I. STRUCTURE OF SUBSTANCES
Atomic number, mass number and chemical element
• Atomic number Z = the number of protons in the nucleus
• Mass number A = the total number of protons and neutrons
• Number of neutrons = N;
N=A–Z
Because the atom electrically neutral, the number of electrons
is also equal to Z.
The three subatomic particles considered, the electron,
proton and neutron, are the only ones involved in chemical
phenomena.
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I. STRUCTURE OF SUBSTANCES
Examples
1. Find the number of protons, neutrons and electrons in the atom of
Cu (Z = 29, A = 64)
p+ =
e- =
no =
2. Find the number of protons, neutrons and electrons in the atom of
Hg (Z = 80, A = 200)
p+ =
e- =
no =
3. Find the number of protons, neutrons and electrons in the atom of
Cl (Z = 17, A = 35)
p+ =
e- =
no =
2
I. STRUCTURE OF SUBSTANCES
All atoms with the same number of protons (same
atomic number Z) reprezent a chemical element. Each
element has a name and a distinctive symbol.
Chemical
symbols
are
one
or
two
letter
abbreviations of the elements name (usually the Latin
name). The first letter, but never the second is capitalized.
For example :
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I. STRUCTURE OF SUBSTANCES
Hydrogenium –
H
Aurum -
Au
Nitrogenium –
N
Cuprum -
Cu
Carbonum –
C
Silicium -
Si
Oxygenium –
O
Ferrum –
Fe
Phosphorus –
P
Tellurium -
Te
Sulphur –
S
Natrium -
Na
Fluorum -
F
Aluminium -
Al
Hydrargirum –
Hg
Strontium -
Sr
Stibium –
Sb
Protactinium -
Pa
Platinum -
Pt
Plutonium –
Pu
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I. STRUCTURE OF SUBSTANCES
To represent the composition of any particular atom,
we need to specify its number of protons, neutrons and
electrons. We can do this with the symbol :
mass number
atomic number
A
E
Z
symbol of the element
Atoms that have the same atomic number Z, but
different mass numbers A are called isotopes.
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I. STRUCTURE OF SUBSTANCES
Hydrogen is the first element of the periodic table. The
nucleus of hydrogen consists of one proton, so the hydrogen
atom has just one electron. Hydrogen exists in nature as 3
isotopes:
- Light hydrogen (protium):
symbol
1
1
H
- Heavy hydrogen (deuterium):
symbol
2
1
H
- super-heavy hydrogen (tritium):
symbol
3
1
H
(1p + 0n)
or D
(1p + 1n)
or T
(1p + 2n)
Natural abundance of hydrogen isotopes is :
1
- 99.985% 2
H
1
1
H
- 0.015% 3 H - insignificant percent
1
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I. STRUCTURE OF SUBSTANCES
Atomic mass unit
Because the mass of protons, neutrons and
electrons is very small and it is difficult to operate with
such small values, the atomic mass unit (amu) has been
introduced.
1 amu is the 12th part of the mass of the isotope
12
6
C
1 amu = 1.6605 · 10-27 kg
The relative atomic mass of an element is the ratio of
the average mass of atoms of an element to 1/12 of the
mass of an atom of 12C.
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I. STRUCTURE OF SUBSTANCES
Relative atomic masses are fractional number because the
natural elements are a mixture of two or more isotopes.
For example, magnesium has 3 stable isotopes:
24
12
Mg (78,70 %), exact atomic mass: 23,98504
25
12
Mg (10,13 %), exact atomic mass: 24,98384
26
12
Mg (11,17 %), exact atomic mass : 25,98259
Knowing the abundance of the stable isotopes one may
calculate the atomic weight of magnesium:
AMg=0.787 x 23.98504 + 0.1013 x 24.98384 + 0.1117 x 25.98259 =
= 24.30934
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Abundance of the elements
What is the most abundant element? This simple
question does not have a simple answer. If we consider the
entire Universe, hydrogen accounts for about 90% of all the
atoms and 75% of the mass, and helium accounts for most
of the rest. If we consider only the elements present on
Earth, iron is probably the most abundant element. However,
most of the iron is in Earth’s core. The currently accessible
elements are those present in Earth’s atmosphere, oceans
and solid continental crust up to 16 km depth. The relative
abundance in these parts of the Earth are called Clark
parameters.
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Nr.
crt.
Element
Clark
Parameter [%]
Nr.
crt.
Element
Clark
Parameter [%]
1
Oxygen
49,4
16
Samarium
510-4
2
Silicon
25,75
17
Gadolinium
510-4
3
Aluminum
7,51
18
Dysprosium
510-4
4
Iron
4,7
19
Ytterbium
510-4
5
Calcium
3,39
20
Erbium
410-4
6
Sodium
2,64
21
Argon
3,610-4
7
Potassium
2,40
22
Praseodymium
3,510-4
8
Magnesium
1,94
23
Lutetium
110-4
9
Hydrogen
0,88
24
Germanium
110-4
10
Titanium
0,58
25
Selenium
810-5
11
Chlorine
0,19
26
Cesium
710-5
12
Phosphor
0,12
27
Terbium
710-5
13
Carbon
0,087
28
Holmium
710-5
14
Manganese
0,085
29
Thulium
710-5
Niobium
410-5
15
Sulfur
0,048
30
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Not all the known elements exist in Earth’s crust.
There are only 88 natural elements. The rest of known
elements can be produced only artificially by nuclear
processes. Moreover, most of the elements do not occur
free in nature, that is, as uncombined element. Only about
20% of them do. The remaining elements occur in chemical
combinations with other elements.
We can see in the last table that oxygen is the most
abundant element in the Earth’s crust (49.4%).
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There are 3 natural isotopes of oxygen:
16
- 99.759%
O
8
17
- 0.037%
O
8
18
- 0.204%
O
8
Large amount of oxygen exists in water and rocks as
well in free state like molecular oxygen (O2) and ozone (O3).
Molecular oxygen (O2) and ozone (O3) are allotropes of
the element oxygen. The second element in Clark’s table is
silicon (Si – 25.75%), but silicon occurs only in chemical
combinations.
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