Here's the definition of the periodic table:

Fig. 1 - The periodic table

The periodic table is handy because it orders elements into rows and columns based on their properties. This means that once you know an element's position in the periodic table, you can predict how it behaves and reacts. Let's start by looking at its structure.

How is the periodic table structured?

First and foremost, the periodic table consists of elements. These elements are assembled in rows, columns, and blocks. It hasn't always been like this, but we'll get into how it has changed later, when we discuss the history of the periodic table. As of now, we'll delve into the ways the periodic table is structured.

Elements in the periodic table

As we defined above, the periodic table is an arrangement of elements. Remember that an element is a pure chemical substance consisting of atoms that all have the same number of protons in their nuclei. Its exact number of protons determines its number of electrons; this is what makes an element, well, an element!

Elements in the periodic table are shown using their chemical symbol. This is a one or two-letter abbreviation, unique to each element. The first letter is always capitalised and the second letter is in lowercase. For example, copper is known as Cu, calcium as Ca, and carbon as C. Elements are also shown with their atomic number and relative atomic mass.

Fig. 2 - An element

Periods in the periodic table

One of the ways elements are assembled in the periodic table is in rows. A row in the periodic table is known as a period; this is where the word periodic comes from. Rows in the periodic table show periodicity.

Elements in a period are ordered by increasing atomic number. Remember that atomic number is the number of protons in an element. Atomic number increases by 1 each time you move from left to right across a period in the table. When you reach the end of a period, you move down and left to the start of a new one, and continue counting atomic numbers from there. In total there are 7 periods in the periodic table.

Fig. 3 - Periods in the periodic table

Fig. 4 - Atomic number increases as you go along a period

Periodic Table Groups

Fig. 5 - Groups in the periodic table

Elements are grouped according to the number of electrons they have in their outer shell. Outer shell electrons are also known as valence electrons. Valence electrons determine the chemical properties and reactivity of an element. This means that elements in the same group all react in a similar way. However, note that each element in a group has one more electron shell than the element above it.

The old numbering system is useful because it gives us an indication of how many valence electrons an element in a particular group has. For example, all of the elements in group I, the alkali metals, have one valence electron. In contrast, all of the elements in group VII, the halogens, have seven valence electrons. The noble gases, group VIII, all have eight valence electrons, giving them complete outer electron shells.

Blocks in the periodic table

You might have noticed that using the old system for numbering groups misses out two large chunks of the periodic table. What about the elements between groups II and III, found in the IUPAC groups 3 to 12? How about those two rows of elements below the periodic table - where do they fit in? Well, another way of looking at the periodic table is by splitting it up into blocks.

There are four blocks in the periodic table:

• s-block elements all have their highest energy valence electron in an s-subshell. The s-block includes groups I and II.
• p-block elements all have their highest energy valence electron in a p-subshell. The p-block includes groups III to VIII, or in IUPAC terms, 13 to 18, and is mostly made up of non-metals.
• d-block elements all have their highest energy valence electron in a d-subshell. The d-block includes groups 3 to 12 and also features the transition metals.
• f-block elements all have their highest energy valence electron in an f-subshell. The f-block includes the lanthanides and the actinides. These are the two rows shown below the main body of the periodic table. We've shown where they fit in relationship to the other elements in the periodic table below.

Fig. 6 - Blocks in the periodic table

Periodic Table Metals, non-metals, and metalloids

The final way of structuring the periodic table that we'll look at today involves splitting the table with a zigzagging line. It starts to the left of boron and meanders its way down and to the right, sneaking between silicon and germanium, then between arsenic and antimony and tellurium and polonium. Finally, it splits astatine from tennessine, before finishing off to the left of oganesson.

This line has various names: the metal-nonmetal line, the amphoteric line, the metalloid line, and the staircase. It divides the table into metals, non-metals, and metalloids.

• The elements to the left of the line are classified as metals.
• The elements to the right of the line (as well as hydrogen) are classified as non-metals.
• Some of the elements touching the line are classified as metalloids.

Fig. 7 - Metals, non-metals, and metalloids in the periodic table

Metals in the periodic table

Metals appear on the left-hand side of the periodic table. They have some characteristic properties.

• Metals typically lose electrons to form positive cations.
• They have high melting and boiling points.
• They have low electronegativity values.
• They have a shiny, lustrous appearance when freshly cut.
• They are malleable and ductile.
• They're good conductors of heat and electricity.

Non-metals in the periodic table

Non-metals are found on the right-hand side of the periodic table (with the exception of hydrogen, which is also a non-metal). As the name suggests, they're the opposites of metals. In fact, you can think of them as simply lacking in metallic characteristics.

• Non-metals typically gain electrons to form negative anions.
• They have high electronegativity values.
• They exhibit a range of melting and boiling points. Some, like silicon, have extremely high melting points whilst others, like oxygen, have low melting points.
• The solid non-metals are brittle.
• They're poor conductors of heat and electricity.

Metalloids in the periodic table

Metalloids are found in the middle of the periodic table. They straddle the dividing line that splits metals from non-metals, and their properties are halfway between the two.

• Typically, metalloids are shiny and lustrous when cut, but are brittle in nature.
• They have medium electronegativity values.
• They are average conductors of electricity.

We now know what the periodic table looks like and how it is structured. But how did it come to be this way?

History of the periodic table

Johann Wolfgang Döbereiner

In 1817, German physicist Johann Wolfgang Döbereiner was the first to attempt to classify the elements. He noticed that you could put certain elements in groups of three, called triads, and that the elements within a triad shared similar properties. In fact, the properties of the second element in the triad fell halfway between those of the first and those of the third. For example, he grouped together lithium, sodium, and potassium, all metals that we now know as being in Group I.

John Newlands

In 1864, British chemist John Newlands also noticed the similarities in properties between certain elements. He saw that if you arranged all of the elements in a table by atomic mass, their properties were repeated at regular intervals. These properties repeated every eight elements, leading to the name 'the law of octaves'.

However, at the time, only 60 or so elements had been discovered. Newlands wrongly assumed that those were the only elements that existed. He didn't leave gaps for any undiscovered elements, so his table didn't really make sense after calcium. He also sometimes put two elements in the same box. In general, his ideas were ridiculed by his peers.

Fig. 8 - Newlands's version of the periodic table

Dmitri Mendeleev

Finally, in 1869, Russian chemist Dmitri Mendeleev arrived at the version of the periodic table that we know today. He took Newlands' idea about the law of octaves but left gaps for undiscovered elements, predicting their properties from the behaviour of the elements around them. Although he mostly arranged the elements in order of atomic mass, he switched a few of them around to better fit with his law of octaves.

For example, argon was much heavier than potassium, but putting potassium before argon would mean the highly reactive metal potassium would be in a group with unreactive non-metal gases, whilst the unreactive argon would be in a group with reactive metals. His table was gradually accepted when new elements were discovered that matched his predicted properties. The issue of atomic mass was solved when subatomic particles were discovered in the 20th century and scientists realised that elements should really be ordered by atomic number, and not mass.

Fig. 9 - Mendeleev's version of the periodic table, with gaps left for undiscovered elements