If opposite charges attract, why are the negative electrons not attracted to the

positive nucleus?

Electrons are always moving.

Electrons have inertia in motion = momentum.

If electrons are always moving and they weigh

very, very little, why don’t they get thrown

away out of the atom?

Because of the electromagnetic

force!

Forces inside the atom

This force of attraction binds the electrons to the atom. The electromagnetic force becomes stronger as the distance between charges

becomes smaller.

If opposite charges attract, how do the

protons get attracted with the neutrons?

Because of the strong nuclear

force!

Forces inside the atom

Protons and neutrons in the nuclei of atoms are held together by the strong force.

If the atomic mass/weight is just the sum of the protons

and neutrons, why are there decimal values for the

atomic weight written on the periodic table?

Atomic Weight is an Average!

It is the weighted average of the individual atomic weights, or mass numbers, of the isotopes.

Example

• Chlorine = 35.48 (atomic weight)Isotopes of ChlorineChlorine-35 = 76% abundanceChlorine-37 = 24% abundance

Σ(mass number x %abundance)

Example

• The mass of Isotope 1 is 6.015 amu and its abundance is 7.5%

• The mass of Isotope 2 is 7.016 amu with a 92.5% abundance.

• What is the average atomic mass?• What is the element?

The Periodic

Table

Johann Dobereiner

(1829)

Triads!

The elements in a triad had similar chemical properties and orderly physical properties.

Calcium, Strontium,

BariumChlorine,

Bromine, Iodine

John Newlands

(1863)

Octaves!

After arranging the elements in order of increasing atomic mass, certain properties

repeated every 8th element.

Lothar Meyer(1867)

Increasing atomic mass!

Increasing atomic mass!

Dmitri Mendeleev

(1869)“Father of the

Periodic Table”

Both scientists left vacant spaces where unknown elements should fit.

Periodic Table,I am your

Father.

Dmitri Mendeleev

(1869)

The weight must be wrong.

Reclassify!

Dmitri Mendeleev

(1869)

Increasing atomic

NUMBER!

Henry Moseley(1913)

Through his work with X-rays, he determined the actual nuclear charge (atomic number) of the elements. He

rearranged the elements in order of increasing atomic number.

Actinide and Lanthanide

Series!

Glenn Seaborg(1944)

He reconfigured the periodic table by placing the actinide series below the

lanthanide series

Periodic Table Geography

George M. McKelvy, Ph.D. 33

The Modern Periodic Table

Columns of elements are called groups.

George M. McKelvy, Ph.D. 34

The Modern Periodic Table

Rows of elements are called periods.

The Alkali Metals

• 1 valence electron

• Highly reactive with water

• Form ionic compounds

• Do not occur in nature as pure elements (always in compounds)

Alkali-Earth Metals

• Have 2 valence electrons• Reactive, but less reactive

than alkali metals • Are ductile, malleable and

have a silvery luster

Transition metals… and inner transition metals

• Are less reactive than groups 1 and 2.

• Tend not to react in water. • Are malleable and ductile, but

still harder than group 1 & 2. • Tend to be solids at room

temperature.• Have variable chemical

properties• Are good conductors of

electricity and heat.

Inner transition metals tend to be radioactive

Nonmetals

• Poor conductors of heat and electricity• Often are found as gases or liquids,

sometimes solids.

Halogens

• Are nonmetals• highly reactive with

metals- most reactive is flourine, lease reactive is astatine

• Mostly exist as gases or liquids (except At -solid)

• Have 7 valence electrons

Noble gases

• At room temperature, exist as gases.

• Are completely unreactive

• Have full s and p orbitals

• Are odorless, colorless, nonflammable

Metalloids

• Tend to be solids• Have properties similar to both metals and

nonmetals• Tend to be semiconductors (which means they are

useful for technological uses)

Periodic Law

When elements are arranged in order of increasing atomic number,

there is a periodic pattern in their physical and chemical properties.

Metals, metalloids (semi-metals) and non-metals

Properties of Metals

• Good conductors of heat and electricity: lots of mobile electrons to move energy and charge around

• Have luster (shiny)• Are ductile (stretchable)

and malleable (bendable)• Solids at room temp.

except for Hg

Properties of Metalloids

• Have properties of both metals and non-metals

• Some of them are semiconductors (like silicon) which behave sometimes as conductors and sometimes as insulators

• Solids at room temperature

Properties of Non-Metals

• Poor conductors of heat and electricity

• Can be solids, liquids or gases (contrary to popular belief not all non-metals are gases!)

• Are not ductile, malleable

Periodic Trends - Reactivity

–how likely or vigorously an atom is to react with other substances

–Usually determined by how easily electrons can be removed and how badly they want to take other atom’s electrons since it is the transfer/interaction of electrons that is the basis of chemical reactions

Periodic Trends - Reactivity

• Metals–Period: DECREASES from left to right–Group: INCREASES down a groupWHY? The farther left and down the

periodic table, the easier it is for elections to be given or taken away, resulting in a higher reactivity

Periodic Trends - Reactivity

• Non-metals–Period: INCREASES from left to right –Group: DECREASES down a group WHY? The farther right and up the

periodic table, the higher the electronegativity, resulting in a more vigorous exchange of electrons