to read from the periodic table the maximum valences of chemical elements of groups 1, 2 and from 13 to 17 of the periodic table in their relationship with hydrogen or oxygen;
to write summary formulas of two‑element chemical compounds on the basis of information on the valences that make up their elements;
to determine the valence of one chemical element in relation to when the valence of the other is known;
to recognize an oxide based on its total formula;
to write the total formula of the oxide, knowing its name;
to draw structural formulas of two‑element chemical compounds, knowing what is the valence of the elements that make them.
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What is the structural formulastructural formulastructural formula of elements?
On the basis of the combined formula of covalent compounds, the composition of their molecules can be determined: the number and type of atoms of elements. However, we are not able to predict how atoms are connected to each other. This information is provided by a different pattern, called structural formulastructural formulastructural formula. It reflects the way atoms are combined in a molecule. In the structural formula, as in the sum formula, we use the symbols of elements to designate the atoms that make up a molecule of a chemical compound. Using dashes, we present bonds between atoms (one dash symbolizes one bond).
For example, a water molecule with the formula sum has the following structural formula:
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Task 1
Based on the above structural formula of the water molecule and a common analysis of the illustrations with the teacher, answer the question „What does the structural formula inform us about”?
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On its basis, we can conclude that in the water molecule, each hydrogen atom is connected to the oxygen atom by a single bond and that hydrogen atoms are not connected with each other. The structural formula does not specify how individual atoms are arranged in space.
Is it possible to describe ionic compounds using structural formulas?
Due to the lack of the possibility to distinguish isolated structures (molecules) of ionic compounds in ionic crystals, we do not describe them using structural formulas.
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What is valence?
The number of bonds that an atom of a given chemical element creates, merging with other atoms, is called valencevalencevalence. When describing it, we use Roman numerals.
The valence of the elements forming the covalent compound can be easily determined on the basis of the structural formula of the compound.
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How do we determine the valence of chemical elements in ionic compounds?
In the case of ionic compounds, the valence of an element is equal to the number of charges of its ion which occurs in the crystal of a compound. When determining the valence of ions, we omit positive and negative signs.
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Can elements have any valence?
There are elements whose valence in most of the chemical compounds they create is constant. For example, sodium or hydrogen always have a valence equal to one (I). There are also elements, which depending on the type of compound may have different valences. For example, a carbon in a compound of the formula it takes the value of two (II) and in carbon dioxide its valence is four (IV).
The periodic table can provide some help in predicting the maximum valence of certain elements in compounds with hydrogen or oxygen.
Table 1. The maximum valence of elements relative to hydrogen
Group number
1.
2.
13.
14.
15.
16.
17.
The highest value in relationships with hydrogen
I
II
III
IV
III
II
I
Table 2. The maximum valence of elements relative to oxygen
Group number
1.
2.
13.
14.
15.
16.
17.
Highest valence in relationships with oxygen
I
II
III
IV
V
VI
VII
On the basis of the data contained in the tables, it can be concluded that the elements belonging to the groups: 1., 2., 13. and 14. have the same highest valences in the compounds with both oxygen and hydrogen. On the other hand, the numbers defining the valence of elements from other groups in relation to oxygen and hydrogen are different. In all compounds, the elements belonging to group 1 have a valence equal to one (I), while the elements from group 2 show a valence of two (II).
Task 2
Determine the highest valence of calcium for hydrogen and oxygen. Use the information in the tables.
Task 3
Watch the presentation of „Values of chemical elements”.
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Exercise 1
Summary
Valence is a feature of all the forming elements and compounds is the number of bonds that forms the atom of an element linking with other atoms. It is described using Roman numerals.
The valence of an element in covalent compounds is equal to the number of bonds that make up its atoms with other atoms.
The valence of the element in the ionic compounds is equal to the charge of its ions, omitting the negative and positive signs.
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Keywords
structural formula, valence
Glossary
structural formula
structural formula
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wzór strukturalny – wzór, który odzwierciedla sposób połączenia atomów w cząsteczce; na jego podstawie można określić liczbę, rodzaj atomów oraz ich wartościowość
valence
valence
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wartościowość – liczba wiązań, które tworzy atom danego pierwiastka chemicznego, łącząc się z innymi atomami