Periodic table

The periodic table, also known as the periodic table of the elements, is an organized chart of the chemical elements. It arranges elements into rows called "periods" and columns called "groups". This table is important in chemistry and physics. Elements in the same group often act similarly, which helps scientists predict how they will react.

The table shows patterns, called trends. For example, elements tend to become more metallic as you move down a group or from right to left across a row.

The first popular periodic table was made by the Russian chemist Dmitri Mendeleev in 1869. He ordered elements by their atomic mass and used his table to guess properties of elements that had not yet been found. Later work, including discoveries about atomic numbers and the contributions of Glenn T. Seaborg, helped create the table we use today. The periodic table now includes all known elements, and scientists continue to study and learn about new, heavier elements.

Structure

Each chemical element has a unique atomic number. This number tells us how many protons are in its nucleus. The atomic number helps us group elements together. For example, hydrogen has an atomic number of 1, helium has 2, and lithium has 3. Elements can also be shown with short symbols like H for hydrogen and He for helium.

The periodic table puts elements in order by their atomic numbers. When a new row starts, it means a new electron shell begins to fill with electrons. Columns, called groups, hold elements that act alike because they have the same number of electrons in their outer shells. For example, oxygen, sulfur, and selenium are in the same group and react in similar ways.

Today, there are 118 known elements. The first 94 are found naturally on Earth. The rest, from americium to oganesson, are made in laboratories. Some elements, like technetium and promethium, were first created in labs before being found in nature.

Variations

The periodic table sometimes has different ways to show the first row and group 3.

In the first row, hydrogen and helium can be hard to place. Hydrogen often goes in group 1 because it has one electron, like the alkali metals. But it can also act more like the halogens. Helium usually sits in group 18 with the noble gases because it does not react, even though its electron setup is different.

For group 3, there is some debate about which elements belong there. Some tables show lanthanum and actinium in group 3. Others show lutetium and lawrencium there. This difference comes from how we understand electron setups and the properties of these elements. Scientists keep talking about the best way to arrange these elements.

Periodic trends

The periodic table shows how elements change in simple, repeating ways. When elements are arranged by their atomic numbers, patterns in their properties repeat. This is called the periodic law. These patterns help scientists understand how elements will react and form new substances.

Elements in the same column, or group, often have similar properties because they have the same number of electrons in their outer shells. For example, the alkali metals in the first group all have one valence electron, making them very reactive. As you move down a group, atomic size increases because electrons are added to higher energy levels. Moving left to right across a period, atomic size decreases because the increasing nuclear charge pulls electrons closer. These trends affect many properties, including how easily an element can gain or lose electrons.

Classification of elements

The periodic table puts elements together that act in similar ways. These groups have special names like alkali metal, alkaline earth metal, halogen, and noble gas. Some groups are also called by the name of their first element or by their group number.

There are also special rows of elements called lanthanides and actinides, which are very similar to each other. Beyond these, there are very heavy and short-lived elements known as transactinides or superheavy elements. Different areas of science may use slightly different ways to describe these elements.

History

Main article: History of the periodic table

See also: Timeline of chemical element discoveries

In 1817, a scientist named Johann Wolfgang Döbereiner tried to group elements by their properties. He found that some elements could be placed in groups of three, with each group having similar traits. These groups were called triads. Other scientists built on his work.

In 1863, John Newlands noticed a pattern when elements were arranged by their weights. Elements with similar properties often appeared at regular intervals.

The big breakthrough came from a chemist named Dmitri Mendeleev in 1869. He arranged the elements by their weights and predicted the existence of elements that had not yet been discovered. In 1871, he shared his predictions about the properties of these missing elements.

In 1913, a physicist named Antonius van den Broek suggested that the position of each element in the table depended on its nuclear charge. Ernest Rutherford called this number the "atomic number." That same year, Henry Moseley used a method called X-ray spectroscopy to show that elements are ordered by their atomic numbers, from aluminium to gold. This atomic number equals the number of protons in an atom and is written as Z.

A scientist named Niels Bohr helped explain why elements behave the way they do by studying the energy levels of electrons.

Later, scientists discovered that some elements, called transition metals and lanthanides, form their own groups.

By 1936, only a few elements were still missing from the table, from hydrogen to uranium. The element number 43, named technetium, was the first to be made in a lab rather than found in nature. It was discovered in 1937.

In 2019, the United Nations celebrated the 150th anniversary of the periodic table. Today, it is one of the most famous symbols in chemistry.

Future extension beyond the seventh period

Main article: Extended periodic table

See also: Island of stability

The last few elements added to the periodic table are nihonium (113), moscovium (115), tennessine (117), and oganesson (118). They finished the seventh row. Future elements would start an eighth row. These new elements might be called by their atomic numbers or special names. So far, scientists have not been able to make these elements.

If the eighth period follows the same pattern as earlier ones, it might have fifty elements. But because of how atoms behave at this level, the usual rules may not work the same way. Scientists are still learning how to place these future elements and what their properties might be.

Alternative periodic tables

Main article: Types of periodic tables

The periodic law can be shown in many ways besides the standard periodic table. Since Mendeleev made his table in 1869, people have created many other versions. Some keep the usual rectangle shape, like Charles Janet’s left-step table. Others look very different, such as spirals, circles, and triangles.

These alternative tables are made to show certain properties of elements more clearly. Because there are so many different styles, some people wonder if there is one best way to arrange the periodic table. There is no agreement on this yet, but Janet’s left-step table is getting more attention.