The Greek philosopher Democritus (460 B. C. – 370 B. C.) was among the first to suggest the existence of atoms

The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms (from the Greek word “atomos”)

• The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms (from the Greek word “atomos”)

• He believed that atoms were indivisible and indestructible
• His ideas did agree with later scientific theory, but did not explain chemical behavior, and was not based on the scientific method – but just philosophy

One change to Dalton’s atomic theory is that atoms are divisible into subatomic particles:

• One change to Dalton’s atomic theory is that atoms are divisible into subatomic particles:

• Electrons
• Protons
• Neutrons

Based on his experimental evidence:

• Based on his experimental evidence:

• The atom is mostly empty space
• All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus”
• The nucleus is composed of protons and neutrons (Discovered by Chadwick)
• The electrons distributed around the nucleus, and occupy most of the volume
• His model was called a “nuclear model”

OBJECTIVES:

• OBJECTIVES:

• Explain what makes elements and isotopes different from each other.

OBJECTIVES:

• OBJECTIVES:

• Calculate the number of neutrons in an atom.

OBJECTIVES:

• OBJECTIVES:

• Calculate the atomic mass of an element.

OBJECTIVES:

• OBJECTIVES:

• Explain why chemists use the periodic table.

1. Draw and name the 4 atomic models.

• 1. Draw and name the 4 atomic models.

• 2. Draw an atom with all of the subatomic particles labeled.

• 3. Convert 29837 to scientific notation.

Atoms are composed of identical protons, neutrons, and electrons

• Atoms are composed of identical protons, neutrons, and electrons

• How then are atoms of one element different from another element?

• Elements are different because they contain different numbers of PROTONS

• The “atomic number” of an element is the number of protons in the nucleus

• # protons in an atom = # electrons

Contain the symbol of the element, the mass number and the atomic number.

• Contain the symbol of the element, the mass number and the atomic number.

Dalton was wrong about all elements of the same type being identical

• Dalton was wrong about all elements of the same type being identical

• Atoms of the same element can have different numbers of neutrons.

• Thus, different mass numbers.

• These are called isotopes.

• Frederick Soddy (1877-1956) proposed the idea of isotopes in 1912

• Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.

• Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials.

We can also put the mass number after the name of the element:

• We can also put the mass number after the name of the element:

• carbon-12
• carbon-14
• uranium-235

How heavy is an atom of oxygen?

• How heavy is an atom of oxygen?

• It depends, because there are different kinds of oxygen atoms.

• We are more concerned with the average atomic mass.

• This is based on the abundance (percentage) of each variety of that element in nature.

• We don’t use grams for this mass because the numbers would be too small.

Instead of grams, the unit we use is the Atomic Mass Unit (amu)

• Instead of grams, the unit we use is the Atomic Mass Unit (amu)

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

• Carbon-12 chosen because of its isotope purity.

• Each isotope has its own atomic mass, thus we determine the average from percent abundance.

Multiply the atomic mass of each isotope by it’s abundance (expressed as a decimal), then add the results.

• Multiply the atomic mass of each isotope by it’s abundance (expressed as a decimal), then add the results.

• If not told otherwise, the mass of the isotope is expressed in atomic mass units (amu)

OBJECTIVES

• OBJECTIVES

• Describe the origin of the periodic table
• Identify the position of groups, periods and the transition metals in the periodic table

Dmitri Mendeleev (1834 – 1907)

• Dmitri Mendeleev (1834 – 1907)

• Listed the elements in columns in order of increasing mass

• Then he arranged the columns so that the elements with the most similar properties were side by side

Periodic Table – an arrangement of the elements according to similarities in properties

• Periodic Table – an arrangement of the elements according to similarities in properties

Henry Moseley (1913)

• Henry Moseley (1913)

• Determined atomic number of the atoms of elements

Period – Horizontal rows of the periodic table – (side to side)

• Period – Horizontal rows of the periodic table – (side to side)

• There are 7 periods

• Periodic law – When the elements are arranged in order of increasing atomic number, there is a periodic repetition of their physical and chemical properties

Group – vertical (up and down) column of elements in the periodic table

• Group – vertical (up and down) column of elements in the periodic table

• The elements in any group of the periodic table have similar physical and chemical properties

• Each group is identified by a number and letter A or B

• Group A elements are called representative elements because they exhibit a wide range of both physical and chemical properties

These elements can be divided into three broad classes

• These elements can be divided into three broad classes

• Metals

• Characteristics

• High electrical conductivity

• High luster when clean

• Ductile (able to be drawn into wire)

• Malleable (able to be beaten into sheets)

With the exception of hydrogen all the representative elements on the left side of the periodic table are metals

• With the exception of hydrogen all the representative elements on the left side of the periodic table are metals

• Group 1A – alkali metals

• Group 2A – alkalaine earth metals

• Most of the remaining elements that are not in Group A are also metals

• Transition metals

• Inner transition metals

Rare Earth metals – the inner transition metals which appear below the main body of the periodic table

• Rare Earth metals – the inner transition metals which appear below the main body of the periodic table

Occupy the upper-right corner of the periodic table

• Occupy the upper-right corner of the periodic table

• Non-metals are elements that are generally non lustrous and poor conductors of electricity

• Many are gases @ room temperature

• Ex.) O, Cl

• Others are brittle solids

• Ex.) sulfur

Group 7A = Halogens ex.) Cl, Br

• Group 7A = Halogens ex.) Cl, Br

• Group 8A = Noble Gases “inert gases” - undergo few chemical changes

border the line between metals and non metals

• border the line between metals and non metals

• Have properties in between metals and non metals

Rather than memorizing more than 100 elements you need to only learn the general behavior and trends within the major groups

• Rather than memorizing more than 100 elements you need to only learn the general behavior and trends within the major groups

• This gives you a working knowledge of the properties of most elements