Presentation on theme: "Periodic Table History"— Presentation transcript:
1 Periodic Table History The periodic table as we have it today has not always existed; it developed much in the same way as atomic theory did. In the early 1800’s scientists began looking for ways to classify the elements that had been discovered.
2 A German chemist, Johann Dobereiner, found, that the properties of three metals (Ca, Ba, Sr) were very similar. He also recognized that the atomic mass of Sr was about midway between that of Ca and Ba. He grouped these three elements together and called them a triad.**Two other triads he found included Cl, Br, and I as well as S, Se, and Te.
3 John Newlands saw that if he put the elements in rows of seven that there would be a repeating pattern every eighth element. This is often referred to as the Law of Octaves.
4 In 1860, Stanislao Cannizaro described a method for accurately determining the atomic weights of elements. This gave scientists a set of standardized measurements to use in all countries. Based on Cannizaro’s work other chemists began working on other ways to organize the elements.
5 Dmitri Mendeleev, a Russian chemist in the mid 1800’s, was looking for a way to organize all known elements. By organizing the elements by density, reactivity, and melting point, he came up with a periodic table that put the elements in order of increasing atomic mass.There were gaps (undiscovered elements) in Mendeleev’s table. For each of these gaps, Mendeleev predicted the properties of the missing elements.
6 In the early 1900’s, English scientist, Henry Moseley found a new pattern in Mendeleev’s table. This pattern showed that the nuclei of each element in the table increased by one positive charge (proton). This observation plus the discoveries of new elements to fill in Mendeleev’s gaps led to the modern periodic table being organized by increasing atomic number.
7 From both Mendeleev and Moseley’s work, the periodic law was developed From both Mendeleev and Moseley’s work, the periodic law was developed. The periodic law states that the chemical and physical properties of the elements are periodic functions of their atomic number.
8 Horizontal rows are called periods Horizontal rows are called periods. Vertical columns are called groups or families.
9 Family Names Group 1 or IA= Alkali Metals (does not include hydrogen) Group 2 or IIA = Alkaline Earth Metals.Group 13 or IIIA = Boron familyGroup 14 or IVA = Carbon familyGroup 15 or VA = Nitrogen family.
10 Family Names continued: Group 16 or VIA = Calcogens.Group 17 or VIIA = Halogens.Group 18 or VIIIA = Noble Gases - once known as the Inert Gases.
11 The General LayoutThe middle section of the periodic table contains the Transition Metals (d-block)The two rows below the main part of the table are the Inner Transition Metals (f-block)
12 General Layout Continued: The metals are found on the left side of the periodic table.The non-metals are found on the right side of the table.The metalloids are found above and below the stair-step line started between boron and aluminum.
13 Where are the phases?The most solid elements are located to the left and below the metalloid line.Two elements are liquid at room temperature: mercury and bromine.Several elements are gases at room temperature: hydrogen, helium, nitrogen, oxygen, fluorine, neon, chlorine, argon, krypton, xenon, and radon.
15 Nonmetals Solids lack luster Many gases at room temperature Poor conductor of electricityLower melting pointsTightly held valence electronsNonmetal Activity Increases
16 Energy Level TrendEnergy levels – Increases down a group, does not change across a periodIn other words, there are more layers of electrons as move down the table (think about an onion)Remember – energy levels are regions of space in which electrons can move around the atom’s nucleus
17 Atomic Radius Trend What is atomic radius? It is one half the distance between 2 adjacent atoms of the same element.It increases as you go down a group.It decreases as you go from left to right in a period.
18 Ionic radiusCation- a positively charged ion because it has LOST electrons. The radius always decreases with a lose of electronsMetals tend to form cations!Anion – a negatively charged ion because it has GAINED electrons. The radius always increases with a gain of electrons.Nonmetals tend to form aniona!
19 Electronegativity Trend What is electronegativity?The measure of the ability of an atom in a chemical compound to attract electronsDoes NOT include the Noble Gases!!Decreases as you move down a group.Increases from left to right across a period.
20 Ionization Energy Trend What is Ionization Energy?The amount of energy necessary to remove an electron from a gaseous atom.Decreases as you move down a group.Increases as you move from left to right across a period.