Understand the Development of the Periodic Table

In this worksheet, students will explore the structure of the Periodic Table and learn how it was developed.

QUESTION 1 of 10

Everything (even us) is made of atoms. At first, people thought that every different thing was made of a different type of atom. Later on, they realised that most things are a few types of atoms joined together in different ways. By about 1800, scientists had discovered about 40 elements (or different types of atom), and knew some of their properties. Then scientists started looking for patterns because that's what scientists do.

The first scientist to get anywhere with this problem was Johann Wolfgang Döbernier. In 1829, he noticed that the known elements mostly fitted into groups of three. The groups of three had similar properties, and each group of three had a light element, a heavy element and one with middleweight. Döbernier called his groups of three Triads (tri means three- like in triangle). For example, lithium, sodium and potassium were a triad of metals which reacted violently with water.

The next scientist in this story was John Newlands. In 1865, he wrote about a Law of Octaves (oct means eight- like in octopus). He took the elements known at the time and ordered them from lightest to heaviest. Some of the time, similar elements appeared every eighth position. This picture shows how that worked; the elements are sorted into order from light to heavy, and similar elements have been highlighted in bright colours.

Then came the key person in this story- Russian scientist Dimitri Mendeleev. He had two ideas which might seem obvious now but were revolutionary at the time.

First, there might be elements which exist but haven't been discovered yet. So leave gaps if it helps make the patterns work better.

Second, if the patterns (like the octaves) work better if pairs of elements are swapped around, then swap them.

The really impressive thing about Mendeleev's idea for the Periodic Table was that the gaps in the table were predictions of new elements to discover. The patterns in the table helped Mendeleev to predict what these elements would be like. After the Periodic Table was published, scientists looked for the missing elements. They all appeared and had the properties which Mendeleev had predicted.

In 1913 a British chemist, Henry Moseley, realised that it would be better to use an atomic number rather than mass, so he put the final touch on the Periodic Table. This resolved some issues that surfaced from Mendeleev's work.

The Periodic Table, as we know it today, is shown below. The different colours show elements with similar characteristics. They sit in columns; these are the triads of Döbernier (except the groups have more than three in them now), and repeat as Newland's octaves (for the first few rows, anyway). The crucial ideas to make the Periodic Table work came from Mendeleev.

Who was the key scientist who had the idea for the Periodic Table?

Sir Isaac Newton

James Joule

Dmitri Mendeleev

John Newlands

What was special about Mendeleev​'s periodic table?

It contained all of the atoms

He left gaps for undiscovered elements

He put atoms with similar properties together

He put the atoms in order of atomic weight.

Why do we now know that Mendeleev's periodic table is the best way of grouping the elements?

We can see the patterns in the electron structures.

He arranged the elements in no particular way

It dons't work for all of the elements.

What did Moseley change in Mendeleev's periodic table which improved it?

He rearranged it in order of atomic mass

He rearranged it according to elements' properties

He rearranged the elements in order of atomic number and properties

Match these ways of sorting elements with the ways they worked.

Column B

Find groups of three elements with similar propert...
Newland's Octaves
Sorting elements by mass, leaving gaps so similar ...
Mendeleev's Periodic Table
Every eighth element (by mass) has similar propert...

One of Döbereiner's Triads was calcium, strontium and barium. The atomic mass of calcium is 40, and the atomic mass of barium is 137. What is the average of these two numbers?

The atomic mass of iodine is 127, and the atomic mass of tellurium is 128. Why did Mendeleev put tellurium before iodine in his Periodic Table?

Mendeleev just wanted to swap them.

Swapping the elements made the groups work better.

The atomic number of iodine is higher than for tellurium.

Mendeleev put the elements in alphabetical order.

Tellurium has mass number 128, and atomic number 52.

Iodine has mass number 127, and atomic number 53.

Which of these statements about tellurium and iodine are true? Tick all the ones which are true.

Tellurium has 76 neutrons

Tellurium has 74 neutrons

Tellurium has 72 neutrons

Iodine has 74 neutrons

Iodine has 76 neutrons

Look at the periodic table in the introduction (or another one if you have it). Sort these elements into pairs in the same column. Chemists call these columns "groups".

