Chapter 4:

Atomic Structure

4.1 Defining the Atom

• An Atom is the smallest particle of an

element that retains its identity in a

chemical reaction.

• The Greek Philosopher

Democritus, was among the

First to suggest the existence

Of atoms.

•Democritus believed that

atoms were indivisible and

indestructible.

•Although Democritus’s ideas

agreed with later scientific

theory, they did not explain

chemical behavior.

•They also lacked experimental

support.

Dalton’s Atomic Theory

• By experimental methods, Dalton

transformed Democritus’s ideas on atoms

into scientific theory.

• Dalton studied the ratios in which

elements combine in chemical reactions.

Based on the results of his experiments,

Dalton formulated hypotheses and

theories to explain his observations.

Dalton’s Atomic Theory

1. All elements are composed of tiny indivisible particles called atoms.

2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element.

3. Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.

4. Chemical reactions occur when atoms are separated, joined, or rearranged.

Sizing up the Atom….

• Copper atoms are very small.

• A copper coin the size of a penny contains

about 2.4 x 10^22 atoms. By comparison,

Earth’s population is only about 6 x 10^9

people.

• If you could line up100,000,000

Copper atoms side by side, they would

produce a line only 1 cm long.

4.2 Structure of the Nuclear Atom

• Much of Dalton’s atomic theory is

accepted today. One important change

however, is that atoms are now known to be divisible.

• Three kinds of subatomic particles are

electrons, protons, and neutrons.

Electrons• In 1897, English

physicist J.J. Thomson discovered the Electron.

• Electrons are negatively charged subatomic particles.

• Thomson called these particles corpuscles; later they were named electrons.

Robert A. Millikan (1868-1953)

• Carried out

experiments to find

the quantity of charge

carried by an

electron.

• An electron carries

exactly 1 negative

charge and its mass

is 1/1840 the mass of

a hydrogen atom.

»You need to understand….

•Atoms have

NO CHARGE!!

»They are electrically

neutral.

• Next……..

•Electric charges are

carried by particles of

matter.

Third……

•There are NO

fractions of charges,

only whole number

multiples!!!

And Finally…….

• When a negative particle combines

with a positive particle an electrically

neutral particle is formed.

+1 + -1 = 0

Eugene Goldstein

• Observed a cathode-ray tube and found

rays traveling in the direction opposite to

that of the cathode rays.

• He called these rays canal rays and

concluded that they were composed of

positive particles.

• These positive particles are called

Protons.

James Chadwick

• In 1932, an English Physicist confirmed

the existence of yet another subatomic

particle: The Neutron.

• A neutron is a subatomic particle with no

charge but with a mass nearly equal to

that of a proton.

Plum Pudding Model

• In Thomson’s atomic

model, known as the

plum pudding model,

electrons were stuck

into a lump of positive

charge, similar to

raisins stuck in

dough.

• This model of the atom turned out to be short

lived, however, due to the groundbreaking

work of Ernest Rutherford (1871-1937), a

former student of Thomson.

Rutherford’s Gold Foil Experiment

• Rutherford’s Gold Foil

Experiment

»Here is a video of what

happened!!!

Rutherford Atomic Model

• He proposed that the atom is mostly empty space, thus explaining the lack of deflection of most of the alpha particles.

• He concluded that all the positive charge and almost all the mass are concentrated in a small region.

• He called this region the Nucleus.• The nucleus is the tiny central core of an atom and is

composed of protons and neutrons.

Key Concept

• In the nuclear atom, the protons and

neutrons are located in the nucleus. The

electrons are distributed around the

nucleus and occupy almost all the volume

of the atom.

• Although it was an improvement over Thomson’s model of the atom, Rutherford’s model turned out to be incomplete.

• In Chapter 5, we will learn how it was revised.

4.3 Distinguishing Among Atoms

•Elements are different because they contain different numbers of protons.

Atomic Number

• The atomic number of an element is the

number of protons in the nucleus of an

atom of that element.

Mass Number

• The total number of protons and neutrons

in an atom is called the mass number.

• Number of neutrons = mass number – atomic number

• The number of neutrons in an atom is the difference

between the mass number and atomic number.

• If you know the

atomic number

and mass

number of an

atom of any

element, you can

determine the

atom’s

composition.

• How do you calculate

mass number?

Isotopes

Isotopes are atoms that have the same number of protons but different numbers of neutrons.

Because isotopes of an element have different numbers of neutrons, they have different mass numbers.

Isotopes Cont…

• Atomic mass unit is defined as one twelfth of the mass of a carbon-12 atom.

• In nature most elements occur as a mixture of two or more isotopes.

• Isotopes of an element has a fixed mass and a natural percent abundance.

Relative Abundance

• The Atomic Mass of an element is a weighted average mass of the atoms in a naturally occurring sample of the element.

• A weighted average mass reflects both the mass and the relative abundance of the isotopes as they occur in nature.

Determine Atomic Mass

1. # stable isotopes

2. Mass of each isotope

3. Natural % abundance of each isotope

Atomic Mass = Mass of Isotope x natural

abundance = add them together

Look on page 117 for practice problems!!

The Periodic Table – A Preview

•Each horizontal row of the periodic table is called a PERIOD.

Periodic Table Cont….

• Each vertical column of the periodic table is called a group.

• Elements within a group have similar chemical and physical properties.

Lets Review!!!

• Atoms are the smallest particle of an element that retains its identity in a chemical reaction.

• Democritus believed that atoms were indivisible and indestructible.

• Dalton’s atomic theory states that

1. All elements are composed of atoms

2. Atoms of different elements combine to form compounds

3. Atoms of the same element are identical

4. Chemical reactions occur when atoms are separated, joined, or rearranged.

Review Cont….

• There are 3 subatomic particles

1. Proton

– Identifies the atom

– Positive charge

2. Electron

– Determines chemical reactions

– Negative Charge

3. Neutron

– Determines Isotope

– Neutral Charge

Review Cont…

• Thomson atomic model, known as the plum pudding model, electrons were stuck into a lump of positive charge.

• Rutherford’s Gold Foil Experiment: He proposed that atoms are mostly empty space.

• The nucleus is the tiny central core of an atom made of protons and neutrons. Electrons surround the nucleus.

• Atomic number is the number of protons.

• Mass number is the # of protons and neutrons.

• Isotopes are atoms that have the same number of protons but a different number of neutrons.

Review Cont…

• Atomic mass unit is defined as one twelfth of the mass of a carbon-12 atom.

• The atomic mass of an element is the weighted average mass of the atoms in a naturally occurring sample of the element.

• To calculate the atomic mass of an element, multiply the mass by its natural abundance and then add the products.

Review Cont…

• Periodic Table allows you to easily

compare the properties of one element to

another element.

• Groups go down!!

• Periods all across!!

• Elements within a group has similar

physical and chemical properties.