CHAPTER 3

Chemical Foundations

3.1 THE ELEMENTS

• 118 elements

•92 occur naturally, the rest are synthesized

•All matter in the universe can be chemically broken down into elements

•Compounds are made by combining atoms of elements like words are formed from letters of the alphabet.

Words Compounds

ABUNDANCE OF ELEMENTS

• 9 elements make up about 98% of Earth

• Elements of the human body: CHNOPS

THE WORD ELEMENT CAN HAVE SEVERAL MEANINGS

Element

Element

Microscopic form

Single atom of that

element

Macroscopic form

Sample of that

element large enough

to weigh on a balance

Element

Generic form

When we say the human body contains the element

sodium or lithium, we do not mean that free

elemental sodium or lithium is present. Rather we

mean that atoms of these elements are present in

some form.

NAME AND SYMBOLS FOR THE ELEMENTS

• Each element has a unique name and symbol

• Symbol usually consists of first one or two letters of elements name • Oxygen = O

• Krypton = Kr

• Sometimes the symbol is taken from the original Latin or Greek name

FORMULAS OF COMPOUNDS

• The types of atoms and the number of each type is shown using chemical symbols and subscripts.

• Write the formula for each of the following, listing the elements in the order given

• One atom of sulfur and three atoms of oxygen

• Two atoms of nitrogen and five atoms of oxygen

• give the number of each atom in the following: • Ca(NO3)2

• AlPO4

• Al2(SO4)3

• 2H2O

3.2 EARLY IDEAS ABOUT MATTER

• Greek philosophers – formed explanations based on life experiences

•Proposed four ‘elements’

DEMOCRITUS – 400 BC

•Believed matter was made of tiny particles called atomos • Cannot be created, destroyed, or divided

•Aristotle disagreed

ALCHEMY

• Origin of the word Chemistry

• Tried to make gold out of abundant elements

• Invented many of the techniques still used today like filtration and distillation

ANTOINE LAVOISIER

• 1743-1794

• French scientist known as the “Father of Chemistry”

• Made Chemistry quantitative by taking accurate measurements

• Wrote 1st chemistry textbook

• Beheaded in 1794 for being funded by taxes

PROUST (1754-1826)

• Proposed the Law of Constant Composition (1794): • A given compound always contains the same proportion by mass of

the elements of which it is composed.

JOHN DALTON (1803)

• Revived and Revised Democritus’ idea

• Came up with his own Atomic Theory • All elements are composed of tiny particles called atoms

which cannot be broken into smaller parts.

• All atoms of the same element are identical, atoms of different elements are different

• Atoms of one element can combine with atoms of other elements to form compounds. A given compound always has the same relative numbers and types of atoms

• Chemical reactions take place when atoms rearrange. Atoms are not created or destroyed during chemical reactions.

3.3 DEFINING THE ATOM

• The smallest particle of an element that retains the properties of the element

•Atoms are very small One Cu atom = 1.28 x 10-10 m

Solid copper penny = 2.9 x 1022 atoms

If 1 atom = size of an orange, an orange would be as big as the earth!

• Scanning tunneling microscope (STM) allows individual atoms to be seen Scans across the surface of atoms

silicon

J.J. THOMSON – 1897

Worked with a cathode ray tube

Discovered cathode ray was attracted to positively charged plate

What 2 things did Thomson learn from this experiment?

By measuring the effects of both magnetic and electric fields on a cathode ray, Thomson determined the mass of the charged particle is less than the hydrogen atom (the lightest atom known)

Why is this significant?

What subatomic particle did Thomson discover?

•Thomson’s model Matter is neutral so some positive charge needs to balance

out the negative electrons

Plum pudding model

ERNEST RUTHERFORD - 1911

• In 1911, Rutherford and his coworkers at the University of Manchester, England, directed a narrow beam of alpha particles at a very thin sheet of gold foil.

• Based on Thomson’s model of the atom he expected the alpha particles to pass through the foil but not all did!

What conclusion can Rutherford draw from this result?

Rutherford concluded: Atom consists of mostly empty space

If an atom had a diameter of 2 football fields the nucleus would be the size of a nickel

Almost all + charge & mass are found in nucleus

Rutherford Model of the atom

ATOMIC STRUCTURE

Ernest Rutherford showed that atoms have internal structure. The nucleus, which is at the center of the

atom, contains protons (positively charged) and neutrons (uncharged).

– Electrons move around the nucleus.

JAMES CHADWICK - 1932

What we knew

1. Electrons & Protons have opposite charges

2. Neutral atom has same number of electrons as protons

3. Mass of atom was too much to just be made of protons & electrons

Showed nucleus contained a particle with the same mass as a proton but no charge – neutron

Particle Symbol Location Charge Mass

Electron

Proton

Neutron

The number of protons in an atom identifies it as a particular element

Number of protons = atomic number

Periodic table is organized in order of increasing atomic number

All atoms are electrically neutral therefore

# of protons =

Determine the number of protons and electrons in an atom of Molybdenum

MASS NUMBER

Mass number = protons + neutrons

Neutrons =

Mass numbers are given in atomic mass units (amu) 1 amu = 1/12 mass of Carbon-12 atom

ATOMIC SHORTHAND

• Beryllium- - -atomic number 4

mass number 9

The atomic number is written as a subscript.

