Chapter 6 The Periodic Table

J. W. Dobereiner In organized elements into “triads” (a group of 3 elements with similar properties)

History John Newlands in 1865 –He was the first to arrange the elements according to their properties –arranged the known elements according to their properties and in order of their increasing atomic mass –He place the elements in a table and he noticed that all of the elements in a given row had similar properties and these properties seemed to repeat every 8 elements, so he called this pattern the law of octaves

Dmitri Mendeleev in 1869 used Newlands observation and other information to produce the first orderly arrangement of all the elements known at the time (63 elements) this arrangement is called a periodic table He arranged the elements in order of increasing atomic mass he started a new row every time he noticed the properties repeated placing elements in the new row directly below elements of similar properties in the preceding row

Dmitri Mendeleev, continued Mendeleev’s table contained gaps he explained these gaps by predicting that there were undiscovered elements he also predicted the properties of these missing elements based on the column they were in

Dmitri Mendeleev, continued he also noticed that some elements didn’t fit in the correct column by property when ordering them by atomic mass he switched the order of these elements to group the elements in the column that matched their properties at first he thought the atomic masses must have been incorrect, but they were correct, so Mendeleev couldn’t explain this irregularity

Henry Moseley - about 40 years after Mendeleev’s periodic table was published found a different physical basis for the arrangement of the elements, discovered atomic number He studied the Xrays of different elements and found that they each had unique wavelengths that correlate to the number of protons in the element this number is now referred to as the atomic number

Henry Moseley, continued when the elements were arranged in the periodic table by atomic number rather than atomic mass, the discrepancies Mendeleev found were no longer there Moseley’s work led to the modern definition of atomic number and showed that atomic number is the basis for organization of the periodic table not atomic mass

The Physical Basis of the Periodic Table The Periodic Law states: the repeating properties of elements change periodically with their atomic number Organization of the Periodic Table elements in each column have the same number of electrons in their outer level This is why they have similar properties We can use this to predict the ending electron configurations on the s and p blocks

outer electrons are called valence electrons it is the valence electrons that participate in chemical reactions, so elements with the same number of valence electrons tend to react in a similar fashion That is why groups of elements in the same column share certain properties

Arrangement on the Periodic Table vertical columns are called groups and horizontal rows are called periods certain groups have specific names and we call them families elements in the same period have the same number of occupied energy levels (1-7) label these levels on the periodic table we can use the period numbers and ending electron configurations for the groups to predict an elements electron configuration

The Main-Group Elements Elements in groups 1,2 and are called the main-group elements they are in the s and p blocks, meaning they are adding electrons to their s and p sublevels respectively their electron configurations are regular and consistent Remember, the elements in each group have the same number of valence electrons and therefore the same ending configuration

groups with family names group 1 – alkali metals, they have a single valence electrons and are very reactive they are never found in nature as pure elements because they are so reactive they are always combined with other elements as compounds group 2 – alkaline-earth metals, they have 2 valence electrons, they must lose 2 electrons to have a stable configuration, they are slightly less reactive than the alkali metals they are usually found in nature as compounds rather than pure elements

groups with family names, continued group 16 – chalcogens, they have 6 outer electrons and are relatively reactive group 17 – halogens, the most reactive group of nonmetals, they have 7 outer electrons, they often gain one electron group 18 – noble gases, have a full set of outer electrons, these elements are very stable, they rarely react, they are also known as inert gases These groups are all nonmetals

Helium is the only noble gas that has only 2 outer electrons -because it is in the first level, and 2 is the maximum number of electrons that level 1 can hold -so it has a full outer level Hydrogen is in a class by itself it is the most common element in the universe it consists of only one proton and one electron, so it behaves differently than any other element it can react with many other elements

Most Elements are Metals when looking at the periodic table; metals are on the left, nonmetals are on the right, and the majority of elements are metals there is a general dividing line, called the staircase line, which separates metals and nonmetals elements along the staircase line that have properties of both metals and nonmetals are referred to as metalloids

Metals Share Many Properties most metals are shiny (not all, and there are some nonmetals that are shiny) all metals conduct electricity (this is one property that distinguishes between metals and nonmetals) metals are conductors of heat (there are some nonmetals that do this) metals are ductile (can be squeezed out into a wire) metals are malleable (can be hammered or rolled into sheets)

Metals Share Many Properties, continued Other Properties of Metals –metals can be mixed with one or more other elements (usually metals) to form alloys this mixing eliminates some of the disadvantages of the independent elements and gives the new alloy properties that are different than the original elements Ex. – Brass is a yellowish alloy of copper and zinc, sometimes including small amounts of other metals, but usually 67 percent copper and 33 percent zinc.

Transition Metals groups 3-12 are referred to as transition elements they are the d group elements, therefore some of them have “irregular” electron configurations they do not always have identical electron configurations within the same group

Transition Metals, continued even if these elements do not have the same electron configurations within a group, there is a pattern to there outer electron configs – the sum of the outer d and s electrons is equal to the group number transition metals are generally less reactive than the alkali and alkaline-earth metals –some transition metals are so unreactive that they rarely form compounds with other elements

Properties of Nonmetals generally gases or brittle solids at room temperature if they are solid their surfaces are dull they are insulators

Lanthanides and Actinides there are 2 periods at the bottom of the periodic table, they are referred to as a series the top period is called the lanthanide series because it follows the element lanthanum their atomic numbers range from they are adding electrons to the 4f orbitals the bottom period is called the actinide series because it follows the element actinium their atomic numbers range from they are adding electrons to the 5f orbitals they are all radioactive

Trends in the Periodic Table Ionization Energy the energy required to remove an electron from an atom is called the ionization energy Decreases as You Move Down a Group the further down the table you go, the more energy levels you add therefore, the pull from the nucleus is less due to the increased distance and the electron shielding affect (the inner electrons shield the outer electrons from the nucleus)

Increases as You Move Across a Period –from left to right across the periodic table, you are adding one proton in the nucleus and one electron in the same outer level –this additional positive charge allows the nucleus to pull harder on the electrons, and the electrons are being added in the same level so the electron shielding is not increasing Ionization Energy, continued

Atomic Radius the size of an atom from its center to the outer edge of its cloud Increases as You Move Down a Group as you move down the table you are adding additional levels, this increases the size of the cloud

–Atomic Radius Decreases as You Move Across a Period from left to right across the periodic table, you are adding one proton in the nucleus and one electron in the same outer level this additional positive charge allows the nucleus to pull harder on the electrons and decrease the size of the cloud Atomic Radius, continue

Electronegativity –atoms often bond together to form compounds, these bonds sometimes involve the sharing of electrons this sharing is not always equal, one atom sometimes pulls the electrons harder than the other –numerical values called electronegativities reflect how a bonded atom attracts the shared electrons the atom with the highest electronegativity value will pull on the electrons harder than the other atom Fluorine is the element with the highest value and it is 4

–Decreases as You Move Down a Group as you move down the table you are adding additional levels, this makes the distance and between the outer electrons and the nucleus more and increases the shielding affect these two things account for the less pull from the nucleus Electronegativity, continued

–Increases as You Move Across a Period from left to right across the periodic table, you are adding one proton in the nucleus and one electron in the same outer level this additional positive charge allows the nucleus to pull harder on the electrons as the shielding affect stays the same; therefore the nucleus can pull harder on the outer electrons Electronegativity, continued

–Electron Affinity relates to a non-bonded atoms attraction for electrons the energy that occurs when a neutral atom gains an electron decreases as you move down the table for the same reasons of electronegativity increases as you move across the table for the same reasons of electronegativity