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Oxidation numbers are a fundamental concept in chemistry that play a crucial role in understanding chemical reactions and reactions’ mechanisms. They are a way of assigning a positive or negative value to an atom in a compound or ion, indicating the electron distribution around it and its role in the reaction. Determining oxidation numbers is an essential skill for chemists and is used to identify the oxidation state of an element, balance redox equations, and predict the behavior of compounds in various reactions. In this article, we will explore the principles and guidelines behind finding oxidation numbers and provide step-by-step approaches to help you master this fundamental concept in chemistry.
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In the field of chemistry, the terms “oxidation” and “reduction” are used to refer to reactions in which an atom (or group of atoms) loses or gains one or more electrons. An oxidation number is a number assigned to an atom (or group of atoms) by which we can keep track of the number of electrons exchanged when a substance is oxidized or reduced in a reaction. The process of determining the oxidation number of an atom can be very simple, but it can also be complex, depending on the charge of the atom as well as the chemical composition of the compound of which the atom is composed. For complex forms of matter, some atoms can have more than one oxidation number. However, the determination of oxidation numbers has been laid out in clear rules that are easy to understand if you already have a basic background in math and chemistry.
Steps
Determination of oxidation number based on chemical rule
- For example, Al (s) and Cl 2 both have an oxidation number of 0 because they are in unbound form.
- Note that S8 sulfur, a rare form of sulfur, also has an oxidation number of zero.
- For example, the Cl ion – has an oxidation number of -1.
- In the compound NaCl, the Cl ion still has an oxidation number of -1. Since the Na ion has a charge of +1 by definition, we also know that the Cl ion has a charge of -1, so the oxidation number of the Cl ion is also -1.
- For example, consider a compound containing the metal aluminum ion. AlCl 3 compound has a total charge of 0. Since we know that Cl ion – which has a charge of -1 and has 3 Cl ions – in the compound, therefore, for the total charge of the compound to be zero, the Al ion must be zero. has a charge of +3. Therefore, the oxidation number of Al is +3.
- When oxygen is in its elemental state (O 2 ), the oxidation number is 0, the same for other elemental atoms.
- When oxygen is in the compound ”peroxide”, the oxidation number of oxygen is -1. Peroxides are a group of compounds with a single bond between two oxygen atoms (or O 2-2 anion). For example, in the molecule H 2 O 2 (hydrogen water), oxygen has an oxidation number (and charge) of -1. Similarly, when the oxygen is in the highly reactive oxidation state (superoxide), the oxidation number of oxygen is -0.5.
- When oxygen is bonded to fluorine, its oxidation number is +2. We will consider the rule for fluorine-containing compounds later. In the compound (O 2 F 2 ), the oxidation number of oxygen is +1.
- For example, in the compound H 2 O, hydrogen has an oxidation number of +1 because oxygen has a charge of -2 and we need two +1 charges to neutralize the total charge. However, in the compound Sodium Hydride, NaH, hydrogen has an oxidation number of -1 because the Na ion has a charge of +1, and the total charge of NaH is 0, so the charge of hydrogen is -1 and also the oxidation number of hydrogen in this case.
- This is a pretty cool way to double-check your results. If the sum of the oxidation numbers of the atoms in the compound is not equal to the sum of the charges of the substance, then you probably assigned or miscalculated somewhere.
Find oxidation numbers for atoms that don’t have their own rules for oxidation numbers
- For example, in the compound Na 2 SO 4 , the charge of sulfur (S) is not known – and the sulfur atom is not in the elemental state at this time, so its oxidation number is different from zero. This is examples that we will use in this section.
- In Na 2 SO 4 , based on the principles stated, we know that the Na ion has a charge (also an oxidation number) of +1, the oxygen atom has an oxidation number of -2.
- In Na 2 SO 4 , we see that there are 2 Na atoms, 4 O atoms. So we have 2 × +1, the oxidation number of Na, get 2, and 4 × -2, the oxidation number of O, get – 8.
- In the example of Na 2 SO 4 , the sum of 2 and -8 is -6.
- In the example of Na 2 SO 4 , we have:
- (Sum of known oxidation numbers) + (oxidation number to find) = (total charge of compound)
- -6 + S = 0
- S = 0 + 6
- S = 6. So S has an oxidation number of +6 in the compound Na 2 SO 4 .
Advice
- In an uncharged compound, the sum of all the oxidation numbers of the atoms in the compound must be zero. If there is an ion consisting of 2 atoms, the sum of the oxidation numbers must equal the charge of that ion.
- Knowing how to read the periodic table of the chemical elements and how to locate elements in the periodic table is very helpful in determining oxidation numbers.
- The atoms in a single element have an oxidation number of 0. A monatomic ion has an oxidation number equal to the charge of that ion. Atoms of group 1A metals in their elemental state, such as hydrogen gas, elemental lithium and sodium, have an oxidation number of +1; Atoms of group 2A metals in their elemental state, such as magnesium and calcium metals, have an oxidation number of +2. The oxidation number of a hydrogen atom and an oxygen atom depends on the atom to which it is bonded and the type of bond.
- Here are some tips to help you identify the difference between oxidation and reduction:
- O (oxidation) donor, reduction accept.
- Metal atoms tend to lose electrons and form positive ions (oxidation).
- Nonmetals and nonmetals tend to gain electrons and form negative ions (reduction).
- An ion can also accept or give away electrons to become an ion with a different charge than the original ion, or to become an electrically neutral atom.
Things you need
- Periodic table of chemical elements
- Internet connection, chemistry textbook
- Paper, pen
- Computer
wikiHow is a “wiki” site, which means that many of the articles here are written by multiple authors. To create this article, 27 people, some of whom are anonymous, have edited and improved the article over time.
This article has been viewed 185,796 times.
In the field of chemistry, the terms “oxidation” and “reduction” are used to refer to reactions in which an atom (or group of atoms) loses or gains one or more electrons. An oxidation number is a number assigned to an atom (or group of atoms) by which we can keep track of the number of electrons exchanged when a substance is oxidized or reduced in a reaction. The process of determining the oxidation number of an atom can be very simple, but it can also be complicated, depending on the charge of the atom as well as the chemical composition of the compound of which the atom is composed. For complex forms of matter, some atoms can have more than one oxidation number. However, the determination of oxidation numbers has been laid out in clear rules that are easy to understand if you already have a basic background in math and chemistry.
In conclusion, determining the oxidation number of an element within a compound involves understanding certain rules and guidelines. By following the steps outlined, one can successfully find the oxidation number of a given element. These steps include identifying the known oxidation numbers of other elements in the compound, using the charges of ions and the overall charge of the molecule, and applying rules such as the sum of oxidation numbers being equal to the charge of the molecule. Additionally, oxidation numbers can also be determined by considering the electronegativity of the elements involved and their position in the periodic table. It is important to note that practice and familiarity with the concept of oxidation numbers are crucial in order to become proficient in identifying them accurately. Moreover, understanding the oxidation state of an element plays a significant role in predicting and balancing chemical reactions as well as understanding the nature of chemical compounds. Overall, finding the oxidation number is an essential skill in chemistry that can contribute to a better understanding of chemical processes and reactions.
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