Chemistry History of the Periodic Table

ITS Chemistry During the nineteenth century, chemists began to categorize the elements according to similarities in their physical and chemical properties. The end result of these studies was our modern periodic table.

ITS Chemistry Döbereiners’ Triads Johann Döbereiner (c. 1829) described the idea of the triad –Triads were groups of three elements that shared similar properties –He also noted that when the elements were arranged in order of their increasing atomic mass, the atomic mass of the middle element was approximately half way between that of the other two elements of the triad

ITS Chemistry Döbereiners’ Triads He listed three triads: –Ca, Sr, Ba –S, Se, Te –Cl, Br, I

ITS Chemistry Newlands Octaves John Newlands (c. 1864) arranged the existing elements by increasing atomic mass He noticed that properties reemerge after 8 elements He called this the law of octaves The Law of Octaves states that the properties of the elements seem to repeat in a periodic fashion after every 8.

ITS Chemistry Newlands Octaves

ITS Chemistry Newlands Octaves The pattern was perfect up to calcium then became less convincing as some metals appeared unlike the non-metals to their left. Note that Newlands did not always stick to a strict increase in number. He exchanged the positions of Zn and Y, presumably because he realised that Y resembled Bo (modern symbol B). On 1st March 1865, he described his ideas at a lecture at the Chemical Society. Many critised his work saying he that he might have equally well listed the elements alphabetically. Chemistry journals refused to publish his paper, supposedly because it was of a purely theoretical nature. Humiliated, Newlands went back to his work as chief chemist at a sugar factory.

Dmitri Mendeleev In 1869 he published a table of the elements organized by increasing atomic mass.

ITS Chemistry Mendeleev Russian teacher Dimitri Mendeleev (c. 1869) proposed the first true version of the periodic table Mendeleev’s table was not complete in that he left “holes” where he thought elements would later be discovered.

ITS Chemistry Mendeleev For example, Mendeleev proposed that there was an element with similar properties as silicon that should exist, though it had not been discovered –This element was called ekasilicon –Later experiments isolated ekasilicon. It is now known as Germanium today. –Properties of germanium were almost the same as predicted

ITS Chemistry

Moseley English scientist Henry Moseley (c. 1913) proposed the modern periodic table He arranged the periodic table in order of increasing atomic number as opposed to atomic mass Came up with periodic law –(def) the physical and chemical properties of the elements are periodic functions of their atomic numbers

ITS Chemistry Moseley The modern periodic law states: The chemical and physical properties of the elements are periodic functions of their atomic numbers. Put in other words, when the elements are arranged in order of increasing atomic number, there is a periodic repetition of their properties.

ITS Chemistry Moseley His research was halted when the British government sent him to serve as a foot soldier in WWI. He was killed in the fighting in Gallipoli by a sniper’s bullet, at the age of 28. Because of this loss, the British government later restricted its scientists to noncombatant duties during WWII.

ITS Chemistry Differences between Mendeleev's Periodic Table and the Modern Periodic Table Mendeleev’s Periodic TableModern Periodic Table Fewer Elements (some not yet discovered) More Elements Gaps presentNo Gaps Arranged in order of Atomic MassArranged in order of Atomic Number Group 0 Elements missingGroup 0 Elements included

ITS Chemistry Mendeleev’s periodic table versus the modern periodic table Mendeleev’s Periodic Table Modern Periodic Table

ITS Chemistry Our periodic table can be arranged into three general sections

The horizontal rows of the periodic table are called PERIODS.

The vertical columns of the periodic table are called GROUPS, or FAMILIES. The elements in any group of the periodic table have similar physical and chemical properties!

Alkali Metals

Alkaline Earth Metals

Transition Metals

Inner Transition Metals These elements are also called the rare-earth elements.

Halogens

Noble Gases