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
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
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