Section 6-1 NotesOrganizing the Elements

Organizing the Elements

As new elements were discovered chemists needed to find a logical way to organize them

Properties of elements were used to sort them in to groups

In 1829, J.W. Dobereiner published a classification system in which the elements were grouped into triads

A group of 3 elements with similar properties

Ex. Chlorine, Bromine, and Iodine

One element in each triad tended to have properties with values that fell midway between those of the other two elements

Problem - some of the elements could not be grouped into triads

Mendeleev’s Periodic Table

1869, Dmitri Mendeleev published a table of elements

The organization he chose was a periodic table, based on a set of repeating properties of the 60 known elements.

Mendeleev arranged the elements in order of increasing atomic mass – see fig. 6.3

He added ? because:

1st because he knew that Bromine needed to be with Chlorine & Iodine.

2nd because he predicted that elements would be discovered that fit in those spaces

Periodic Law

Mendeleev arranged his element based on increasing atomic mass, but he placed tellurium(127.6amu) before iodine (126.9amu) in his table.

He did this because it placed iodine in the same group as chlorine and bromine.

He knew that these elements had similar properties

1913, Henry Moseley, British physicist, determined an atomic number for each known element

By using the atomic number, it makes sense that Tellurium should come before Iodine

Therefore, in the modern periodic table, elements are arranged in order of increasing atomic number not mass

Periodic Law

There are seven rows (periods) in the table

Each period of the table corresponds to a principal energy level

The elements within a column (group) have similar properties

These properties repeat in each period, a pattern known as the periodic law

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

Metals, Nonmetals and Metalloids

The elements have been grouped into 3 broad classes based on their

characteristics

Metals

Nonmetals

Metalloids

Properties of Metals

Good conductors of heat and electricity

High luster or sheen

Solid at room temperature

Exception - Mercury

Ductile - can be drawn into wires

Malleable - can be hammered into thin sheets

Properties of Non Metals

Most are gases at room temperature

Sulfur and Phosphorus are solids

Bromine is a dark-red liquid

Poor conductors of heat and

electricity

Carbon is an exception

Solid nonmetals tend to be brittle

Metalloids

Metalloids generally have properties similar to those of metals and nonmetals

Depending on the conditions a metalloid can act as a metal or a nonmetal

Example

Pure silicon is a poor conductor of electric current, similar to nonmetals

But, when mixed with boron it is a good conductor of electric current, similar to metals

Section 6-2 NotesClassifying the Elements

Squares in the Periodic Table

The periodic table displays the symbols and names of the elements, along with information about the structure of their atoms.

Each square include the symbol, atomic mass, and atomic number of the element

There is a vertical column that which lists the number of electrons in each energy level

Also some tables provide a color coordinated chart to distinguish some specific groups of elements

Groups of Elements

Alkali metals- group 1A elements

Arabic for “the ashes”, Na & K are common in wood ashes

Alkaline earth metals- group 2A elements

Halogens- group 7A elements

Hals- Greek for salt genesis- Latin for to be born

These elements can be produced from their salts

Chalcogens- group 6A elements

Noble Gases – group 8A elements

Electron Configurations in Groups

Elements can be sorted into:

Noble gases

Representative elements

Transition metals

Inner transition metals

Based on their electron configurations.

The noble gases all have a full outer shell so they rarely take part in a reaction

The Representative Elements

Groups 1A-7A are the representative elements

They are referred to as representative elements because they display a wide range of physical and chemical properties

Some are metals, metalloids, and nonmetals

Most are solids, some are gases and one is a liquid at room temperature

The s and p sublevels are the highest occupied energy levels that are being filled

Transition Elements

The B groups on the periodic table are known as transition elements

These elements are all metals

These elements are characterized by the presence of electrons in the d sublevel

Inner transition metals are characterized by the presence of electrons in the f sublevel

Section 6-3 NotesPeriodic Trends

Trends in Atomic Size

To determine atomic size we use two atoms of the same element joined together

Since the atoms in each molecule are identical, the distance between the nuclei of the atoms can be used to estimate the size of the atoms

The atomic radius is ½ the distance between the two nuclei

In general, atomic size increases from top to bottom within a group

Atomic size decreases from left to right across a period

Group Trends in Atomic Size

What happens to the # of protons we move through a group (column)?

The increase in positive charge draws the electrons closer to the nucleus

The increase in # of occupied orbitals shields electrons in the highest energy level from the attraction of protons

Called the shielding effect

The shielding effect is greater then the effect of the increase in nuclear charge

As a result, the atomic size increases

Ions

An ion is an atom or group of atoms that has a positive or negative charge

Positive and negative ions form when electrons are transferred between atoms

An ion with a positive charge is called a cation

Na+, Ca2+

An ion with a negative charge is called an anion

Cl-, O2-

Trends in Ionization Energy

The energy required to remove an electron from an atom is called ionization energy

Always measured when the element is in its gaseous state

First ionization energy tends to decrease from top to bottom

Recall the shielding effect

Tends to increase from left to right across a period

Group Trends in Ionization Energy

Recall that the atomic size increases

as the atomic number increases

within a group

As the size increases, nuclear charge

has a smaller effect on the electrons

in the highest energy level.

Therefore, it takes less energy to

remove an electron, causing the first ionization energy to be lower

Periodic Trends in Ionization Energy

Energy generally increases as you

move left to right due to an increase

in nuclear charge and the shielding effect remaining the same for the

period

Since there is an increase in the

attraction of the nucleus for an

electron, it takes more energy to remove an electron from an atom

Trends in Ionic Size

Cations are always smaller than the atoms from which they form.

Anions are always larger then the atoms from which they form

When a sodium atom loses an electron, the attraction between the remaining electrons and the nucleus is increasedAs a result, the electrons are drawn closer,

making the size smaller

This is opposite for nonmetals that gain electrons because the attraction is decreasedMore negative charge then positive charge

Trends in Electronegativity

Electronegativity is the ability of an atom of an element to attract electrons when the atom is in a compound

Scientists use factors such as ionization energy to calculate values for electronegativity

In general, electronegativity values decrease from top to bottom within a group

The values tend to increase from left to right across a period

Cesium is the least electronegative element

Fluorine is the most electronegative element

The electronegativity of transition metals is irregular