Chapter 4 Atomic Structure

OBJECTIVES:

• Understand the history of atom

• Differentiate between different models of the atom

• Identify the no. of protons and neutrons in a neutral atom based on atomic number Z and mass number A

KEY TERMS• Atom• Atomic mass• Atomic mass unit• Atomic number• Cathode ray• Dalton’s atomic theory• Electron• Group• Isotopes• Mass number• Neutron• Nucleus• Period• Periodic table• Proton

SECTIONS

• 4.1 Defining the Atom

• 4.2 Structure of the Nuclear Atom

• 4.3 Distinguishing Among Atoms

4.1 Defining the Atom Early Models of the Atom

Democritus’s Atomic Philosophy 400 BC (430?)

4.1

“Everything is made up of a few simple parts called atomos.” Atomos means “uncuttable” in Greek.

He envisioned atomos as small, solid particles of many different sizes and shapes.

Democritus’s ideas were limited because they didn’t explain chemical behavior and they lacked experimental support. His ideas were rejected because Aristotle supported *the “earth, air, water, and fire” concept of matter.

Democritus believed that atoms were solid particles that are indivisible and indestructible.

1.1.ALL MATTER IS COMPOSED OF ALL MATTER IS COMPOSED OF EXTREMELY SMALL PARTICLES EXTREMELY SMALL PARTICLES CALLED CALLED ATOMSATOMS

2.2. ATOMS OF A GIVEN ELEMENT ATOMS OF A GIVEN ELEMENT ARE ARE IDENTICAL IN SIZE, MASSIDENTICAL IN SIZE, MASS, , AND OTHER PROPERTIES; AND OTHER PROPERTIES; ATOMS OF DIFFERENT ATOMS OF DIFFERENT ELEMENTS DIFFER IN SIZE, ELEMENTS DIFFER IN SIZE, MASS, & OTHER PROPERTIESMASS, & OTHER PROPERTIES

ELEMENT2

ELEMENT2

ELEMENT 3

ELEMENT 3

ELEMENT 1

ELEMENT 1

ELEMENT4

ELEMENT4

3.3. ATOMS CANNOT BE ATOMS CANNOT BE SUBDIVIDED, CREATED, OR SUBDIVIDED, CREATED, OR DESTROYEDDESTROYED

4.4. ATOMS OF DIFFERENT ATOMS OF DIFFERENT ELEMENTS COMBINE IN ELEMENTS COMBINE IN SIMPLE SIMPLE WHOLE # RATIOSWHOLE # RATIOS TO FORM CHEM COMPDSTO FORM CHEM COMPDS

5.5. IN CHEMICAL RXNS, IN CHEMICAL RXNS, ATOMS ARE COMBINED, ATOMS ARE COMBINED, SEPARATED, OR SEPARATED, OR REARRANGEDREARRANGED

++++ ++++

Sizing up the Atom

• The radii of most atoms fall within the range of 5x10-11m to 2 x 10-10m. Individual atoms are observable with instruments such as scanning tunneling microscopes.

Discovery of the ElectronDiscovery of the ElectronIn 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively

charged particle: the electron

Subatomic ParticlesCathode Ray Tube

4.2

A cathode ray is deflected by a magnet.

A cathode ray is deflected by electrically charged plates.

Thomson concluded that a cathode ray is a stream of electrons. Electrons are parts of the atoms of all elements.

Subatomic Particles

In 1886, Eugen Goldstein (1850–1930) observed a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays. He concluded that they were composed of positive particles.

Such positively charged subatomic particles are called protons.

4.2

In 1932, the English physicist James Chadwick (1891–1974) confirmed the existence of yet another subatomic particle: the neutron.

Neutrons are subatomic particles with no charge but with a mass nearly equal to that of a proton.

• Plum Pudding Model– JJ Thomson – 1897

• Most of the atom was pos. charged• Atoms had negative charges embedded in pos.

charged material

• Rutherford’s Model– Ernest Rutherford – 1911

• Gold Foil Experiment – Rutherford fired positively charged particles at a sheet of gold foil

• Rutherford discovered the nucleus as a result of his experiment

The Atomic NucleusRutherford’s Gold-Foil Experiment

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.

4.2

Alpha particles scatter from the gold foil.

The Atomic Nucleus

The Rutherford Atomic Model

Ernest Rutherford concluded that the atom is mostly empty space. All the positive charge and almost all of the mass are concentrated in a small region called the nucleus.

The nucleus is the tiny central core of an atom and is composed of protons and neutrons.

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.

4.2

4.3 Distinguishing Among Atoms

Just as apples come in different varieties, a chemical element can come in different “varieties” called isotopes.

4.3

The Periodic Table—A PreviewA periodic table is an arrangement of elements in

which the elements are separated into groups based on a set of repeating properties.

A periodic table allows you to easily compare the properties of one element (or a group of elements) to another element (or group of elements).

Each horizontal row of the periodic table is called a period. Within a given period, the properties of the elements vary as you move across it from element to element.

Each vertical column of the periodic table is called a group, or family. Elements within a group have similar chemical and physical properties.

4.3

The Periodic Table—A Preview

• A Period

4.3

Period goes across

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 also have different mass numbers.

4.3

Despite these differences, isotopes are chemically alike because they have identical numbers of protons and electrons.

Isotopic notation

• Atomic number = # p– Elements are put in order of atomic number

on the periodic table.

Mass number = p + n (in the nucleus)

EA

Z

Element symbol

Ex: An atom of carbon with 7 neutrons:

An atom of lead with 125 neutrons:

13C6

207Pb82

Mass Number

Au is the chemical symbol for gold.

4.3

How many protons, electrons, and neutronsdoes a gold atom have?

The atomic number is 79. Therefore, there are 79 protons and 79 electrons. The mass number is 197, which is the total number of protons and neutrons. Therefore, 197-79= 118 neutrons.

Practice #2 (on worksheet)

Atomic Mass

An atomic mass unit (amu) is defined as one twelfth of the mass of a carbon-12 atom.

It is useful to to compare the relative masses of atoms to a standard reference isotope. Carbon-12 is the standard reference isotope. Carbon-12 has a mass of exactly 12 atomic mass units.

4.3Some elements and their isotopes

Atomic Mass

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

4.3

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

Atomic MassTo calculate the atomic mass of an element,

multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products.

For example, carbon has two stable isotopes: Carbon-12, which has a natural abundance of 98.89% and

a mass of 12.000 amu

Carbon-13, which has a natural abundance of 1.11% and a mass of 13.003 amu.

4.3

Silver is found in two isotopes with atomic masses 106.9041 and 108.9047 amu, respectively. The first isotope represents 51.82% and the second 48.18%. Determine the average atomic mass of silver.

(106.9041)(.5182)= 55.398(108.9047)(.4818)= 52.470

55.398 + 52.470 = 107.868 amu

• Planetary Model – Bohr Model– Neils Bohr – 1913

• Electrons move in definite orbits• Orbits are referred to as energy levels

4.2 The Structure of the Atom

• Atoms consist of subatomic particles– Protons

• Positive charge• Located in the nucleus

– Neutrons• No charge• Located in the nucleus

– Electrons• Negative Charge• Located in the electron cloud

Atomic Number and Mass Number

• Atomic Number Z– # of protons in the nucleus of an atom– Different elements have different numbers of

protons

• Mass Number A– Sum of an atoms protons and neutrons– n0 = A - Z

Isotopes

• Isotopes are atoms of the same element with different numbers of neutrons and different mass numbers.