1.2.1.A Elements, Compounds & Mixtures
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1.2.1 Elements, Compounds & Mixtures
Elements
History
In the early ages, only nine elements (carbon, gold, silver, tin, mercury, lead, copper, iron, and sulfur) were known. At that time, it was considered that:
"Elements were the substances that could not be broken down into simpler units by ordinary chemical processes."
Until the end of the nineteenth century, sixty-three elements had been discovered.
Evolution
Now 118 elements have been discovered, out of which 92 are naturally occurring elements.
The modern definition of the element is that:
"An element is a substance made up of the same type of atoms, having same atomic number and cannot be decomposed into simple substances by ordinary chemical means."
It means that each element is made up of a unique type of atom that has very specific properties.
Occurrence
Elements occur in nature in free or combined form. All the naturally occurring elements found in the world have different percentages in the earth's crust, oceans, and atmosphere. The following figure shows natural occurrence in percentage by weight of some major elements around us. It shows concentrations of these major elements found in the three main systems of our environment.
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| On Earth's Crust |
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| On Atmosphere |
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| On Ocean |
Existence
Elements may be solids, liquids, or gases. The majority of the elements exist as solids e.g. sodium, copper, zinc, gold, etc. There are very few elements that occur in a liquid state e.g. mercury and bromine. A few elements exist as gases e.g. nitrogen, oxygen, chlorine, and hydrogen.
Elements may be solids, liquids, or gases. The majority of the elements exist as solids e.g. sodium, copper, zinc, gold, etc. There are very few elements that occur in a liquid state e.g. mercury and bromine. A few elements exist as gases e.g. nitrogen, oxygen, chlorine, and hydrogen.
Division
Based on their properties, elements are divided into metals, non-metals, and metalloids. About 80 percent of the elements are metals.
Based on their properties, elements are divided into metals, non-metals, and metalloids. About 80 percent of the elements are metals.
Representation
Elements are represented by symbols, which are abbreviations for the name of elements. A symbol is taken from the name of that element in English, Latin, Greek, or German. If it is one letter, it will be capital as H for Hydrogen, N for Nitrogen, and C for Carbon, etc. In the case of two letters symbol, the only first letter is capital e.g. Ca for Calcium, Na for Sodium, and Cl for Chlorine.
Elements are represented by symbols, which are abbreviations for the name of elements. A symbol is taken from the name of that element in English, Latin, Greek, or German. If it is one letter, it will be capital as H for Hydrogen, N for Nitrogen, and C for Carbon, etc. In the case of two letters symbol, the only first letter is capital e.g. Ca for Calcium, Na for Sodium, and Cl for Chlorine.
Valency
The unique property of an element is valency. It is combining the capacity of an element with other elements. It depends upon the number of electrons in the outermost shell.
The unique property of an element is valency. It is combining the capacity of an element with other elements. It depends upon the number of electrons in the outermost shell.
Valency of Covalent Compounds
In simple covalent compounds, valency is the number of hydrogen atoms that combine with one atom of that element or the number of bonds formed by one atom of that element e.g. in the following compounds. The valency of chlorine, oxygen, nitrogen, and carbon is 1, 2, 3, and 4, respectively.
In simple covalent compounds, valency is the number of hydrogen atoms that combine with one atom of that element or the number of bonds formed by one atom of that element e.g. in the following compounds. The valency of chlorine, oxygen, nitrogen, and carbon is 1, 2, 3, and 4, respectively.
Valency of Ionic Compounds
In simple ionic compounds, valency is the number of electrons gained or lost by an atom of an element to complete its octet. Elements having less than four electrons in their valence shell; prefer to lose the electrons to complete their octet. For example, atoms of Na, Mg, and Al have 1, 2, and 3 electrons in their valence shells respectively. They lose these electrons to have a valency of 1, 2, and 3, respectively. On the other hand, elements having five or more than five electrons in their valence shells, gain electrons to complete their octet. For example, N, O, and Cl have 5, 6, and 7 electrons in their valence shells respectively. They gain 3, 2, and 1 electron respectively to complete their octet. Hence, they show valency of 3, 2, and 1, respectively. A radical is a group of atoms that have some charge.
In simple ionic compounds, valency is the number of electrons gained or lost by an atom of an element to complete its octet. Elements having less than four electrons in their valence shell; prefer to lose the electrons to complete their octet. For example, atoms of Na, Mg, and Al have 1, 2, and 3 electrons in their valence shells respectively. They lose these electrons to have a valency of 1, 2, and 3, respectively. On the other hand, elements having five or more than five electrons in their valence shells, gain electrons to complete their octet. For example, N, O, and Cl have 5, 6, and 7 electrons in their valence shells respectively. They gain 3, 2, and 1 electron respectively to complete their octet. Hence, they show valency of 3, 2, and 1, respectively. A radical is a group of atoms that have some charge.
Variable Valency
Some elements show more than one valency, i.e. they have variable valency.For example: in ferrous sulfate (FeSO4) the valency of iron is 2. In ferric sulfate 4 (Fe2(SO4)3), the valency of iron is 3. Generally, the Latin or Greek name for the element 2 4 3 (e.g., Ferrum) is modified to end in 'ous' for the lower valency (e.g. Ferrous) and to end in 'ic' for the higher valency (e.g. Ferric).
Some elements show more than one valency, i.e. they have variable valency.
For example: in ferrous sulfate (FeSO4) the valency of iron is 2. In ferric sulfate 4 (Fe2(SO4)3), the valency of iron is 3. Generally, the Latin or Greek name for the element 2 4 3 (e.g., Ferrum) is modified to end in 'ous' for the lower valency (e.g. Ferrous) and to end in 'ic' for the higher valency (e.g. Ferric).
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