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Compounds & Formulas . Compounds. There are about There are about 100 million pure substances that have been identified Out of these pure substances, only 118 of them are elements, the rest are compounds
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Compounds • There are about There are about 100 million pure substances that have been identified • Out of these pure substances, only 118 of them are elements, the rest are compounds • Elements are substances that cannot be separated into simpler substances. They are made entirely from one type of atom. • A compound is a substance formed when two or more different kinds of elements are chemically joined.
Chemical Formulas • A chemical formula acts as a recipe for a compound. • It tells us… the numbers and kinds of atoms that are in the compound, and the type of compound that it is. • Ex.
Reading a Chemical Formula • Ex. 1 • Consider Table Salt – Sodium Chloride – NaCl • Count the atoms: • Na atoms = one • Cl atoms = one
Subscripts • Sometimes there are subscripts….These are numbers, written smaller and lower than the letters in the formula, that tell how many of a certain kind of atom are present. The subscript one (1) is always assumed and never written. • Ex. 2 • Consider water – Dihydrogen Oxide – H2O • H atoms = 2 • O atoms = 1 • In this formula “2” is a subscript for Hydrogen
Brackets and Subscripts • Sometimes there are brackets with a subscript…the subscript tells how many atoms of EACH atom that are written inside the bracket • Ex. Calcium Hydroxide – Ca(OH)2 • Ca atoms = 1 • O atoms = 2 • H atoms = 2
Sometimes there are subscripts in the brackets… • Ex. Calcium Nitrate – Ca(NO3)2 • Ca atoms = one • O atoms = 6 (2x3) • N atoms = 2 (2x1) • In this case the subscripts that are inside and outside the bracket are multiplied together. Each atom is considered separately.
Review – Counting Atoms • 1. Symbol Ex. Ca • 2. Subscript Ex. N2 • 3. Bracket and Subscript Ex. Ba3(PO4)2 • 4. Coefficient Ex. 3C
More on Coefficients… • A coefficient is the number written in front of a chemical formula and indicates the number of molecules of that compound • It multiplies the number of atoms of each element in the formula • 2 H2O (there are 2 molecules of water) • 3 FeSO4 (there are 3 molecules of FeSO4) • 4 Cu(NO3)2 (there are 4 molecules of this cpd)
Worksheets • Complete the next two pages of worksheets • Counting Atoms • Subscripts and Coefficients
Chemical Bonding – Definitions – page 6 • Chemical Bonding – Atoms must gain, lose or share electrons to obtain a full valence shell and become stable • Chemical Bond – the attraction/force that holds atoms together. There are covalent bonds and ionic bonds. • Compound – a neutral substance containing 2 or more elements bonded together. There are covalent compounds and ionic compounds. The elements bonded together can be the same or different.
Molecule – a neutral group of atoms held together with bonds. There can be more than one molecule of a substance in a compound H2O – a compound consisting of one molecule 2H2O – the same compound as above, but with 2 molecules In both examples, the compound is water. Just the number of molecules of water is different.
Atom – smallest particle of an element that can exist by itself Ion – a positive or negative charged atom. An atom that has lost or gained electrons Element – a pure substance that cannot be broken down into simpler substances Chemical Formula – a shorthand representation of the atoms found in a compound. Shows the numbers of each kind of element that is in the compound.
Valence – the combining capacity of an atom. The number of electrons in the outer most shell of an atom that can be involved in the chemical combining of elements to form compounds. It is the number of electrons that can be lost, gained or shared during the making of a chemical bond.
