How to determine the valence?
Valency is the ability of atoms of some chemical elements to attach to themselves the exact number of atoms of other elements or atomic groups. Thanks to this concept, we can find out how many atoms of each element are part of the molecule, and also make up its graphical formula. Therefore, to successfully write formulas, reaction equations, as well as to correctly solve problems, it is important to know well how to determine the valence of an element.
Chemical elements can have constant or variable valence. It is necessary to memorize all the elements with a constant valence. Here is a list of them:
- Hydrogen, halogens and alkali metals are always monovalent.
- A valence of two is always manifested by oxygen and alkaline earth metals.
- Always trivalent B and Al.
How to determine the valence on the periodic table
If you for some reason, for example, having become agitated on the exam, have forgotten this list, you can determine the valence on the periodic table.To do this, we need to find out which group the chemical element in which we are interested in, i.e. find out the group number, as well as determine whether it is located in the main group or in a secondary group. Higher valence is always equal to the group number.
To determine the lowest variable valence, which non-metals most often possess, it is necessary to subtract the group number from 8. The result will be the desired value.
To make it more clear how to determine the valence on the periodic table, we give a few examples:
- All alkali metals belong to the main subgroup of the first group and have a constant valence - I.
- For alkaline earth metals (the main subgroup of the second group), the valence will be II.
- Most non-metals have a variable valence. The highest degree of their valence is equal to the number of the group, and the lowest is determined, as has already been written above. Take for example sulfur. Since This element is located in the 6th group - its highest valence is equal to VI, and the lowest - II (8 - 6 = 2).
- Unlike all other non-metals, halogens that are part of the main subgroup of the eighth group have a constant valence equal to I.
- For the rest of the elements included in the side groups, the valence will have to be remembered. Most often, these elements are represented by metals having a valence from I to III.