1 © 2006 Brooks/Cole - Thomson ATOMS & ELEMENTS: COMPOSITION AND MASS

2 © 2006 Brooks/Cole - Thomson ATOMIC COMPOSITION ProtonsProtons –+ electrical charge –mass = x g –relative mass = atomic mass units (u) ElectronsElectrons – negative electrical charge –relative mass = u NeutronsNeutrons – no electrical charge –mass = u

3 © 2006 Brooks/Cole - Thomson ATOM COMPOSITION protons and neutrons in the nucleus.protons and neutrons in the nucleus. the number of electrons is equal to the number of protons.the number of electrons is equal to the number of protons. electrons in space around the nucleus.electrons in space around the nucleus. extremely small. One teaspoon of water has 3 times as many atoms as the Atlantic Ocean has teaspoons of water.extremely small. One teaspoon of water has 3 times as many atoms as the Atlantic Ocean has teaspoons of water. The atom is mostly empty space

4 © 2006 Brooks/Cole - Thomson Atomic Number, Z All atoms of the same element have the same number of protons in the nucleus, Z 13 Al Atomic number Atom symbol Atomic weight

5 © 2006 Brooks/Cole - Thomson Isotopes Atoms of the same element (same Z) but different mass number (A).Atoms of the same element (same Z) but different mass number (A). Boron-10 has 5 p and 5 n: 10 5 BBoron-10 has 5 p and 5 n: 10 5 B Boron-11 has 5 p and 6 n: 11 5 BBoron-11 has 5 p and 6 n: 11 5 B 10 B 11 B

6 © 2006 Brooks/Cole - Thomson Hydrogen Isotopes Hydrogen has _____ isotopes 11H11H11H11H 21H21H21H21H 31H31H31H31H __ proton and __ neutrons, tritium radioactive __ proton and __ neutrons, deuterium __ proton and __ neutrons, protium

7 © 2006 Brooks/Cole - Thomson Isotopes & Their Uses Heart scans with radioactive technetium Tc Emits gamma rays

8 © 2006 Brooks/Cole - Thomson Isotopes Because of the existence of isotopes, the mass of a collection of atoms has an average value.Because of the existence of isotopes, the mass of a collection of atoms has an average value. Average mass = ATOMIC WEIGHTAverage mass = ATOMIC WEIGHT Boron is 19.9% 10 B and 80.1% 11 B. That is, 11 B is 80.1 percent abundant on earth.Boron is 19.9% 10 B and 80.1% 11 B. That is, 11 B is 80.1 percent abundant on earth. For boron atomic weightFor boron atomic weight = _____ (_____ u) + _____ (_____ u) = _____ u = _____ (_____ u) + _____ (_____ u) = _____ u 10 B 11 B

9 © 2006 Brooks/Cole - Thomson Masses of Isotopes determined with a mass spectrometer

10 © 2006 Brooks/Cole - Thomson Mass spectrum of C 6 H 5 Br

11 © 2006 Brooks/Cole - Thomson Isotopes & Atomic Weight Because of the existence of isotopes, the mass of a collection of atoms has an average value.Because of the existence of isotopes, the mass of a collection of atoms has an average value. 6 Li = 7.5% abundant and 7 Li = 92.5% 6 Li = 7.5% abundant and 7 Li = 92.5% –Atomic weight of Li = ______________ 28 Si = 92.23%, 29 Si = 4.67%, 30 Si = 3.10% 28 Si = 92.23%, 29 Si = 4.67%, 30 Si = 3.10% –Atomic weight of Si = ______________

12 © 2006 Brooks/Cole - Thomson Atomic Weight The mass of one atom of an element relative to one atom of another element.The mass of one atom of an element relative to one atom of another element. OR — the mass of 1000 atoms of one relative to 1000 atoms of another.OR — the mass of 1000 atoms of one relative to 1000 atoms of another. For example, an O atom is approximately 16 times heavier than an H atom.For example, an O atom is approximately 16 times heavier than an H atom. Define one element as the standard against which all others are measuredDefine one element as the standard against which all others are measured Standard = carbon-12Standard = carbon-12 C atom with ____ protons and ___ neutrons is the mass standardC atom with ____ protons and ___ neutrons is the mass standard C-12 = 12 atomic mass units (u) 1 u = 1/12th the mass of a carbon-12 atom 1 u = 1/12th the mass of a carbon-12 atom

13 © 2006 Brooks/Cole - Thomson Counting Atoms Mg burns in air (O 2 ) to produce white magnesium oxide, MgO. How can we figure out how much oxide is produced from a given mass of Mg?

14 © 2006 Brooks/Cole - Thomson Counting Atoms Chemistry is a quantitative science—we need a “counting unit.” 1 mole is the amount of substance that contains as many particles (atoms, molecules) as there are in 12.0 g of 12 C. MOLE 518 g of Pb, 2.50 mol

15 © 2006 Brooks/Cole - Thomson Particles in a Mole x Avogadro’s Number There is Avogadro’s number of particles in a mole of any substance. Amedeo Avogadro

16 © 2006 Brooks/Cole - Thomson Molar Mass 1 mol of 12 C = _______ g of C = _______ atoms of C g/mol of 12 C is its MOLAR MASS Taking into account all of the isotopes of C, the molar mass of C is ______ g/mol

17 © 2006 Brooks/Cole - Thomson One-mole Amounts

18 © 2006 Brooks/Cole - Thomson

19 PROBLEM: How many moles of Mg are represented by g? Mg has a molar mass of ___________. How many atoms in this piece of Mg?

20 © 2006 Brooks/Cole - Thomson PROBLEM: What is the mass of 4.2 x atoms of sodium?

21 © 2006 Brooks/Cole - Thomson MOLES mol PARTICLES atoms molecules formula units MASS g VOLUME of L       6.02 x particles/mol 22.4 L/mol g/mol