Column B

Na (sodium)
Ge (germanium)
Si (silicon)
Br (bromine)
Cl (chlorine)
Ar (argon)
He (helium)
Rb (rubidium)

Look at these elements and their electronic structures. Which of them are in the same column of the periodic table as breyllium (Be) [2.2]?

calcium (Ca) [2.8.8.2]

chlorine (Cl) [2.8.7]

magnesium (Mg) [2.8.2]

silicon (Si) [2.8.4]

strontium (Sr) [2.8.18.8.2]

• Question 1

Who was the key scientist who had the idea for the Periodic Table?

Dmitri Mendeleev
EDDIE SAYS
Dmitri Mendeleev created the first edition of the Periodic Table. He now has an element named after him (Mendelevium)
• Question 2

What was special about Mendeleev​'s periodic table?

He left gaps for undiscovered elements
EDDIE SAYS
Mendeleev realised that we were working with an incomplete list of the elements and so he left gaps in his table. he described 'eca-aluminium' 20 years before it was discovered and re-named galium.
• Question 3

Why do we now know that Mendeleev's periodic table is the best way of grouping the elements?

We can see the patterns in the electron structures.
EDDIE SAYS
Even after his death, we now know that he must have been correct as when we look at the atoms we see that there is patterns in the electron structure as well as their properties.
• Question 4

What did Moseley change in Mendeleev's periodic table which improved it?

He rearranged the elements in order of atomic number and properties
EDDIE SAYS
In 1913 a British chemist, Henry Moseley, realised that it would be better to use atomic number rather than mass, so he put the final touch on the Periodic Table, after resolving some issues that surfaced from Mendeleev's work.
• Question 5

Match these ways of sorting elements with the ways they worked.

Column B

Find groups of three elements wit...
Newland's Octaves
Every eighth element (by mass) ha...
Mendeleev's Periodic Table
Sorting elements by mass, leaving...
EDDIE SAYS
You can see traces of Döbereiner's and Newland's ideas in Mendeleev's Periodic Table. They weren't totally wrong, but they missed some key ideas to make the ordering of elements work.
• Question 6

One of Döbereiner's Triads was calcium, strontium and barium. The atomic mass of calcium is 40, and the atomic mass of barium is 137. What is the average of these two numbers?

88.5
89
EDDIE SAYS
To work out the average, you do ½ (40 + 137). (The actual average is 88.5, but if you round this to the nearest whole number you get 89). The actual atomic mass of strontium is 88, so the mass of the middle element is the average of the heavy and light element in the triad.
• Question 7

The atomic mass of iodine is 127, and the atomic mass of tellurium is 128. Why did Mendeleev put tellurium before iodine in his Periodic Table?

Swapping the elements made the groups work better.
EDDIE SAYS
The idea of atomic numbers came much later than Mendeleev. All he had to work with was atomic masses and patterns of behaviour. The clever thing Mendeleev did was to concentrate on the patterns, and do what he needed to make them work as well as possible.
• Question 8

Tellurium has mass number 128, and atomic number 52.

Iodine has mass number 127, and atomic number 53.

Which of these statements about tellurium and iodine are true? Tick all the ones which are true.

Tellurium has 76 neutrons
Iodine has 74 neutrons
EDDIE SAYS
Because we know about protons and neutrons now, it makes much more sense to sort elements by atomic number than mass number. We\'re lucky to have that information, but Mendeleev didn\'t know about them, because they hadn\'t been discovered.
• Question 9

Look at the periodic table in the introduction (or another one if you have it). Sort these elements into pairs in the same column. Chemists call these columns "groups".

Column B

Na (sodium)
Rb (rubidium)
Si (silicon)
Ge (germanium)
Cl (chlorine)
Br (bromine)
He (helium)
Ar (argon)
EDDIE SAYS
There are some activities where you will learn about these groups in more detail- how they behave and why. For example, He and Ar are very unreactive gases, called noble gases.
• Question 10

Look at these elements and their electronic structures. Which of them are in the same column of the periodic table as breyllium (Be) [2.2]?

calcium (Ca) [2.8.8.2]
magnesium (Mg) [2.8.2]
strontium (Sr) [2.8.18.8.2]
EDDIE SAYS
Notice that all the elements in the same column as beryllium have got 2 electrons in their outermost shell. The same thing happens for other groups of the Periodic Table. Mendeleev\'s Periodic Table and Bohr\'s model of electron shells help explain each other.
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