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I. STRUCTURE OF SUBSTANCESAtomic 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 SUBSTANCESExamples

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 =

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I. STRUCTURE OF SUBSTANCESAll 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 :

AZE

mass number

atomic number

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

Natural abundance of hydrogen isotopes is :

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:

11H - 99.985% 2

1H - 0.015% 31H - insignificant percent 1

1H - 99.985% 21H - 0.015% 3

1H - insignificant percent

- Light hydrogen (protium): symbol (1p + 0n)

- Heavy hydrogen (deuterium): symbol or D (1p + 1n)

- super-heavy hydrogen (tritium): symbol or T (1p + 2n)

H1

1

H2

1

H3

1

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I. STRUCTURE OF SUBSTANCESAtomic 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 C126

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

For example, magnesium has 3 stable isotopes:

Mg2412

Mg2512

Mg2612

(78,70 %), exact atomic mass: 23,98504

(10,13 %), exact atomic mass: 24,98384

(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

Relative atomic masses are fractional number because the

natural elements are a mixture of two or more isotopes.

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Abundance of the elementsWhat 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.

ElementClark Parameter [%]

Nr.crt.

ElementClark Parameter [%]

1 Oxygen 49,4 16 Samarium 510-4

2 Silicon 25,75 17 Gadolinium 510-4

3 Aluminum 7,51 18 Dysprosium 510-4

4 Iron 4,7 19 Ytterbium 510-4

5 Calcium 3,39 20 Erbium 410-4

6 Sodium 2,64 21 Argon 3,610-4

7 Potassium 2,40 22 Praseodymium 3,510-4

8 Magnesium 1,94 23 Lutetium 110-4

9 Hydrogen 0,88 24 Germanium 110-4

10 Titanium 0,58 25 Selenium 810-5

11 Chlorine 0,19 26 Cesium 710-5

12 Phosphor 0,12 27 Terbium 710-5

13 Carbon 0,087 28 Holmium 710-5

14 Manganese 0,085 29 Thulium 710-5

15 Sulfur 0,048 30 Niobium 410-5

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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:

168 O - 99.759%

178 O - 0.037%

188 O - 0.204%

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.