Lecture 0202 The Atom & Isotopes

Atomic Number Z # protons in nucleus

Mass Number A # protons + # neutrons

Isotope particles with same number of protons but different number of neutrons particles with same atomic number (Z), but different mass number

hydrogen deuterium tritium

Masses of Isotopes determined with a mass spectrometer

AMU Atomic Mass Unit: a more convenient way to measure atomic mass (because the number of grams is incredibly small). Atomic Mass Unit: a more convenient way to measure atomic mass (because the number of grams is incredibly small). 1 amu = 1/12 mass of a Carbon-12 atom. 1 amu = 1/12 mass of a Carbon-12 atom. 1 proton = 1amu. 1 neutron = 1 amu 1 proton = 1amu. 1 neutron = 1 amu 1 electron = 0 amu. 1 electron = 0 amu. Amu are used for avg. atomic mass. Amu are used for avg. atomic mass.

Atomic Mass Unit By definition, 12 C has a mass of exactly 12 amu

6 protons 6( amu) = neutrons 6( amu) =

Atomic Mass atomic weight A “weighted” average of the naturally occurring isotopes of an atom

Because of the existence of isotopes, the mass of a collection of atoms has an average value. Average mass = ATOMIC WEIGHT Boron is 20% 10 B and 80% 11 B. That is, 11 B is 80 percent abundant on earth. To determine the atomic weight of boron: 0.20 (10 amu) (11 amu) = 10.8 amu (Mass %) + (Mass %)+…. = atomic weight

Atomic Mass The weighted average of each isotope of an element according to its relative abundance on earth. Example: carbon = The weighted average of each isotope of an element according to its relative abundance on earth. Example: carbon = The value of the average is closest to the most abundant isotope. The value of the average is closest to the most abundant isotope. C-12 = 98.89% C-13 = 1.11% ( x.9889) + (13.03 x.0111) =

Calculating an Average Atomic Mass Avg. Atomic Mass = (mass % abundance) + mass % abundance) Find the avg. atomic mass of chlorine: Cl-35 Mass = % Abundance = Cl-37 Mass = % Abundance = ( x.7577) + ( x.2423) = amu

Calculating an Average Atomic Mass Avg. Atomic Mass = (mass % abundance) + mass % abundance) Find the avg. atomic mass of nitrogen: N-14 Mass = % Abundance = N-15 Mass = % Abundance = 0.37 ( x.9963) + ( x.0037) = amu amu

Exercise % of all chlorine in nature is 35 Cl with a mass of amu and the rest is 37 Cl with a mass of amu. What is the atomic mass of Cl?

Mole Central concept in Chemistry Fundamental definition of amount of material

Counting Atoms Chemistry is a quantitative science—we need a “counting unit.” MOLE

What is a mole? A mole is a counting unit used in chemistry. A mole is a counting unit used in chemistry. Other examples of counting units: Other examples of counting units: 1 dozen = 12 1 dozen = 12 1 reem = reem = gross = gross = 144 1Mole = 602,000,000,000,000,000,000,000 or 6.02 x Mole = 602,000,000,000,000,000,000,000 or 6.02 x 10 23

How big is a mole? If I had a mole of pennies and divided them equally among everyone alive in the world, how much money would each person have? If I had a mole of pennies and divided them equally among everyone alive in the world, how much money would each person have? $860,000,000, $860,000,000, (860 Billion dollars!) (860 Billion dollars!)

How big is a mole? If I had a mole of marshmallows how much space would they occupy? (What size container would I need to hold them?) If I had a mole of marshmallows how much space would they occupy? (What size container would I need to hold them?) They would cover the entire earth to a depth of 12 miles. They would cover the entire earth to a depth of 12 miles.

How big is a mole? If I had a mole of animal moles lined up end to end how far would they reach? If I had a mole of animal moles lined up end to end how far would they reach? They would reach to the star nearest the sun, Alpha Centauri, which is 25 trillion miles away….. They would reach to the star nearest the sun, Alpha Centauri, which is 25 trillion miles away… over 2,000,000 times over 2,000,000 times. That’s 500 quintillion miles That’s 500 quintillion miles 500,000,000,000,000,000,000 miles 500,000,000,000,000,000,000 miles 4.3 light years 4.3 light years

How big is a mole? If I had a mole of water…. If I had a mole of water…. It would fit in a small cup. It would fit in a small cup. Molecules are WAY SMALL! Molecules are WAY SMALL!

Why That Number? The unit for measuring the mass of individual atoms is The unit for measuring the mass of individual atoms is amu (atomic mass unit) 1 carbon atom has a mass of amu. 1 carbon atom has a mass of amu. A mole is the number of carbon atoms it takes to get a mass in grams = the mass of 1 carbon atom in amu A mole is the number of carbon atoms it takes to get a mass in grams = the mass of 1 carbon atom in amu

Summary 1 atom 1 atom amu amu 1 atom 1 atom Atomic mass Atomic mass 1 mole 1 mole grams grams 6.022e23 atoms 6.022e23 atoms Molar mass Molar mass

1 atom of Zn  amu 1 mol of Zn  g

The molar mass of an element is an amount in grams numerically equal to the atomic mass in amu.

…why that number? So… 1 atom of C is amu So… 1 atom of C is amu But… 1 mole of C is grams But… 1 mole of C is grams The value stays the same. Only the unit changes. What is the mass of 1 O atom? 16.00amu What is the mass of a mole of oxygen atoms? grams.

So What? Because we know the relationship between moles and the mass of atoms and molecules, we can calculate the number of atoms present just by knowing the mass! Because we know the relationship between moles and the mass of atoms and molecules, we can calculate the number of atoms present just by knowing the mass!

1. How many atoms of sodium are in 0.5 mole of sodium metal?

One-mole Amounts

Exercise 2.5 What is the mass, in grams, of 1.5 mol of silicon?