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You can think of a chemical reaction like the process of baking cookies. When you mix ingredients like flour, butter, salt, sugar and eggs, then bake, you’ll see the mixture transform into cookies. In chemical terms, the equation is the recipe for the cake, the ingredients are the “reactants” and the cookies are the “products”. All chemical equations have the form “A + B -> C (+ D..)”, where each letter is an element or molecule (a collection of atoms linked together by bonds). chemistry). Arrows represent the reaction or change that takes place. There are some important rules that you need to know to write chemical equations.
Steps
Write the chemical formula of a covalent compound
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Remember the atomic number prefix. When naming compounds, Greek prefixes are used to indicate the atomic number of each element. Covalent compounds have the first element written in its full name, and add the suffix “ua” after the name of the second element. For example, diphosphate trisulphide is the name of a substance with the chemical formula P 2 S 3 . [1] X Research Source Here are the prefixes 1-10:
- 1: Mono-
- 2: Di-
- 3: Tri-
- 4: Tetra-
- 5: Penta-
- 6: Hexa-
- 7: Hepta-
- 8: Octa-
- 9: Nona-
- 10: Deca-
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Write the chemical symbol for the first element. When writing the formula of a compound, you must recognize the elements and know their chemical symbol. The first element is the “first name” of that compound. Use the periodic table to find the element’s chemical symbol. [2] X Research Source
- Example: Dinitrogen hexefluoride. The first element is nitrogen and the chemical symbol for nitrogen is N.
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Write the atomic number as subscript. To find out the atomic number of each element, you just need to look at the element’s prefix. Remembering the Greek prefix will help you write chemical formulas quickly without having to look up. [3] X Research Sources
- For example, Dinitrogen has the prefix “go-” which means 2, so there are two nitrogen atoms in the compound.
- Write the dinitrogen as N 2 .
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Write the chemical symbol for the second element. The second element is the “last name” of the compound and is written after the first element. For covalent compounds, the element name will end with “ua”, if the element name ends in the sound o, add r before ua for readability. [4] X Research Sources
- Example: Dinitrogen hexefluoride. The second element’s name is fluorine. Simply add “ua” after the element’s actual name. The chemical symbol for fluorine is F.
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Write the atomic number as subscript. As with the first element, you can tell the atomic number of the second element by looking at the prefix. Use this prefix to write the atomic number as the subscript to the right of the chemical symbol. [5] X Research Sources
- For example, Hexafluoride has the prefix “hexa-” meaning 6, so there are 6 fluorine atoms.
- Write hexafluoride as F 6 .
- The final chemical formula of dinitrogen hexafluoride is N 2 F 6 .
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Practice with some examples. When you first learn chemistry, you need to memorize a lot of things. Similar to learning a new language, the more you practice, the easier it will be to read chemical formulas, and the easier it will be to learn the language of chemistry.
- Sulfur dioxide: SO 2
- Carbon tetrabromide: CBR 4
- Diphosphate pentaoxide: P 2 O 5
Write the chemical formula of the ionic compound
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Determine the chemical symbols for cations and anions. All chemical compounds have first and last names. The first name is the name of the cation (positive ion) while the last name is the name of the anion (negative ion). The cation name is the element name, while the anion name is the element name with the ending “ua”. [6] X Research Source
- The chemical symbol for each element can be found on the periodic table.
- Unlike covalent compounds, the Greek prefix is not used to indicate the atomic number of each element. You have to balance the elemental charges to determine the atomic number.
- Example: Lithium oxide is Li 2 O.
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Identify polyatomic ions. Sometimes the cation or anion is a polyatomic ion. This is a molecule with two or more atoms bonded together to form a group of charged atoms. There is no way to memorize them, you only have to memorize them. [7] X Research Sources
- There are only three polyatomic cations: ammonium (NH 4+ ), hydronium (H 3+ ), and mercury(I) (Hg 22+). All have a charge of +1.
- The remaining polyatomic ions all have negative charges from -1 to -4. A few common ions are carbonate (CO 32- ), sulfate (SO 42- ), nitrate (NO 3– ), and chromate (CrO 42- ).
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Determine the electrovalence of each element. Electrovalence can be determined by the element’s position on the periodic table. There are a few rules to remember to help you determine the charge: [8] X Research Source
- Group 1 elements have a charge of +1.
- Group 2 elements have a charge of +2.
- The transition element has a charge specified in Roman numerals.
- Silver is 1+, zinc is 2+, and aluminum is 3+.
- Group 17 element has a charge of 1-.
- Group 16 elements have a charge of 2-.
