Quizzes and Exams• Please write in non-erasable blue or black pen, no pencil!

(Will lose 1 point for pencil or erasable ink)

• I will put periodic table on backs of quizzes, if you need them.

• BOX answers! If it’s not obvious where (or which) the answer is that I’m supposed to grade, it will get no credit.

• Remember to bring your non-programmable calculators to class on Wednesdays.

• Make sure your answers make sense (can there be 23 grams of zinc in a 4.75 g sample of zinc oxide?).

• You may only use approved calculators on quizzes; others will be confiscated for the duration of the quiz or exam.

Chapter 2: Atoms and Elements1. Dalton’s Atomic Theory is based on empirical observations, formulated as Laws of

Conservation of Mass

Definite Proportions

Multiple Proportions

No detectable gain or loss of mass occurs in chemical reactions.

In a compound, elements are always combined in the same proportion by mass.

Whenever two elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in the ratioof small whole numbers.

2. Summary of Dalton’s Theory

--Matter consists of tiny particles called atoms.

--Atoms are indestructible. In chemical reactions, the atoms rearrange but they do not themselves break apart.

--In any sample of a pure element, all the atoms are identical in mass and other properties.

--The atoms of different elements differ in mass and other properties.

--In a given compound the constituent atoms are always present in the same fixed numerical ratio.

Sample Problems

1. A sample of caffeine contains 96.1 g of carbon for every 10.1 g of hydrogen. If another sample of caffeine contains 30.0 g of carbon, how many g of hydrogen does it contain?

2.* Phosphorus forms two compounds with chlorine. In 5.5 g of one of these compounds, there were 1.24 g of P. In 5.5 g of the other compound, there were 0.818 g of P. Explain how these compounds exhibit the law of multiple proportions.

“Modern” View of an Atom

nucleus has protons(+) and neutrons

electrons(-) form a diffuse cloud around the nucleus

Massproton neutron 1 amuelectron 10-4 amu

(amu = 1.66 x 10-24 g = 1/12 of mass of 12C)

~If there is a different number of protons, it is a different ___________?~If there is a different number of neutrons, it is a different __________?

Atomic number, Z = # of protonsMass number, A = # of protons + number of neutronsIsotopes = atoms of the same element with different mass numbers

Matter is neutral, so # of electrons = # of protons

AEZ

~Where is most of the mass of an atom?

Sample Problems

If a beryllium atom has 4 protons, then it should weigh 4 amu; but it actually weighs 9 amu! Where is the extra mass coming from?

How many protons, electrons, and neutrons are in an atom of 52Cr?24

Some Notes on Charge

• Two kinds of charge called + and –

• Opposite charges attract– + attracted to –

• Like charges repel– + repels +– – repels –

• To be neutral, something must have no charge or equal amounts of opposite charges

The Periodic Table

Average among all isotopes

Decides ordering of elements

Inner transition elements

Transition metals

Types of Elements

Metals:Shiny, malleable, ductile solids with

high mp and bpGood electrical conductors

Nonmetals:Gases, liquids, or low-melting solidsNon-conductors of electricity

Metalloids:Intermediate properties, often semiconductors

Diatomic elements: H2, O2, N2, F2, Cl2, Br2, I2

Predicting Ionic Charges

Li+1

Na+1

K+1

Rb+1

Cs+1

Mg+2

Ca+2

Sr+2

Ba+2

Al+3

O-2

S-2

Se-2

Te-2

F-1

Cl-1

Br-1

I-1

N-3

1A

2A 3A 7A6A5A He

Ne

Ar

Kr

Xe

Rn

F?

Rb?

N?

Noble gases have very favorable electron configurations. The other elements are jealous…If an ion forms, it will commonly match the nearest noble gas.

Calculating Atomic Mass (Weighted Avg)

Atomic Mass = ∑ (fraction of isotope n) x (mass of isotope n)

= (fraction1∙mass1) + (fraction2 ∙ mass2) + (fraction3 ∙ mass3) + …

n

Example

63Cu has mass = 62.9396, natural abundance 69.17%,and 65Cu has mass = 64.9278 and natural abundance 30.83%. What is the atomic mass?

Answer: Atomic mass = (62.9396)(0.6917) + (64.9273)(0.3083)

= 63.55 amu (4 sig fig!)

Sample Problems1. In a class A is worth 4 points and B is worth 3. Out of 20 grades, if youhave 4 A’s (20%) and 16 B’s (80%), what is your average? If an element has two isotopes, A (Z = 4 amu) and B (Z = 3 amu), and their natural abundance is 20% A and 80% B, what is the average molecular weight?

2. Naturally occurring boron (Z = 10.8110 amu) is composed of two isotopes,10B and 11B. Atoms of 10B have a mass of 10.0129 amu and those of11B have a mass of 11.0093 amu. Calculate the percentages by mass of the individual isotopes, 10B and 11B.

The Mole; How Chemists “Count”1. Avogadro’s Number -- The Chemists’ “Dozen”

N0 = number of atoms in exactly 12 g of carbon-12

= 6.022 x 1023 “things” ( a very large number!)This is a conversion factor, just like 12 things per dozen, e.g.Mass of one atom of carbon-12 = (12.0 g)/6.022 x 1023 atoms)

= 1.99 x 10-23 g/atom

2. The MoleOne Mole of a substance contains an Avogadro’s

number of formula units.

e.g. 1 mole of H2O = 6.022 x 1023 H2O

molecules

Sample Problems• How many atoms of gallium are there in 0.2398 moles of

gallium?

• How many molecules of sugar are there in 0.2398 moles of sugar?

• How many marbles are there in 0.2398 moles of marbles?