History of the Periodic Table L.O: To know what some early versions of the periodic table were, and to understand how the periodic table is arranged now Friday, 16 November 2018

Starter Look at the periodic table you’ve been given, and highlight any patterns you see. Think about the number of protons, the properties of the elements, and where they are in the table.

Early Versions of the Periodic Table In the early 1800s, scientists had no idea of atomic structure or of protons or electrons, so there was no such thing as atomic number to them. Back then, the only thing they could measure was relative atomic mass. As a result, the known elements were originally arranged in order of atomic mass. Q: What is relative atomic mass?

John Newlands Born: 26 November 1837, London, England Died: 29 July 1898 (aged 60) London, England Nationality: British Fields: Analytical chemistry Notable awards: Davy Medal (1887)

Law of Octaves As you move notes on a piano, the sound changes in frequency slightly. When you get to the eighth note, it sounds very similar to the first. This is similar to the way in which properties of elements change every time you go up by 8 elements (in order of relative atomic mass). Click on the image to try it out!

Newlands’ Law of Octaves Newlands had the first good stab at arranging the elements like this in 1864. After he arranged them in order of relative atomic mass, he noticed a pattern in the properties of the elements — every eighth element had similar properties. These sets of eight elements were called Newlands’ Octaves.

The 1st Periodic Table Newlands listed his Octaves in rows of seven. This meant that the elements were in a table, arranged so that elements with similar properties were in columns, known as groups. Because similar properties occurred periodically (at regular intervals) the table was called the periodic table.

Presenting his Ideas Newlands presented his ideas to the Chemical Society in 1865. But his work was criticised for a number of reasons: Some of his groups contained elements that didn’t have similar properties, e.g. carbon and titanium were in the same group but they don’t have similar properties. He mixed up metals and non-metals, e.g. oxygen (a non-metal) and iron (a metal) were in the same group. He didn’t leave any gaps for elements that hadn’t been discovered yet, so any new elements that were discovered would not fit into Newlands’ system.

Dmitri Mendeleev Born: 8 February 1834, Tobolsk Governorate, Russian Empire Died: 2 February 1907 (aged 72), Saint Petersburg, Russian Empire Nationality: Russian Fields: Chemistry, physics and adjacent fields Notable awards: Davy Medal (1882)

Mendeleev’s Periodic Table In 1869 in Russia, Dmitri Mendeleev arranged about 60 known elements into his Table of Elements. Mendeleev put the elements in order of atomic mass (like Newlands). But Mendeleev found he had to leave gaps in order to keep elements with similar properties in the same vertical columns (groups) — and he was prepared to leave some very big gaps in the first two rows before the transition metals come in on the third row. The gaps were the really clever bit because they predicted the properties of so far undiscovered elements. When they were found and they fitted the pattern it proved that Mendeleev’s system of classifying elements was a good one.

Modern Periodic Table It was only in the 20th century after protons and electrons were discovered that it was realised the elements were best arranged in order of atomic number. Then, all the elements were put into groups to create the modern periodic table…

Importance of Periodic Table When the periodic table was first released, many scientists thought it was just a bit of fun. At that time, there wasn’t all that much evidence to suggest that the elements really did fit together in that way — ideas don’t get the scientific stamp of approval without evidence.

Mendeleev’s Evidence After Mendeleev released his work, newly discovered elements fitted into the gaps he left. This was convincing evidence in favour of the periodic table. Once there was more evidence, many more scientists realised that the periodic table could be a useful tool for predicting properties of elements. It really worked. Sc Ga Ge Tc Hf Re

Modern Periodic Table In the late 19th century, scientists discovered protons, neutrons and electrons. The periodic table matches up very well to what’s been discovered about the structure of the atom. Scientists now accept that it’s a very important and useful summary of the structure of atoms.

Questions What did Newlands and Mendeleev arrange the elements in order of? How did Newlands arrange the elements into a table? Give two reasons why Newlands’ version of the periodic table was heavily criticised. Why did Mendeleev leave gaps in his periodic table? Give one piece of evidence that strongly suggested Mendeleev’s system of classifying elements was correct. How are elements arranged in the modern periodic table? Suggest why many scientists didn’t initially take the periodic table seriously. What happened to make scientists believe that the periodic table is a useful summary of atomic structure?

Answers Their relative atomic masses. He noticed that every eighth element has similar properties and arranged the elements in rows of seven so that elements with similar properties were aligned in columns/groups. Any two from: e.g. some of his groups contained elements that didn’t have similar properties. / Some of his groups contained a mixture of metals and non‑metals. / He didn’t leave any gaps in the table for elements that hadn’t been discovered yet. To leave space for undiscovered elements.

Answers The gaps in Mendeleev’s table predicted the properties of elements not discovered at the time. The gaps were filled when new elements were discovered. According to atomic number/proton number. E.g. there wasn’t much evidence to suggest that elements really did fit together in that way. Protons, neutrons and electrons were discovered and the periodic table matched up very well to what was discovered about the structure of the atom.

Homework Use the comprehension given to you to create a timeline of the periodic table throughout time. Due in: Next lesson