- Group 15 elements have a 3-charge.
- Remember that when working with polyatomic ions, we only use the charge of the ion.
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Balance the negative and positive charges of ions. Once you have determined the charge of each element (or polyatomic ion), you will use them to determine the atomic number of each element. Since the compound has zero charge, you have to add more atoms to balance the charge. [9] X Research Source
- Example: Lithium oxide. Lithium is a group 1 element, so it has a +1 charge. Oxygen is a group 16 element, so it has a 2-. To balance the 2- charge of oxygen, you need 2 lithium atoms; so, the chemical formula of lithium oxide is Li 2 O.
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Practice with some examples. The best way to learn to write chemical formulas is to do lots of examples. You should use examples in chemistry books or find exercises online. Practice as much as you can until you feel confident writing chemical formulas.
- Calcium nitride: The chemical symbol for calcium is Ca and the chemical symbol for nitrogen is N. Ca is a group 2 element so it has a charge of +2. Nitrogen is a group 15 element so it has a 3- charge. For balance you need 3 calcium atoms (6+) and 2 nitrogen atoms (6-): Ca 3 N 2 .
- Mercury(II) phosphate: The chemical symbol for mercury is Hg and phosphate is the polyatomic ion PO 4 . Mercury has a charge of 2+ defined in Roman numerals near it. The phosphate ion has a charge of 3-. To balance you need 3 mercury atoms (6+) and 2 molecules of phosphate ion (6-): Hg 3 (PO 4 ) 2 .
Determination of products from reactants
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Identify all cations and anions in the reactants. A basic exchange equation would have two cations and two anions. The general equation takes the form AB + CD –> AD + CB, where A and C are cations, and B and D are anions. You also need to determine the charge of each ion. [10] X Research Source
- For example: AgNO 3 + NaCl –> ?
- The cation is Ag +1 and Na +1 . Anions are No 31- and Cl 1- .
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Converts ions to form products. Once you have identified all the ions and their charges, simply arrange them as follows: the first cation combines with the second anion and the second cation combines with the first anion. Recall the equation: AB + CD –> AD + CB. [11] X Research Source
- Remember to balance the charge when forming new compounds.
- For example: AgNO 3 + NaCl –> ?
- Ag +1 combines with Cl 1- to form AgCl.
- Na +1 combines with NO 31- to form NaNO 3 .
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Write the full equation. After writing the products formed, you can write the entire equation with products and reactants. Write reactants on the left side of the equation, and products on the right with a plus sign between them. [12] X Research Source
- For example: AgNO 3 + NaCl –> ?
- AgNO 3 + NaCl –> AgCl + NaNO 3
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Equation balance . When writing equations with enough products and reactants, you must make sure the entire equation is balanced. The equation is only balanced when all elements have the same number of atoms on both sides. [13] X Research Source
- Example: AgNO 3 + NaCl –> AgCl + NaNO 3
- Count the number of atoms on each side: 1 Ag on the left, 1 Ag on the right; 1 N left side, 1 N right side; 3 O on the left, 3 O on the right; 1 Na on the left side, 1 Na on the right side; 1 Cl on the left, 1 Cl on the right
- This equation is balanced because the number of atoms on both sides of the equation is equal.
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Practice a few examples. The only way to write chemical equations fluently is to practice. Make these examples yourself to make sure you understand how to balance an equation.
- NiCl 2 + (NH 4 ) 2 S –> ?
- Cation: Ni 2+ and NH 4+
- Anions: Cl 1- and S 2-
- Combine ions to form the product: NiS + NH 4 Cl
- Write the equation: NiCl 2 + (NH 4 ) 2 S –> NiS + NH 4 Cl
- Balance the equation: NiCl 2 + (NH 4 ) 2 S –> NiS + 2NH 4 Cl
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This article is co-authored by a team of editors and trained researchers who confirm the accuracy and completeness of the article.
The wikiHow Content Management team carefully monitors the work of editors to ensure that every article is up to a high standard of quality.
This article has been viewed 103,096 times.
You can think of a chemical reaction like the process of baking cookies. When you mix ingredients like flour, butter, salt, sugar and eggs, then bake, you’ll see the mixture transform into cookies. In chemical terms, the equation is the recipe for the cake, the ingredients are the “reactants” and the cookies are the “products”. All chemical equations have the form “A + B -> C (+ D..)”, where each letter is an element or molecule (a collection of atoms linked together by bonds). chemistry). Arrows represent the reaction or change that takes place. There are some important rules that you need to know to write chemical equations.
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