Mr. Chapman Chemistry 20. Grade 9: Counting Atoms in a Compound Rule #1: An element can have either one or two letters for its symbol. The first letter.

REVIEW: ATOMS & CHEMICAL

FORMULAS

Mr. Chapman

Chemistry 20

Grade 9: Counting

Grade 9: Counting Atoms in a Compound

Rule #1: An element can have either one or two letters for its symbol. The first letter is always capitalized, and the second one is never capitalized.

Example: Potassium, Oxygen, Fluorine have the respective symbols K, O & F.

Example: Sodium, Iron and Neon have the respective symbols Na, Fe & Ne.

Grade 9: Counting Atoms in a Compound

Rule #2: When looking at a chemical formula, you will sometimes see small numbers written beside some elements. These numbers are called subscripts, and they represent how many of that atom is present in the chemical formula.

Example: The formula for water is H2O. This means that there are 2 hydrogen atoms and 1 oxygen atom in EVERY MOLECULE OF WATER.

Grade 9: Counting Atoms in a Compound

Rule #3: If some chemical formulas, there are brackets. There is always a subscript outside of the bracket. To count the number of atoms in these formulas, multiply the number outside the brackets by everything inside the brackets.

The formulas in brackets are called polyatomic ions. You can find them on the List of Common Ions, which you now need as badly as the Periodic Table.

Example: Mg(NO3)2

Grade 10: Ionic vs. Molecular Compounds

Grade 10: Ionic vs. Molecular Compounds Ionic compounds are compounds that form

between a metal and a non-metal.

Molecular compounds form between two non-metals.

Ionic compounds form by transferring electrons to form ions.

Molecular compounds form by sharing electrons in order for each atom to have a full valence shell.

Grade 10: Ionic Compounds Recall from a few classes ago that all

atoms want to be like the nearest noble gas – these are the group 18 elements.

Atoms will transfer electrons to each other so that each of them can have an electron configuration like their nearest noble gas.

Copy the white board example of sodium and chlorine, and then hear their love story.

Grade 10: Ionic Compounds When ions come together to form compounds,

their goal is ALWAYS TO HAVE A TOTAL CHARGE OF ZERO AFTER COMBINING. When a metal and a non-metal form a compound, it is called an ionic compound.

For example, magnesium forms the Mg2+ ion and has an overall charge of +2. Chloride ions, Cl-, have a charge of -1. If two chloride ions combine with one magnesium ion, their combined charge will be 0.

Sodium and ChlorineA touching love story of atoms & ions.

Sodium and Chlorine: Salt

Storytime: A Touching Love Story of Atoms and Ions

A chlorine (Cl) atom is walking down the street thinking as usual about being noble like the nearest noble gas ________. “I wish I could just gain one more electron to become like _______” the sad chlorine atom sighed. Meanwhile, a sodium (Na) atom is walking the other way down the street thinking as usual about being like the nearest noble gas _________. “If only I could get rid of one of my electrons to become like _________” the unhappy sodium atom said. Now they were both hanging their heads because of their sadness so they accidentally bumped into each other! “Hey,” said the sodium atom to the chlorine atom, “would you like to have one of my electrons?” The sodium atom was used to getting rejected by most elements so he wasn’t very hopeful.

“Why YES!” exclaimed the chlorine atom. So the sodium atom gave the chlorine atom one electron. Being a very honest element, sodium said, “Now that I have a charge of ____ and you have a charge of ____, I am suddenly very attracted to you.” “Me too, after all, opposites attract!” replied chlorine. So chlorine bonded with sodium to become a molecule of sodium chloride (NaCl). Their family and friends were delighted that they had finally reacted to become an ionic compound. They were proud of their new nickname, too, as some people now called them Salt.

Grade 10: Molecular Compounds Non-metals will share electrons with each

other in order to fill their outer valence shells.

Bonds formed between two non-metals through the sharing of electrons are called covalent bonds, or molecular bonds.

Copy the whiteboard example showing the formation of methane, CH4.