Periodic Table

Periodic Table

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The periodic table is like a jigsaw puzzle. Imagine you're trying to finish a puzzle before the clock runs out. You match each piece together until finally, you form a picture. But what if you were missing half the pieces, with no picture to help you? That's what chemists must have felt like when they were creating the periodic table.

In physical chemistry, the periodic table is a way of organising elements. It's structured by atomic number. We'll explore the history of the periodic table and how it's changed over time. By the end of this article, you'll know how the periodic table is structured, be able to compare rows, columns, and blocks, and describe its evolution.

The periodic table

The periodic table is a useful tool because it organises elements into rows and columns based on their properties. This organisation makes it possible to predict how elements behave and react simply by knowing their position in the table. Let's take a closer look at how the periodic table is structured.

Elements are the building blocks of the periodic table. Each element is a pure chemical substance made up of atoms with the same number of protons in their nuclei. This number of protons determines the element's number of electrons, which is what makes each element unique. Elements are represented in the periodic table by their chemical symbols, which are one or two-letter abbreviations unique to each element. For example, copper is represented as Cu, calcium as Ca, and carbon as C. The periodic table also shows each element's atomic number and relative atomic mass. Elements in the periodic table are grouped into rows, which are called periods. The word "periodic" comes from the fact that there are repeating trends, or periodicity, as you move across a period. Elements are arranged in a period in order of increasing atomic number, with atomic number increasing by one with each element. 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. There are seven periods in total in the periodic table.

Periods in the periodic table.
Atomic number increases as you go along a period
Atomic number increases as you go along a period

Elements in the same period have the same number of electron shells. For example, the elements in period 1 have just one electron shell, whereas the elements in period 5 all have five electron shells.

Periodic Table Groups

Elements in the periodic table are also assembled in columns. These columns are called groups. In total, there are 18 groups in the periodic table, with the official IUPAC numbering of 1-18. However, for the purpose of your course, we will be following the traditional numbering system which is 1-0 or 1-8. This is often shown in roman numerals. The groups are: Alkali metals (group I)Alkaline earth metals (group II)Boron group (group III)Carbon group (group IV)The pnictogens (group V)The chalcogens (group VI)The halogens (group VII)Noble gases (group VIII)

Groups in the periodic table

Elements in the periodic table are also grouped based on the number of electrons they have in their outer shell, which are called valence electrons. Valence electrons determine an element's chemical properties and reactivity. Elements in the same group have the same number of valence electrons and therefore 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 for groups is still useful because it gives 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. Group VII, the halogens, have seven valence electrons, while Group VIII, the noble gases, have eight valence electrons, giving them complete outer electron shells. Hydrogen, with an atomic number of 1, is the lightest element in existence and is included in group I, even though it doesn't behave like the other members of the group. The next lightest element, helium, is found in group VIII because its valence electron shell only has room for two electrons, giving it a full outer shell like other elements in that group. The number of groups in the periodic table is 18, as recommended by the International Union of Pure and Applied Chemistry (IUPAC) in 1988. This superseded the older numbering system, 1-0 or 1-8, which excluded the d- and f-block elements. The older system is still in use, especially in exams, and is important to know.


In addition to the old numbering system and the new numbering system, the periodic table can also be divided into blocks based on the subshell in which an element's highest energy valence electron is found. There are four blocks in the periodic table: s-block, p-block, d-block, and f-block.

The s-block elements, which include groups I and II, have their highest energy valence electron in an s-subshell. The p-block elements, which include groups III to VIII or 13 to 18, are mostly made up of non-metals and have their highest energy valence electron in a p-subshell. The d-block elements, including groups 3 to 12, feature the transition metals and have their highest energy valence electron in a d-subshell. Finally, the f-block elements, known as the lanthanides and actinides, have their highest energy valence electron in an f-subshell and are found in the two rows shown below the main body of the periodic table. It's worth noting that while helium is found in Group VIII and usually considered a noble gas, it is actually an s-block element because its outer shell only has room for two electrons, which are found in an s-subshell. All of the other elements in Group VIII have a p-subshell as well.

