2,
where n = the number of the shell.
Shell
Designation
Electron capacity
1 st
K-shell
2
2 nd
L -shell
8
3 rd
M-shell
18
4 th
N-shell
32
The shells are said to represent different energy levels. This is the resultant energy due to the attraction and
repulsion forces of all charged particles. If we use as a guiding principle the fact that electrons always occupy
shells of lowest energy we can build up the electronic configuration of the various atoms. Thus the first (K)
shell is filled first, then the second (L) shell and so on. This is shown in the table on the following page. The
outermost energy level is called the valence level, and the electrons in this level are called valence electrons.
The valence electrons are the electrons that are primarily involved in chemical reactions, and similarities among
the configurations of the valence electrons (the valence shell configurations) account for similarities of the
chemical properties among groups of elements.
Energy level
1
K
2
L
3
M
other energy levels
nucleus
maximum of 2
electrons
stable at 8
electrons
maximum of 8 maximum of
18 electrons
electrons
Details Of The First Three Electron Energy Levels
When a shell is filled a new electron shell is started for the remaining electrons. There is a rule, however,
which does not allow the outermost shell (valence shell) of an atom of an element to hold more than 8 electrons.
For example, as the third (M) shell has a capacity of 18 electrons you would assume that potassium, containing
19 electrons, should have an electronic configuration of 2 8 9. This, however would mean that the valence shell
contains more than eight electrons. Therefore once the M shell contains the stable number of eight electrons,
the next electron enters the N shell resulting in the configuration 2 8 8 1 for potassium.
In view of the inert (very stable) nature of the Group VIII (noble gases), it is clear that the optimum number of
electrons is eight - an octet of electrons. This is the basis of the octet rule, which states that any element is most
stable when it has a set of eight valence electrons.
7
Chapter 1: Structure & Properties of the Atom
Energy level
1
K
10P
10N
Energy level
1
K
2
L
2e -
2
L
18P
22N
8e -
3
M
2e -
8e -
8e -
Argon
atom
Neon
atom
Energy level
Energy level
1
K
11P
12N
2
L
2e -
1
K
3
M
8e -
1e -
Sodium
atom
19P
22N
2
L
2e -
3
M
8e -
4
N
8e -
1e -
Potassium
atom
Electronic Structure of Four Elements:
Neon, Sodium, Argon & Potassium
QUESTIONS:
16.
State the maximum number of electrons which each of the first four main energy shells of an atom may
contain.
17.
Write the electronic configuration of:
18.
(a)
oxygen:
(b)
calcium:
(c)
beryllium:
(d)
fluorine:
(e)
magnesium:
Deduce the electronic configuration of the element whose atomic number is 18. Give the name of this
element. If the predominant isotope of this element contains 22 neutrons in the nucleus of an atom, state
its mass number.
Electronic Configuration:
Name:
Mass No:
8
SACE Stage 1 Chemistry - The Essentials
19.
Which of the following pairs are isotopes? Explain your answers.
(a)
50Ti
(b)
12C
(c)
40Ar
20.
and 50V
and 13C
and 40K
Explain why the atomic number of an element is more important than the mass number in identifying an
element.
Electronic Configuration Of The First 20 Elements
Electronic Configuration
Element
Symbol
At. No.
1 (K)
2 (L)
3 (M)
4 (N)
Hydrogen
H
1
1
Helium
He
2
2
Lithium
Li
3
2
1
Beryllium
Be
4
2
2
Boron
B
5
2
3
Carbon
C
6
2
4
Nitrogen
N
7
2
5
Oxygen
O
8
2
6
Fluorine
F
9
2
7
Neon
Ne
10
2
8
Sodium
Na
11
2
8
1
Magnesium
Mg
12
2
8
2
Aluminium
Al
13
2
8
3
Silicon
Si
14
2
8
4
Phosphorus
P
15
2
8
5
Sulfur
S
16
2
8
6
Chlorine
Cl
17
2
8
7
Argon
Ar
18
2
8
8
Potassium
K
19
2
8
8
1
Calcium
Ca
20
2
8
8
2
9