The Periodic Table and Its Development

The periodic table lists every known element in order of increasing atomic number — the number of protons in each atom. Reading left to right along each row, the atomic number goes up by one at every step: hydrogen is 1, helium 2, lithium 3, and so on.

Arranging elements this way does something powerful. Elements with similar chemical properties end up in the same vertical column. This repeating pattern of properties is why the table is called periodic: similar properties recur at regular intervals as you move through the elements.

Term	Meaning
Atomic number	Number of protons; sets the order of elements
Group	Vertical column of the table
Period	Horizontal row of the table

A student is always supplied with a periodic table in the exam, so the skill being tested is reading position and relating it to structure and reactivity, not memorising the whole table.

The columns and rows of the table carry specific meaning, and the two are often confused.

A group is a vertical column. The group number tells you how many electrons are in the outer shell of each atom in that group. Group 1 elements have one outer electron; Group 7 elements have seven; Group 0 (the noble gases) have a full outer shell. Because chemical reactions involve the outer electrons, elements in the same group react in similar ways.

A period is a horizontal row. Moving across a period, each element has one more proton and one more electron than the last, and the outer shell gradually fills up. The period number tells you how many occupied electron shells the atoms have.

Group 1 element	Electronic structure	Outer electrons
Lithium (Li)	2, 1	1
Sodium (Na)	2, 8, 1	1
Potassium (K)	2, 8, 8, 1	1

Each of these sits in Group 1 because each has exactly one outer electron, which is the reason they share the reactions of the alkali metals.

Because the group number equals the number of outer-shell electrons, position in the table lets you predict how an element behaves before you have seen any data on it.

An atom is most stable when its outer shell is full. Elements react to gain, lose or share electrons until they reach that stable arrangement. A Group 1 atom has one outer electron to lose, so it forms a 1+ ion. A Group 7 atom needs one more electron to fill its outer shell, so it forms a 1− ion. The diagram below highlights Group 1, the reactive metals whose single outer electron drives their chemistry.

The group number gives the number of outer-shell electrons (except Group 0, which has a full outer shell). This single fact explains why a whole group reacts alike.

Knowing this, you can predict, for example, that any Group 1 metal reacts vigorously with water, or that reactivity changes in a steady trend down a group.

Before protons, neutrons and electrons were discovered, chemists had no atomic number to work with, so early attempts at a periodic table classified elements in order of their atomic weight.

This caused problems. Ordering strictly by weight sometimes placed elements in a group with different properties from the rest of the column. Many elements were still undiscovered, leaving the table incomplete, and forcing every known element into a fixed weight order meant some ended up in the wrong place. Early tables were therefore both incomplete and, in places, wrong.

The breakthrough was to stop treating atomic weight as an unbreakable rule and instead prioritise grouping elements by their properties. That shift is credited to Dmitri Mendeleev.

In 1869 the Russian chemist Dmitri Mendeleev produced a table that overcame the earlier problems in two clever ways.

First, he left gaps for elements he believed existed but had not yet been discovered, rather than forcing the known elements into a continuous sequence. He even used the gaps to predict the properties of the missing elements. When elements such as gallium and germanium were later found, their properties matched his predictions closely, which convinced other scientists his table was correct.

Second, he was willing to change the order of a few elements, placing a slightly heavier element before a lighter one where their properties demanded it, rather than following atomic weight blindly.

Mendeleev's two key moves: leaving gaps for undiscovered elements (and predicting their properties), and swapping a few pairs out of strict atomic-weight order to keep similar elements together.

His predictions being confirmed was the strongest evidence that the periodic arrangement reflected something real about the elements.

Mendeleev's occasional need to swap pairs of elements out of weight order looked like an untidy fix at the time. The discovery of isotopes later explained why it was necessary.

Isotopes are atoms of the same element with different numbers of neutrons, so the same element can contain atoms of different masses. The atomic weight shown for an element is an average across its isotopes. This means atomic weight does not always increase neatly from one element to the next: an element with heavier isotopes can appear before a lighter neighbour if ordering by weight alone. Ordering instead by atomic number (proton number), which is always a whole number that increases by one from element to element, removes these anomalies completely.

Ordering rule	Problem
By atomic weight	Averaged over isotopes; occasionally out of sequence, so some elements land in the wrong group
By atomic number	Always a whole number increasing by one; every element falls into the correct group

This is why the modern table is arranged by atomic number, and why Mendeleev's swaps turned out to be correct all along.

The periodic table splits into metals and non-metals, and the exam defines the difference in terms of ions, not just physical appearance.

An element that reacts to form positive ions is a metal. An element that does not easily form positive ions is a non-metal. Metals sit on the left and towards the bottom of the table, while non-metals sit towards the right and top. Most elements are metals.

This chemical definition links back to electron arrangement. Metals have few outer electrons and lose them easily to form positive ions; non-metals have nearly full outer shells and tend to gain or share electrons instead.

Property	Metals	Non-metals
Ion formed	Positive ions	Do not easily form positive ions
Position	Left and bottom	Right and top
Typical state at room temperature	Mostly solid	Mixed (gases, liquid, solids)
Conduct electricity	Yes	Mostly poor conductors

1. Confusing groups and periods

A group is a vertical column; a period is a horizontal row. Only group number tells you the number of outer-shell electrons. Mixing the two loses easy marks.

2. Saying the table is ordered by atomic weight

The modern table is ordered by atomic number (proton number). Atomic weight was the old rule and caused the anomalies that atomic number later fixed.

3. Forgetting Group 0's outer shell

The group number equals the outer electrons for Groups 1–7, but Group 0 elements have a full outer shell (eight electrons, or two for helium), not zero. Do not write "zero outer electrons".

4. Defining a metal by shininess alone

The mark-scheme definition is chemical: a metal reacts to form positive ions. Physical properties like shine and conductivity support the answer but are not the defining test.

5. Crediting Mendeleev only with "making a table"

Explain how he improved it: he left gaps for undiscovered elements and predicted their properties, and he changed the order of some pairs rather than following atomic weight strictly.

6. Saying isotopes were what Mendeleev used

Mendeleev worked before isotopes were known. Isotopes were discovered later and explained why ordering by atomic weight sometimes failed, justifying his swaps.