Periodic Table Ernst Homework 1

All the different elements are arranged in a chart called the periodic table. A Russian scientist called Dmitri Mendeleev produced one of the first practical periodic tables in the 19th century. The modern periodic table is based closely on the ideas he used:

  • the elements are arranged in order of increasing atomic number
  • the horizontal rows are called periods
  • the vertical columns are called groups
  • elements in the same group are similar to each other

The main groups are numbered from 1 to 7 going from left to right, and the last group on the right is group 0. The section in the middle of the table is called the Transition Metals. You may also see all the groups numbered (including the transition metals), this time from 1 to 18. If you know what one of the elements in a group is like, you can make predictions about the other elements in a group. For example, all the elements in group 1 are reactive metals, and all the elements in group 0 are unreactive non-metals.

The zig-zag line in this diagram separates the metals, on the left, from non-metals, on the right. Hydrogen is a non-metal but it is often put in the middle.

Notice that most elements are metals, rather than non-metals.

Each element has its own chemical symbol, made from letters. Remember that you will only find elements in the periodic table and never compounds. So you won’t find substances like water or copper sulfate in the periodic table.

Making predictions using the periodic table

Groups in the periodic table contain elements with similar chemical properties. But there are usually trends in properties that allow us to make predictions. For example, in group 1:

Melting pointDensityReactivity
LithiumDecreases down the groupIncreases down the groupIncreases down the group
SodiumDecreases down the groupIncreases down the groupIncreases down the group
PotassiumDecreases down the groupIncreases down the groupIncreases down the group
RubidiumDecreases down the groupIncreases down the groupIncreases down the group

Caesium is the next element in group 1, and it can be found below rubidium. You can accurately predict that it will have the lowest melting point, the highest density and the highest reactivity of all the elements in group 1.


Periodic Table Key

X
Liquids or melt at close to room temp.
Lanthanides
& Actinides


"If all the elements are arranged in the order of their atomic weights, a periodic repetition of properties is obtained. This is expressed by the law of periodicity."
Dmitri Mendeleev, Principles of Chemistry, Vol. 2, 1902, P. F. Collier, p17.

"We have here a proof that there is in the atom a fundamental quantity, which increases by regular steps as one passes from one element to the next. This quantity can only be the charge on the central positive nucleus, of the existence of which we already have definite proof."
Henry Moseley, Philosophical Magazine, Vol. 26, 1913, p1030.

"The chemistry of an atom depends only on the number of electrons, which equals the number of protons and is called the atomic number. Chemistry is simply numbers, an idea Pythagoras would have liked. If you are an atom with one proton, you are hydrogen; two, helium;....."
Carl Sagan, Cosmos, 1980, Random House, p223. Photo: NASA.

The Periodic Table

The periodic table we use today is based on the one devised and published by Dmitri Mendeleev in 1869.

Mendeleev found he could arrange the 65 elements then known in a grid or table so that each element had:

1. A higher atomic weight than the one on its left. For example, magnesium (atomic weight 24.3) is placed to the right of sodium (atomic weight 23.0):

2. Similar chemical properties to other elements in the same column - in other words similar chemical reactions. Magnesium, for example, is placed in the alkali earths' column, with other elements whose reactions are similar:

Mendeleev realized that the table in front of him lay at the very heart of chemistry. And more than that, Mendeleev saw that his table was incomplete - there were spaces where elements should be, but no-one had discovered them.

Just as Adams and Le Verrier could be said to have discovered the planet Neptune on paper, Mendeleev could be said to have discovered germanium on paper. He called this new element eka-silicon, after observing a gap in the periodic table between silicon and tin:

Similarly, Mendeleev discovered gallium (eka-aluminum) and scandium (eka-boron) on paper, because he predicted their existence and their properties before their actual discoveries.


Although Mendeleev had made a crucial breakthrough, he made little further progress. With the benefit of hindsight, we know that Mendeleev's periodic table was underpinned by false reasoning. Mendeleev believed, incorrectly, that chemical properties were determined by atomic weight. Of course, this was perfectly reasonable when we consider scientific knowledge in 1869.

In 1869 the electron itself had not been discovered - that didn't happen for another 27 years.

In fact, it took all of 44 years for the correct explanation of the regular patterns in Mendeleev's periodic table to be found...

Read More...The Periodic Table continued

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