The Periodic Table and Periodic Law



The Periodic Table and Periodic Law

Periodic Table History:

In the late 1790’s Antoine Lavoisier compiled a list of 23 known elements of his time. They included silver, carbon, gold which had been known for thousands of years.

(We often laugh at the alchemists pursuit of turning lead into gold but during the Middle Ages but useful information was discovered about the elements and what reactions they could make.) The information gathered allowed Lavoisier and others to compile knowledge about the elements. With the use of the invention electricity however, there were tremendous advances in the number of known elements.

Scientists were able to melt compounds then run electricity through them and break them down to their constituent elements. Imagine melting a compound like table salt. Sodium chloride is a white crystalline solid before melting. When melted it is a clear liquid and then when electricity is added clouds of yellow chlorine gas and a silver metal are produced. (Remember they often had labs in their homes and I’m sure everyone was thrilled with the stinky chemical reactions they were producing. Chlorine is a toxic gas and sodium will react violently with water so I’m sure the discoveries were very exciting and shared with their fellow researchers.)

The industrial revolution was occurring at the same time. Chemical industries such as petrochemicals, soaps, dyes, and fertilizers also drove the demand for new chemical knowledge. By the mid 1800’s there were 70 known elements. Industries demanded consistent information about elements and chemical processes. Because of this demand chemists agreed on a method to accurately determine the masses of elements. A consistent mass allowed chemists to reproduce each others experiments and confirm or deny discoveries and new processes.

Scientists needed a way to organize the tremendous information being generated about the elements. Melting point, mass, chemical reactivity was known in a piecemeal trivial fashion. Dobereiner came up with the idea of “triads” which were groups of 3 elements with similar properties. John Newlands proposed the “law of octaves” because he noticed a repetition of properties for every eighth element. This was known as a periodic pattern because it repeated in a specific pattern. Unfortunately when the idea was presented it didn’t fit all of the known elements and also many scientists were insulted by the musical analogy feeling it was “unscientific”. Newlands organization was on the right path but Mendeleev improved on his organization and his ideas were accepted by the scientific community.

Mendeleev organized his elements in rows and columns and based their position on the known properties of the elements. His table forced the recalculation of elemental tin’s mass because it didn’t fit the table the way it should according to his organizational method. Mendeleev arranged his table in order of increasing atomic mass. Part of the reason his table was accepted so quickly was the blank spots he left in the table along with the predictions of what the physical properties would be. The elements gallium,

germanium, and scandium were quickly discovered once scientists knew what they were looking for. This led to a widespread acceptance of his organization of the elements.

A problem recognized in Mendeleev’s table was that elements masses don’t always increase as you move across a period. Mendeleev had demanded tin’s mass be recalculated and was found to be correct unfortunately cobalt & nickel’s masses appeared to be reversed and the masses were found to be correct. Because of the tremendous usefulness of the table it was accepted and over time electrons, protons, and neutrons were discovered which explained why the masses didn’t always increase the protons did.

The problems of the periodic tables masses were resolved by Henry Moseley. He placed the elements in rows and columns by their properties as Mendeleev did but he organized them in order of increasing atomic number not increasing atomic mass. This resolved all of the problems in Mendeleev’s table. Moseley’s arrangement led to a clear periodic pattern of properties. The periodic law is a statement of the periodic repetition of chemical and physical properties when they are arranged in order of increasing atomic number.

The Modern Periodic Table:

The modern periodic table is arranged in rows and columns in order of increasing atomic number. It is subdivided into the representative elements, the transition elements, and the inner transition elements. (There are many versions of the periodic table currently being used in the world. The periodic table you made for your project began the f block with lanthanum and actinium where the Glencoe book begins them with cerium and thorium.) The representative elements are the group A elements. Their group number is equal to their valence electrons. The transition elements are the group B elements. Their elements can have multiple charges because the d electrons often join the s electrons to participate in bonding so that the electron sublevel will be either half – full or completely filled. Remember the two iron ions Fe 2 + and Fe 3 + . Iron (II) is formed when the noble gas configuration for iron is [Ar] 3d6. Iron III is formed when the noble gas configuration for iron is [Ar] 3d5. 3d5 is a more stable arrangement for the electron cloud. The f block is made up of the inner transition elements and they contain the lanthanide and actinide series. The inner transition elements also have many charges for the same reasons as the transition elements.

