Periodic table

The periodic table, also known as the periodic table of the elements, is an ordered arrangement of the chemical elements into rows ("periods") and columns ("groups"). It is a key tool in chemistry and physics, helping scientists understand how elements behave. The table shows that elements in the same group often have similar properties, making it easier to predict how they will react with other substances.

Vertical, horizontal, and diagonal trends help describe patterns in the table. For example, metallic character tends to increase as you move down a group or from right to left across a row, while nonmetallic character increases from the bottom left to the top right of the table.

The first widely accepted periodic table was created by the Russian chemist Dmitri Mendeleev in 1869. He arranged elements by their atomic mass and used his table to predict some properties of some of the missing elements. Later discoveries, such as the role of atomic numbers and the work of Glenn T. Seaborg, helped shape the modern form of the table. Today, the periodic table includes all known elements, with ongoing research to explore and understand even heavier, newly created elements.

Structure

Each chemical element has a unique atomic number, which tells us how many protons are in its nucleus. This number helps us classify elements into the chemical elements. For example, hydrogen has an atomic number of 1, helium has 2, and lithium has 3. These elements can also be represented by short symbols like H for hydrogen and He for helium.

The periodic table organizes elements by their atomic numbers. When a new row starts, it means a new electron shell begins to fill with electrons. Columns, called groups, group elements with similar chemical properties because they have the same number of electrons in their outer shells. For example, oxygen, sulfur, and selenium are in the same group because they behave similarly in chemical reactions.

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

Variations

The periodic table sometimes shows different arrangements for the first row and group 3. In the first row, hydrogen and helium can be tricky to place. Hydrogen often goes in group 1 because it has one electron, like the alkali metals, but it also behaves differently, sometimes more like the halogens. Helium usually sits in group 18 with the noble gases because it is unreactive, even though its electron setup is different from the other noble gases.

For group 3, there is debate over which elements belong there. Some tables place lanthanum and actinium in group 3, while others place lutetium and lawrencium there. This difference comes from how we understand electron configurations and the properties of these elements. Scientists continue to discuss the best way to arrange these elements to match their chemical behaviors.

Periodic trends

The periodic table shows how elements change in predictable 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 compounds.

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 groups elements that behave similarly. 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, German physicist Johann Wolfgang Döbereiner began one of the earliest attempts to classify the elements. He found that he could form some of the elements into groups of three, with the members of each group having related properties. He termed these groups triads. Various chemists continued his work and were able to identify more relationships between small groups of elements.

John Newlands published a letter in 1863 on the periodicity among the chemical elements. In 1864 Newlands published an article showing that if the elements are arranged in the order of their atomic weights, those having consecutive numbers frequently either belong to the same group or occupy similar positions in different groups.

The definitive breakthrough came from the Russian chemist Dmitri Mendeleev. On 17 February 1869, Mendeleev began arranging the elements and comparing them by their atomic weights. When elements did not appear to fit in the system, he predicted that either valencies or atomic weights had been measured incorrectly, or that there was a missing element yet to be discovered. In 1871, Mendeleev published a long article, including an updated form of his table, that made his predictions for unknown elements explicit. Mendeleev predicted the properties of three of these unknown elements in detail.

After the internal structure of the atom was probed, amateur Dutch physicist Antonius van den Broek proposed in 1913 that the nuclear charge determined the placement of elements in the periodic table. The New Zealand physicist Ernest Rutherford coined the word "atomic number" for this nuclear charge. In van den Broek's published article he illustrated the first electronic periodic table showing the elements arranged according to the number of their electrons.

The same year, English physicist Henry Moseley using X-ray spectroscopy confirmed van den Broek's proposal experimentally. Moseley determined the value of the nuclear charge of each element from aluminium to gold and showed that Mendeleev's ordering actually places the elements in sequential order by nuclear charge. Nuclear charge is identical to proton count and determines the value of the atomic number (Z) of each element.

The Danish physicist Niels Bohr applied ideas of quantization to the atom. He concluded that the energy levels of electrons were quantised: only a discrete set of stable energy states were allowed. Bohr then attempted to understand periodicity through electron configurations.

The quantum theory clarified the transition metals and lanthanides as forming their own separate groups, transitional between the main groups.

By 1936, the pool of missing elements from hydrogen to uranium had shrunk to four. Element 43 eventually became the first element to be synthesized artificially via nuclear reactions rather than discovered in nature. It was discovered in 1937 by Italian chemists Emilio Segrè and Carlo Perrier, who named their discovery technetium.

In celebration of the periodic table's 150th anniversary, the United Nations declared the year 2019 as the International Year of the Periodic Table. Today, the periodic table is among the most recognisable icons of chemistry.

Future extension beyond the seventh period

Main article: Extended periodic table

See also: Island of stability

The most recently named elements – nihonium (113), moscovium (115), tennessine (117), and oganesson (118) – completed the seventh row of the periodic table. Future elements would begin an eighth row. These elements may be referred to by their atomic numbers or by special names based on their numbers. So far, attempts to create these elements have not succeeded.

If the eighth period follows the pattern of earlier periods, it would contain fifty elements. However, because of how atoms work at this level, the usual rules might not apply. Scientists are still figuring out how to arrange these future elements and how their properties might look.

Alternative periodic tables

Main article: Types of periodic tables

The periodic law can be shown in many different ways besides the standard periodic table. Since Mendeleev created his table in 1869, people have made 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 or most correct way to arrange the periodic table. There is no agreement on this yet, but Janet’s left-step table is getting more attention as a strong candidate.