Atomic number

The atomic number is a special number that helps us identify each chemical element. It tells us how many protons are in the center, or nucleus, of an atom. Protons are tiny particles that have a positive charge, and every atom of a certain element has the same number of protons. For example, all carbon atoms have 6 protons, so carbon's atomic number is 6.

This number is very important because it shows an element's place in the periodic table, a chart that organizes all the elements based on their properties. In a normal atom that isn't charged, the atomic number is also the same as the number of electrons, which are negatively charged particles that orbit the nucleus.

Atoms that have the same atomic number but different numbers of neutrons are called isotopes. These isotopes behave almost the same chemically but can have different properties, such as weight. The atomic number helps scientists understand and predict how elements will react and interact in the world around us.

Notation

The atomic number is used in special notation to show details about an element. We write the mass number above the element symbol and the atomic number below it. For example, we can write ¹²
₆C for carbon. Sometimes, we only write the mass number above the symbol, like ¹²C, because the atomic number is already known from the element symbol.

People often say these notations in a different way than they are written. For example, ⁴
₂He is usually said as "helium-four" instead of "four-two-helium."

History

In the 1800s, people used the term "atomic number" to mean the number of atoms in a certain space. Today, scientists think of it differently.

In 1913, a scientist named Antonius van den Broek suggested that the charge of an atom’s nucleus matched its position on the periodic table of elements. Another scientist, Ernest Rutherford, helped make this idea popular. After Rutherford discovered the proton in 1920, the atomic number came to mean the number of protons in an atom. This idea was later supported by experiments, especially one done by Henry Moseley in 1913. He showed that the atomic number matches the charge of the nucleus, helping us understand the order of elements on the periodic table.

The periodic table lists elements in order, and they can be numbered this way. Dmitri Mendeleev, who created one of the first periodic tables in 1869, arranged elements by their weight but sometimes changed the order to fit their chemical behavior. Later, it was found that ordering by the number of protons worked better, especially for elements that have very similar weights.

In 1911, Rutherford described atoms as having a central nucleus with a positive charge. Van den Broek later suggested that this charge exactly matched an element’s position on the periodic table. Moseley’s experiments in 1913 tested this idea and confirmed that the atomic number is very close to the nucleus’s charge. His work also helped identify how many elements are in the lanthanide series.

After Moseley’s passing in 1915, scientists checked the atomic numbers of all known elements. They found gaps where elements had not yet been discovered. There are also ideas about elements that might exist but have not been observed yet.