Blocks in the periodic table
Blocks in the periodic table

Periodic Table Metals, non-metals, and metalloids

In addition to the previous three methods of organizing the periodic table, it can also be divided by a zigzagging line that starts to the left of boron and meanders down and to the right, separating silicon and germanium, arsenic and antimony, tellurium and polonium, and astatine and tennessine before ending to the left of oganesson. This line has various names, including the metal-nonmetal line, amphoteric line, metalloid line, and staircase.

Elements on the left side of the line are classified as metals, while those on the right side (including hydrogen) are non-metals. Elements that touch the line, including boron, silicon, germanium, arsenic, antimony, tellurium, and polonium, are classified as metalloids. Metalloids have properties of both metals and non-metals, and their behavior can depend on conditions such as temperature and pressure. They are often used in semiconductors and electronic devices. By dividing the periodic table into metals, non-metals, and metalloids, this zigzagging line helps to group elements with similar chemical and physical properties.

Metals, non-metals, and metalloids in the periodic table


The modern periodic table was first proposed by Russian chemist Dmitri Mendeleev in 1869. He arranged the elements in order of increasing atomic mass and grouped them into columns based on their chemical and physical properties. This arrangement allowed him to predict the properties of elements that had yet to be discovered.

Since then, the periodic table has been refined and expanded to include more elements. In 1913, Henry Moseley used X-ray spectroscopy to determine the atomic number of elements, which allowed for more accurate placement of elements in the table.

In the 1950s, Glenn Seaborg and his colleagues proposed the actinide and lanthanide series, which are now known as the inner transition elements.

Today, the periodic table is used to classify and understand the properties of elements, and to predict the properties of new elements.

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 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.

Newland's version of the periodic table

Dmitri Mendeleev

Mendeleev's periodic table was a major breakthrough in the study of chemistry. He arranged the elements in order of increasing atomic mass and put them in rows and columns based on their chemical and physical properties. He also left gaps in the table for undiscovered elements and predicted their properties based on the behavior of the elements around them.

Mendeleev's table was not perfect, however, as some elements did not fit neatly into his system. Nevertheless, his table was gradually accepted as new elements were discovered that matched his predicted properties. The discovery of subatomic particles in the 20th century, particularly the proton and the neutron, led to a new understanding of the structure of the atom and the arrangement of the elements. Scientists realized that elements should be ordered by atomic number, rather than atomic mass, which led to the modern periodic table that we use today. The periodic table remains a fundamental tool in the study of chemistry, allowing scientists to understand the properties of elements and predict the behavior of chemical reactions.

Mendeleev's version of the periodic table

Those are all great key takeaways about the periodic table! To summarize:

  • The periodic table is based on the order of elements' atomic numbers.
  • The periodic table is organized into rows (periods) and columns (groups).
  • Periods show periodicity, meaning that elements in the same period have similar properties.
  • Atomic number increases as you move across a period in the periodic table.
  • Elements in the same group have the same number of valence electrons and react in similar ways.
  • The periodic table is divided into blocks based on the subshell that the highest energy valence electron is in.
  • Elements can be classified as metals, non-metals, or metalloids based on their properties.
  • The modern periodic table was created by Dmitri Mendeleev in 1869, and it is based on atomic number rather than atomic mass.

Periodic Table

How many elements are there in the periodic table?


What is the periodic table?

A tabular arrangement of chemical elements that is arranged by increasing atomic number, and groups elements according to recurring properties.

How are elements arranged in the periodic table?

The elements in the periodic table are arranged by increasing order of atomic number.

Who invented the periodic table?

The chemist Dmitri Mendeleev invented the periodic table.

What are the rows of the periodic table called?

The rows are called periods.

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