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A group or family is a vertical column on the periodic table. The representative elements (A groups) all have family names. They are:

Group I A Alkali metals Li, Na, K, Rb, Cs, & Fr

Group IIA Alkaline Earth metals Be, Mg, Ca, Sr, Ba, & Ra

Group IIIA Boron Family (often called Al family) B, Al, Ga, In, Tl

Group IV A Carbon Family C, Si, Ge, Sn, Pb, Uuq

Group V A Nitrogen Family N, P, As, Sb, Bi

Group VI A Oxygen Family O, S, Se, Te, Po, Uuh

Group VIIA Halogen Family F, Cl, Br, I, At

Group VIIIA Noble Gas He, Ne, Ar, Kr, Xe, Rn, Uuo

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Periods are horizontal rows on the periodic table. There are 7 periods and

they correspond to the 7 energy levels in the electron cloud. Periods stretch from the metals to the metalloids to the nonmetals to the noble gases. As you move from left to right the chemical behavior changes drastically from one side of the table to the other. (Think about metals …what do they want to do with their valence electrons…nonmetals ?)

The periodic table is divided into 3 major categories: metals, nonmetals, and metalloids. (See figure below) Metals are blue, metalloids are pink, and nonmetals are yellow. Remember metalloids are the elements that touch the stair step activity line on either side with the exception of aluminum (aluminum has no nonmetallic properties.)

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Metals make up the majority of the periodic table. Metals lose electrons when they form ions to gain a noble gas or filled sublevel electron configuration. Metals are solid at room temperature (except mercury), shiny (luster), malleable, ductile, good conductors of heat and electricity. Malleable refers to the ability of metals to be hammered without breaking. Ductile refers to metals ability to be drawn into wire. The larger the metal the more reactive it is in a family.

Nonmetals are found to the right of the activity line. Metals gain electrons when they form ions to gain a noble gas or filled sublevel electron configuration. Nonmetals are not shiny, malleable, or ductile. They are poor conductors of heat and electricity.

They are generally gases or brittle, dull looking solids at room temperature. (Remember bromine is a liquid at room temperature.)

Metalloids are found on either side of the activity line. Except for aluminum the elements that are metalloids contain both metallic and nonmetallic properties.

The periodic table is arranged in 4 blocks s, p, d, & f.

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There are 4 ways to write electron configurations: electron filling diagrams, electron configurations, noble gas configurations, ending configurations.

Ex. Phosphorus has 15 electrons

Electron filling diagram Ε Ε ΕΕΕ Ε 555

1s 2s 2p 3s 3p

(Remember Hund’s rule says that every orbital in a sublevel must contain 1 electron before any can contain 2 electrons. There are 3 orbitals in the p sublevel.)

Electron configuration: 1s2 2s2 2p6 3s2 3p3

Noble gas configurations: [Ne] 3s2 3p3

Ending configurations: 3p3

The bonding electrons are the electrons that matter in a chemical reaction. They allow chemists to predict the behavior of an element in a chemical reaction. A new way of showing valence electrons are called Lewis dot diagrams and they were developed by

Gilbert Lewis. He was searching for a way to show the electrons that participate in bonding. The valence electrons are found in the s & p sublevels of the elements.

To draw Lewis dot diagrams first write the element symbol and place the electrons (dots) one at a time around the symbol. Because electrons hybridize the s orbital is treated no differently than the p orbitals.

Ex. Carbon has 4 valence electrons

Calcium has 2 valence electrons

Oxygen has 6 valence electrons

* **

* C * * Ca * * O **

* *

Electrons repel each other so when they are unpaired they get as far apart as possible. If there are 3 electron or more start at the top and go around until you run out of electrons

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