Chemical Reactions Using valence numbers to write formulas for chemical compounds and coefficients to balance reactions.

Let’s review: Remember unbonded atoms: have the same number of protons as electrons are unstable – they do not complete the octet rule can cause ‘trouble’ if they remain unstable are at a high energy state will try to become stable – complete that outer shell with 8 electrons can be classified as metals – less that 4 valence electrons nonmetals – more than 4 valence electrons Some vocabulary terms: valence – the ability of an atom to bond in order to become stable valence shell – the last energy shell of the atom valence electrons – the electrons occupying the valence shell valence number – the number of electrons lost, gained, or shared in order to become stable or complete the octet rule

More on valence numbers: Valence number has to do with the number of electrons that are making the atom unstable. That number of electrons may be electrons that the atoms wants to get rid of or lose (metals), it may be the number of electrons the atoms needs to make 8 electrons on the last shell or the number of electrons the atoms needs to gain (nonmetals), or the number of electrons the atom needs to share because of the availability of atoms it has to bond (carbon, hydrogen, or nonmetal to nonmetal) Remember: Metals need to lose electrons – their valence number is positive Nonmetals need to gain electrons – their valence number is negative Carbon and Hydrogen because they almost always share electrons – their valence number is both positive and negative.

We can use these valence numbers to write chemical formulas. Remember a chemical formula is a way chemists write compounds, using the chemical symbol for the element and subscripts to show the ratio of atoms bonded. ex: MgCl2 shows the formula for magnesium chloride the formula tells us this compound contains 1 magnesium (subscript of 1 on magnesium, Mg) and 2 chlorines (subscript of 2 on chlorine, Cl) But how did we get to MgCl2 ? Mg is a group 2 element meaning it has 2 valence electrons. Its valence number is +2. Cl is a group 7 element meaning it has 7 valence electrons. Its valence number is -1. If we crisscross valence numbers, Mg+2 Cl-1 , so that the valence number of Mg Becomes Cl’s subscript, and the valence number of Cl becomes Mg’s subscript, we Have the formula Mg1Cl2

We are going to study 4 chemical reactions over the next few weeks. Starting with the first reaction which is the SYNTHESIS reaction, we can also learn the LAW OF CONSERVATION OF MATTER which states that matter cannot be created nor destroyed, only changed in its form. SYNTHESIS reaction by its name states that something is being made. It will follow the general formula: A + B AB where metal nonmetal compound – formed by crisscrossing valence numbers Let’s look at an example

Li + Cl2 Li Cl First: Li is a group 1 element, has 1 valence electron, has a valence number of +1 Cl is a group 7 element, has 7 valence electrons, has a valence number of -1 When we crisscross valence numbers we get LiCl. Second: Remember the 7 diatomic molecules: F, Cl, Br, I, H, N, O. These elements must be written with a subscript of 2 if they are NOT bonded to another element. That is why on the left side of the arrow, the reactant side, there is a subscript of 2 on chlorine. Notice on the right side of the arrow, the product side, the subscript of 2 is gone because chlorine is now bonded to lithium. Third: There is a problem. And the problem is that we have “destroyed” a chlorine. On the left side of the arrow, the reactant side, there are 2 chlorines. On the right side of the arrow, the product side, there is only 1. According to the Law of Conservation of Matter we cannot leave the reaction this way. We have to balance the reaction. We balance the reaction by using coefficients. Just like in math, coefficients mean to multiply. So let’s see how that works:

The 2nd type of reaction is the reverse of the SYNTHESIS reaction – it is the DECOMPOSITION reaction. Instead of something being made, something is being broken down. It follows the general reaction: AB A + B where compound metal nonmetal In this reaction, you do not have to worry about the valence numbers. Just break the Compound up between the elements to give you a metal and a nonmetal on the Product side. BUT REMEMBER, diatoms!!!

The 3rd kind of reaction is a SINGLE REPLACEMENT reaction The 3rd kind of reaction is a SINGLE REPLACEMENT reaction. It follows the general Formula: A + BC AC + B where metal compound new compound metal The metals have replaced each other. **Reactivity series on the back of your periodic table is necessary for this reaction.

The 3rd reaction is a DOUBLE REPLACEMENT reaction The 3rd reaction is a DOUBLE REPLACEMENT reaction. It follows the general formula: AB + CD AD + CB where Compound compound compound compound Remember: A and C are the metals B and D are the nonmetals In order to write new compounds as products on the right side of the arrow, valence numbers need to be used for each new compound. Because we are writing new compounds, subscripts from the left side of the reaction are left on the left side. MgCl2 + Al2O3 MgO + AlCl3 This reaction is not balanced yet, but it shows the new subscripts for the new compounds.