Chapter 4

The Structure of the Atom

Section 4.1

Early Theories of Matter

Greek Philosophies of Matter

• A few thousand years ago, people did not know what a controlled experiment was

• Scientific ideas came not from experimentation, but from intellectual thought

• Early people thought that matter was compose of earth, water, air, and fire

• They also thought that matter could be infinitesimally to smaller and smaller pieces

Democritus and His Theory• Democritus was the first person to propose that matter

could not be divided into infinitesimally small pieces • Proposed that

– Matter is composed of empty space through which atoms move– Atoms are solid, homogeneous, indestructible, and indivisible– Different kinds of atoms have different shapes and sizes– The different properties of matter are due to the different size,

shape, and movement of atoms– Apparent changes in matter result from changes in the way

atoms are group together, and not from changes in the atoms themselves

Democritus v. Aristotle

• “What holds atoms together?” was a common criticism for Democritus’ theory which he could not explain

• Aristotle did not agree with Democritus’ view because the theory clashed with his idea of nature

• Aristotle asked how “nothingness,” or empty space could exist and how atoms could move through it, which Democritus could not explain

• Because of these challenges, Democritus was eventually rejected by the intellectual community

John Dalton• Used experimentation to back up

his atomic theory– All matter is composed of extremely

small particles called atoms– All atoms of a given element are

identical, having the same size, mass, and chemical properties. Atoms of a specific element are different from those of any other element

– Atoms cannot be created, divided into smaller particles, or destroyed

– Different atoms combine in simple whole-number ratios to form compounds

– In a chemical reaction, atoms are separated, combined, or rearranged

Daltons Theory Cont’d.

• By using experimentation to back his theory, Daltons theory became a major step towards our current understanding of the atom

• Was Daltons theory completely correct?

Defining Atomic Size

• What is the size of an atom?• The world’s population is nearly 7,000,000,000

and a typical penny has nearly 29,000,000,000,000,000,000,000 copper atoms

• The diameter of a copper atom is 1.28x10-10m• 7billion copper atoms side by side would

result in a line only about one meter long

Section 4.2

Subatomic Particles and the Nuclear Atom

Discovering the Electron

• Scientists began to study the relationship between electric charge and matter in the 1800’s

• Scientists were studying this relationship with a newly invented apparatus called the cathode ray tube

Electrons, cont’d.

Electrons, cont’d.

• By observing the deflection of the stream in the CRT, Thomson was able to conclude two things– The cathode rays were actually charged particles,

not just a wave of energy– The charge of the particles was negative (but the

exact value of the charge was not yet known)• These particles were called electrons

Thomson’s Atomic Model• Thomson’s concluded that

electrons were common to all types of matter

• His model of the atom placed electrons evenly throughout a positive uniform charge in the atom

• It was called the plum pudding model

• Now it is better known as the chocolate chip cookie model

Rutherford and the Nuclear Atom

• Rutherford was a Kiwi, working at Cambridge University

• He was studying how positively charged alpha particles interacted with solid matter

• Rutherford made hypotheses based on Thomson’s plum pudding model of the atom

• He expected most of the alpha particles to pass straight through the matter, will very little deflection

Results

• Rutherford was astounded to find his results• As he predicted, most alpha particles passed straight

through the material with only slight deflection• However, some particles were deflected at very

extreme angles, and some even bounced straight back

• He compared this to firing an artillery cannon at a sheet of paper, and having it bounce back toward you

Explanation for the Results• Rutherford concluded that the plum pudding model was

incorrect based on his findings• He developed a new model of the atom based on his findings• He concluded that most of the atom is empty space, thus

allowing most of the alpha particles to pass straight through• However, there is a tiny, dense, positively charged region,

called the nucleus at the center of every atom• The nucleus contains nearly all of an atoms mass• The nucleus is the cause of some alpha particles being

strongly deflected

Completing the Atom

• In 1920, Rutherford searched for the reason the nucleus was positively charged.

• He discovered the proton, a subatomic particle carrying a charge equal to and opposite of that of an electron (proton has a +1 charge)

• In 1932, the nucleus was also shown to contain a neutron, a particle that has nearly identical mass to a proton, but is neutral in charge

Properties of Subatomic Particles

Particle Symbol Location Relative electrical

charge

Relative mass

Actual mass (g)

Electron e- In the space surrounding the nucleus

1- 1/1840 9.11x10-28

Proton p+ In the nucleus

1+ 1 1.673x10-24

Neutron n0 In the nucleus

0 1 1.675x10-24

Section 4.3

How Atoms Differ

Atomic Number

• In the early 1900’s, shortly after Rutherford’s gold foil experiment, Henry Moseley discovered that atoms of each element contain a unique positive charge in the nuclei

• After Rutherford discovered the proton, Moseley concluded that the number of protons is unique to each element

• The number of protons in the nucleus of an element is known as the atomic number

• This can be found on any periodic table

• Because all atoms are neutral (how do we know this?), the number of protons is also equal to the number of electrons, which is equal to the atomic number

• Atomic number = number protons = number electrons

Composition of Several Elements

Element Atomic Number Protons Electrons

a. Pb 82

b. 8

c. 30

Isotopes and Mass Number

• Dalton’s atomic theory told us that atoms of the same element were identical

• However, we later learned that this was incorrect• An example being potassium. There are three types

of potassium atoms. All three types contain 19 protons, but one type of potassium contains 20 neutrons, another 21 neutrons, and the third has 22 neutrons

• Atoms that have the same number of protons, but different numbers of neutrons are called isotopes

Isotopes

• Do you think the three different isotopes of potassium have the same mass?

• In order to identify the different isotopes from one another, chemists use the atoms mass number

• Mass number is equal to the number of protons plus the number of neutrons in the nucleus

• The isotope of potassium that has 20 neutrons has a mass number of 39 (19p+ + 20n0)

• Number of neutrons = mass number – atomic number

Isotope Composition Data

Element Atomic Number Mass Number Number of Neutrons

a. Neon 10 22

b. Calcium 20 46

c. Oxygen 8 17

d. Iron 26 57

e. Zinc 30 64

f. Mercury 80 204

• There are two ways in which we indicate the isotope of an element

• Potassium-39 the 39 indicates the mass number. How many protons does potassium have? How many neutrons?

• The second way is to use the elemental symbol where the top number is the mass

number and the bottom number is the atomic number

K3919

Atomic Mass

• Atomic mass of an element is the weighted average of the isotopes of that element

• For example, chlorine exists naturally as a mixture of about 75% chlorine-35 and 25% chlorine-37

• Will the weighted atomic mass be closer to 35 or 37? Why?

• What is the atomic mass of chlorine?

Complete the following ChartComposition of Various Isotopes

Isotope Atomic number

Mass number

Number of protons

Number of neutrons

Number of electrons

32 16

24 20

Zn-64

9 10

11 23

Find the atomic mass of chromium using the following data

Chromium Isotope Data

Isotope Percent abundance Mass (amu)

Cr-50 4.35% 49.946

Cr-52 83.79% 51.941

Cr-53 9.50% 52.941

Cr-54 2.36% 53.939