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❶Polyatomic Examples Let's try an example:

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If the cation has a Roman numeral after it, that is the charge on that cation. Cations receive Roman numerals when they can take more than one ionic form. If there is no Roman numeral, you can determine the charge from the cation's position on the periodic table. Such is the case with magnesium. Chlorine is in column VIIA so it has a charge of Write the two symbols together and determine how to make the compound neutral by finding the lowest common multiple of the charges on each ion.

Then figure out how atoms of each element are needed to make that charge. The lowest common multiple is two. So, just one magnesium. So you will have two chloride ions, or Cl 2. The magnitude of the charge on the ion becomes the number of the opposite ion needed. If the charge on the ion is simply one, you can ignore it.

Now swap the subscripts' places: Now you have your formula. Binary Examples Let's try an example: Write Na and O. So you will have two sodium atoms or Na 2. So just one O. The final formula is Na 2 O. Let's try a more difficult one: Want to learn more? Select a subject to preview related courses: Write Fe and O. The lowest common multiple between three and two is six. So the final formula is Fe 2 O 3 , and this produces a neutral compound.

Polyatomic Ionic Compounds Writing formulas for polyatomic ionic compounds isn't hard when you know the formula for the polyatomic ion. Polyatomic Examples Let's try an example: Write K for potassium and PO 4 for phosphate. I know that the charge on phosphate is 3- because I looked at a chart.

This gives the formula K 3 PO 4. If I'm using the drop and swap, I'll start from step two. Since the charge on potassium is one, I ignore that number.

Careful here to only swap the number three and not the number four. That's part of the polyatomic ion phosphate: Now I swap the subscripts' place: Let's try another slightly more difficult example: Write Fe and CrO 4. The polyatomic ion has parenthesis around it to denote it as its own ion. Notice that I put parenthesis around the chromate ion. This indicates that everything inside these parentheses is multiplied by that number.

Fe 2 CrO 4 3 is my final formula. If we do drop and swap, we start with step two. It's helpful to put parenthesis around your polyatomic ion during this step - Fe 3 , then in parenthesis CrO 4 2. Now swap the subscripts. Fe 2 CrO 4 3 is the final formula. Lesson Summary Ionic compounds are neutral compounds made of positive cations and negative anions.

Write the cation symbol, then the anion symbol. Write the respective charges for each one. Find the lowest common multiple for the ions to make the compound neutral. An alternative to the third step is the drop and swap method. Learning Outcomes You should have the ability to do the following after watching this video lesson: Define ionic compounds, cations and anions List the steps for writing the chemical formula for binary and polyatomic compounds Explain how to treat polyatomic ions when writing the formula for polyatomic compounds.

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You add all of the electrons together and divide by two. Three Ca ions combine with two P ions and eight O ions. Print Writing Ionic Compound Formulas: Why do some ionic compound names have Roman numerals in them? To indicate the charge on the cation.

To indicate how many anions are present. To indicate the charge on the ionic compound. To indicate the total number of atoms in a compound. How can you determine the charge of an ion formed by a representative element? The charge of the ion formed is related to the element's group number. The charge of the ion is related to the element's period number. The charge of the ion is the number of valence electrons minus the number of core electrons.

Create your account to access this entire worksheet. Create an account to get started Create Account. The composition of an electronically neutral compound Lithium fluoride's formula Magnesium oxide's formula Skills Practiced Practiced skills include: Information Recall - access your knowledge regarding the meaning of Roman numerals in ionic compound names Reading Comprehension - ensure that you draw the most important information from the lesson on ionic compound formulas Knowledge Application - use your knowledge to answer questions about how to determine the charge of an ion formed by a representative element Additional Learning Review the lesson, Writing Ionic Compound Formulas: Define ionic compound Understand element groups Know the steps to writing ionic compound formulas Review polyatomic examples.

You are viewing lesson Lesson 5 in chapter 5 of the course:. General Chemistry 14 chapters lessons 11 flashcard sets. Like this lesson Share. Browse Browse by subject. Upgrade to Premium to enroll in Chemistry Enrolling in a course lets you earn progress by passing quizzes and exams. Take quizzes and exams. Earn certificates of completion. You will also be able to: Create a Goal Create custom courses Get your questions answered.

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Ionic Compound Formula Writing Worksheet Write chemical formulas for the compounds in each box. The names are found by Naming Ionic Compounds – Answer Key Give the name and molar mass of the following ionic compounds: Name 1) Na 2 CO 3 .

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Ionic Compound Formula Writing Worksheet. Write chemical formulas for the compounds in each box. The names are found by finding the intersection between the.