The mass number is written as a superscript.

Be

Element Atomic

Number

Protons Electrons Neutrons Mass

number

19 20

35 80

10 20

ISOTOPES

Atoms with the same number of protons but different number of neutrons

• Most naturally occurring elements have two or more stable isotopes One is usually more common than the others

Oxygen – 16, oxygen – 17, oxygen - 18

• mass number vs. atomic mass • Mass number = protons + neutrons

• Atomic mass (on periodic table) = weighted average of all the isotopes of an element

CALCULATING ATOMIC MASS

Unknown element X has two isotopes. 6X has an atomic mass of 6.015 amu and a 7.59 % abundance. 7X has an atomic mass of 7.016 amu and a 92.41% abundance. What is the atomic mass of the unknown element? What is the identity of the unknown element?

IONS •Atoms can form ions by gaining or losing electrons. Metals tend to lose one or more electrons to form positive ions called

cations.

– Cations are generally named by using the name of the parent atom.

IONS • Nonmetals tend to gain one or more electrons to form negative ions called anions.

• Anions are named by using the root of the atom name followed by the suffix

–ide.

Element Atomic

# Protons Electrons Neutrons Mass

number

O2-

Al3+

19 18

146C

Potassium - 41

Ion charges can be predicted from the periodic table

WHAT CAN CHANGE IN AN ATOM?

Protons – NEVER!!!

Neutrons – if changed an isotope is formed

Electrons – if changed an ion is formed

PROTON CHANGED

Whole new atom!!!

Oxygen loses a proton it becomes Nitrogen

Oxygen gains a proton it becomes Fluorine

NEUTRON CHANGED

Different version of the same atom is formed (isotope)

Oxygen – 16 has 8 Neutrons

Oxygen – 17 has 9 Neutrons

Oxygen – 18 has 10 Neutrons

ELECTRONS CHANGED

Charged version of the same atom is formed (ion)

Oxygen gains 2 electrons – 10 electrons & 8 protons Anion

Oxygen loses 2 electrons – 6 electrons & 8 protons Cation

COMPOUNDS THAT CONTAIN IONS

•Ions combine to form ionic compounds.

•Properties of ionic compounds High melting points

Conduct electricity

If melted

If dissolved in water

Ionic compounds are electrically neutral Total positive charge must cancel out total negative charge

FORMULAS FOR IONIC COMPOUNDS

Write cation symbol followed by anion symbol

The number of cations and ions must be correct so their charge sums to zero

• Write the formula for compounds containing the following ions: Potassium and oxygen

Calcium and sulfur

Magnesium and nitrogen

3.4 INTRODUCTION TO THE PERIODIC TABLE

• In 1913 our current periodic table was developed by Henry Moseley who arranged the atoms by atomic number

• Periodic law – when elements are arranged according to increasing atomic number, there is a periodic repetition of chemical and physical properties

THE MODERN PERIODIC TABLE

•Columns = groups Numbered 1 – 18

Correspond to the number of outermost electrons

• Have similar properties

Some have special names

•Rows = periods Numbered 1 – 7

Correspond to outermost energy level

PHYSICAL STATES AND CLASSES OF ELEMENTS

•Most elements are solid at room temperature

•Br & Hg are liquid

•N, O, F, Cl, and Noble gases are gas

METALS

•Shiny

•Solid at room temperature

•Good conductors of heat & electricity

•Malleable

•Ductile

•Found to the left of the staircase • 1 exception =

Alkali metals – group 1 elements Except for

Very reactive

# of valence electrons =

Alkaline earth metals – group 2 elements Also highly reactive

# of valence electrons =

Transition metals – group 3 – 12 (d block) Number of valence electrons varies

Inner transition metals – f block Number of valence electrons varies

Lanthanide & actinide series

NONMETALS

•Gasses or dull looking solids • 1 exception =

•Poor conductors of heat and electricity

•Found to the right of the staircase

Halogens – group 17 Highly reactive

# of valence electrons =

Nobel gases – group 18 Extremely unreactive

# of valence electrons =

METALLOIDS

Border the staircase

Have properties of both metals and nonmetals

Semi-conductors = conduct electricity only under certain conditions

NATURAL STATES OF THE ELEMENTS

Most elements are very reactive.

Elements are not generally found in uncombined form. Exceptions are:

Noble metals – gold, platinum and silver

Noble gases – Group 8

NATURAL STATES OF THE ELEMENTS

Diatomic Molecules

Nitrogen gas contains

N2 molecules.

Oxygen gas contains

O2 molecules.

NATURAL STATES OF THE ELEMENTS

Diatomic Molecules

NATURAL STATES OF THE ELEMENTS

Elemental Solids

Diamond Graphite Buckminsterfullerene

Carbon

atoms