CHAPTER 2: ATOMS AND MOLECULESChemistry 140

HCC/TCHS

LEARNING OBJECTIVES/ASSESSMENT

• 1. Use symbols for chemical elements to write formulas for chemical compounds.

• 2. Identify the characteristics of protons, neutrons, and electrons. • 3. Use the concepts of atomic number and mass number to determine

the number of subatomic particles in isotopes and to write correct symbols for isotopes.

• 4. Use atomic weights of the elements to calculate molecular weights of compounds.

• 5. Use isotope percent abundances and masses to calculate atomic weights of the elements.

• 6. Use the mole concept to obtain relationships between number of moles, number of grams, and number of atoms for elements, and use those relationships to obtain factors for use in factor‐unit calculations.

• 7. Use the mole concept and molecular formulas to obtain relationships between number of moles, number of grams, and number of atoms or molecules for compounds, and use those relationships to obtain factors for use in factor‐unit calculations.

2.1 Symbols and Formulas• All elements in the Periodic Table have symbols that are recognized world wide.

• It does not matter which country you are in or the language you speak.

• Example: The element Iron is always identified by “Fe” and Oxygen is “O”.

• The names are not always the same but the symbols are. “Fe” is iron in Canada, fer in France and fier in Romania.

Chemical Symbols• 1. The first letter of a symbol is always capitalized. • 2. If there are two or three letters in a symbol, they are

never capitalized.• 3. Some symbols were derived from other languages.

• Examples: sodium – Na, tungsten – W, lead – Pb • Sodium comes from the word sodanum, a headache remedy,

and it’s symbol (Na) comes from the Latin word Natrium.

• There are 36 common elements you should know the symbol for. A quiz will be given soon!

Chemical FormulasA chemical formula is the combination of symbols that

represent a particular compound.The chemical formula indicates which elements are

present in the compound and in what proportions.Ex. 1: Water molecule

H2O: 2 atoms of hydrogen, 1 atom of oxygenEx. 2: Iron Oxide molecule

Fe2O3: 2 atoms of iron, 3 atom of oxygen

Chemical Formulas - Examples

• Calcium Carbonate (chalk) – CaCO3

• Sodium Chloride (salt) – NaCl• Acetylsalicylic acid (aspirin) – C9H4O8

• Acetic acid (vinegar) – C2H4O2

Chemical Formulas

• NOTES:

- Each symbol in a formula represents an element.

- If only one atom of an element is present in the compound, no subscript is used.

- If more than one atom of an element is used, then the symbol is followed by a number indicating how many atoms are used. This is called the subscript.

Chemical Formula Quantification • Subscripts

2.2 Inside the Atom• Particle Symbol Charge Location

Size(relative)• electron e- - electron shell S• proton p+ + nucleus M• neutron n0 0 nucleus L• Protons and neutrons are made up of smaller particles called

quarks. • So far, scientists have confirmed the existence of six uniquely

different quarks.

2.3 Isotopes• isotope - atoms of the same

element having the same atomic number but different mass numbers.

• atomic number - the number of protons in an atom.

• mass number - the number of protons and neutrons in an atom.

• Models of two isotopes of boron are shown. Because the numbers of neutrons in the isotopes are different, the mass numbers are also different.

• You use the name of the element followed by the mass number of the isotope to identify each isotope: boron-10 and boron-11.

2.4 Relative Masses of Atomsand Molecules

The formula mass of sucrose, C12H22O11, is equivalent to the molar mass or the mass of one mole of the compound in grams.

• Element # atoms mass total• C 12 x 12 = 144• H 22 x 1 = 22• O 11 x 16 = 176• -------------------------------------------------------------------------• formula mass.....342g/mol

2.5 Isotopes and Atomic Weights• The average atomic mass of an element is the weighted-

average mass of the mixture of its isotopes. • For example, four out of five atoms of boron are boron-11, and

one out of five is boron-10. • To find the weighted-average or the average atomic mass of

boron, you would solve the following equation:

Atomic Mass• The actual mass of an atom is extremely small. • For example the mass of the oxygen atom is 2.65 X 10-23 g.

• The mass of atomic particles is expressed in atomic mass units, amu. One amu is equal to 1.6605655 X 10-24 g. This is 1/12 the mass of the carbon 12 isotope. The atomic masses of all atoms is based on this concept.

• atomic mass - the actual mass of an atom expressed in amu

Isotopes of Hydrogen

• Example: hydrogen has three isotopes; protium, deuterium, and tritium. Each of these three isotopes has a single electron located in the first orbit or shell. These shells or orbits are also referred to as energy levels.

• The shells are labeled with the letters K,L,M,N,O,P, and Q.

2.6 Avogadro’s Number: The Mole

• mole - that amount of a substance containing Avagadro's number(6.02 X 1023) of chemical units

• If a substance is elemental, such as iron, one mole of iron contains 6.02 X 1023 iron atoms.

• If a substance is a compound, such as water, one mole of water contains 6.02 X 1023 water molecules.

2.7 The Mole and ChemicalFormulas

• Atomic weight or molar mass is the average weight of all the available isotopes of a particular element, which occur in nature, in amu. We will mainly be concerned with the gram-atomic weight of the elements.

• Molar mass – the mass of one mole of a substance expressed in grams

Molar Mass

• The mass of one mole of hydrogen is ____ g.• The mass of one mole of C is 12.011 or 12 g.• What is the mass of 0.5 moles of carbon?• mass = # moles x molar mass• How many moles in 24g of carbon?

Sample Problems

• Sample Problem 1: Find the number of moles in 96 grams of oxygen.

• Sample Problem 2: Find the mass of 2.5 moles of magnesium.

• Sample Problem 3: Find the number of moles in 50 grams of calcium carbonate, CaCO3.

• Sample Problem 4: Find the mass of 0.25 moles of CaO.