Being able to name chemical compounds can save lives! Therefore, Episode 5 reviews the major naming rules as well as how to write chemical formulas.
Being able to name chemical compounds can save lives! Therefore, Episode 5 reviews the major naming rules as well as how to write chemical formulas. We start with naming ionic compounds (1:30) and writing the formulas for ionic compounds as formula units (3:00). Then, using greek prefixes we are naming molecular compounds (6:05) and we are discussing the naming of one “special” group of chemical compounds: binary acids (8:00).
Question: What is the chemical formula for sulfuric acid? (9:14)
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap chemical formulas and naming.
Let’s Zoom Out
A rather famous comic strip shows the following story: Two men walk into a bar. “A glass of H2O, please”, says the first man. The second one adds “a glass of H2O, too”. As soon as the second man takes a sip, he starts choking. What has happened? H2O2 is commonly known as hydrogen peroxide and quite harmful when ingested. If he would have ordered “a glass of water”, this wouldn’t have happened! As you can see, being able to write chemical formulas and use their names, in appropriate situations, might actually save lives! Therefore, in today’s episode, we will recap chemical formulas and naming chemical compounds.
Let’s zoom in:
When writing chemical formulas and naming compounds, our first step has to be to determine the type of compound. A quick reminder from our chemical bonding episode: two nonmetals form molecular compounds by sharing one or more valence electrons. A metal and a nonmetal form ionic compounds, which are the result of the attractive forces between oppositely charged ions.
Let’s start with ionic compounds. Ionic compounds are crystal lattices made up of positively charged cations and negatively charged anions. (Think the t of cation like a plus sign). The overall lattice is not charged, because the sum of the positive charges is equal to the sum of the negative charges. The chemical formula for ionic compounds doesn’t tell us the exact number of ions, but it does show us the ratio between cations and anions. But how do we know what type of ion an element forms? To determine this, we are using our “cheat sheet”, the periodic table. We can determine the number of valence electrons of an element using the group in which they are: group 1 elements have 1 valence electron, group 2, two, group 13, three and so on. Now we have also already discussed that atoms are most stable when they have noble gas configuration – meaning having eight valence electrons in their outermost shell. To form an ion, atoms can either lose or gain electrons. When we look at group 1 elements with 1 valence electron, it is easier for them to lose that one electron than to gain seven. Therefore, the alkali metals form “one plus” ions. Group 2 elements lose 2 valence electrons and form 2+ ions. On the other hand, for elements in group 17 with 7 valence electrons, it is easier to accept one than lose seven. Therefore, halogens form anions with a charge of 1-.
To form an ionic compound with a net charge of zero, we have to have a balanced ratio between cations and anions. For example: If I have Na+ and Cl- the chemical formula would be NaCl. I need one of each. If I have Al3+ and O2- I am looking for the LCM, or least common multiple. In this case, that’s 6. So, to get to 6 charges for each, I need two Al3+ ions and three O2- ions. My chemical formula is therefore Al(subscript)2O(subscript)3. The subscripts indicate the ratio: for two Aluminum cations, there are three oxygen anions. The metal ion is always first and no charges are written in the chemical formula, since the net charge is zero. This is a bit like playing with LEGOs. Imagine the oxygen ion as being a two-stud piece and the aluminum as being a three-stud piece. You want to combine them to form a block with no open studs between the two. Therefore, you will need 2 of the aluminums and three of the oxygens. If you have some LEGO lying around, try it! It is a great visualization – just don’t step on them! (Ouch, that hurts even when just thinking about it)
A helpful trick here is the “crossing over” method. You can write the two ions, Al3+ and O2- next to another and then the 3+ crosses down to become the subscript of oxygen and the 2- crosses down and becomes the subscript of Aluminum. Because the chemical formulas for ionic compounds are the smallest whole number ratios, you then have to check if you can cancel.
Now that we know how to determine the ionic formula, let’s talk about naming ionic compounds. To do so, we have to take a closer look at the metal and determine if the metal is a main-group metal or a transition metal. Transition metals, as well as some post-transition metals, can form ions of different charge. Iron, for example, can form Fe2+ as well as Fe3+. It is important to indicate this, because it affects the chemical properties of the compound formed. Therefore, these ions are named with a roman numeral in parentheses behind the element name: Iron(two) cation and Iron(three) cation. Main-group metals are just called with their element name plus cation, for example, sodium cation.
The nonmetal, which is the anion, receives an -ide ending. The ion of chlorine is called chloride, the oxygen-ion oxide. To name your ionic compound you just put the cation and anion name together, for example NaCl is sodium chloride, Al2O3 is aluminum oxide and Fe3N2 is Iron(Roman numeral II) nitride. Be careful that you are aware of the difference between ionic charge of a transition metal and the subscript! This might need some practice!
OK - Let’s switch to molecular compounds. Remember that molecular compounds form between non-metals. In comparison to ionic compounds, there can be several chemical formulas for compounds being formed between the same two elements. One example we heard about in the beginning: H2O and H2O2. Both consist of hydrogen and oxygen, but water has two hydrogens and one oxygen and hydrogen peroxide has two hydrogens and two oxygens. And that makes a huge difference and can obviously create a “life and death” situation! Therefore, molecular formulas are not ratios, but they show us the exact number of atoms that are bonded together to form the molecule. This has to reflect in the process of naming: When naming molecular compounds, we use greek prefixes for numbers to indicate how many atoms of an element are in the molecule – which is our subscript. The greek prefixes are: mono – one; di – two; tri – three; tetra – four (think of tetra pak with its four corners); penta – five (think of the pentagon with five corners), hexa – six, hepta – seven; octa – eight; nona – nine and deca – ten; a decade has 10 years. Since these are pre-fixes we combine them before the element name and the second element gets an -ide again!
Let’s look at a few examples: PCl5 is phosphorus pentachloride; the prefix mono is not used if the first element is one. We also do not write the one as a subscript. If we have N2O it would be Dinitrogen monoxide. Here we are using the “mono” because it is the second element. Water (or wooder if you’re from Philly) is therefore dihydrogen monoxide – try to order that in the restaurant next time!
There is one “special” group of chemical compounds that is made up of nonmetals: acids. You can easily identify them, because their chemical formulas start with “H”. Let’s first focus on binary acids, which are made of only two elements, like HCl. According to naming rules it would be “hydrogen monochloride”. To reflect its acidic properties the name that is used for HCl in aqueous solution is “hydrochloric acid”. Hydro – for the hydrogen, chlor – for chlorine and to indicate the acid we add “-ic acid”. Having that said, water should actually be hydroxic acid. Uhm, no, that’s weird!
To recap……
For ionic compounds, the formulas show the ratio between cations and anions that are forming the ionic compound with a net charge of zero. When naming, you combine the name of the cation and the name of the root of the anion and add -ide to the anion. The formulas of molecular compounds show us the exact number of atoms that are bonded and is reflected in the chemical name by using greek prefixes. A subgroup are binary acids, which end in -ic acid.
Coming up next on the Apsolute RecAP Chemistry Edition: Types of Chemical Reactions
Today’s Question of the day is about Names of Chemical Compounds.
Question: What is the chemical formula for sulfuric acid?
A. H2S
B. H2SO4
C. H2SO3