TLDR: The atomic number is the number of protons in an atom's nucleus, which determines its chemical properties.

The atomic number is a special number that tells us how many protons are in the nucleus of an atom. It's like a unique ID for each element. For example, hydrogen has an atomic number of 1 because it has one proton in its nucleus. Oxygen, on the other hand, has an atomic number of 8 because it has eight protons. The atomic number is important because it determines the chemical properties of an element. Elements with the same atomic number have similar properties. So, if you know the atomic number of an element, you can predict how it will behave in chemical reactions.

The atomic number is also equal to the number of electrons in a neutral atom. Electrons are negatively charged particles that orbit around the nucleus. They balance out the positive charge of the protons, so the atom is electrically neutral. For example, hydrogen has one proton and one electron, while oxygen has eight protons and eight electrons.

The atomic number is used to identify elements and organize them in the periodic table. The periodic table is a chart that arranges all the known elements in order of their atomic numbers. It's like a big family tree for elements. Each element has its own spot on the table based on its atomic number. This helps scientists understand the relationships between different elements and how they interact with each other.

Scientists have been studying atomic numbers for a long time. In the early days, they used to arrange elements based on their atomic weights, but that didn't always work perfectly. It wasn't until the early 20th century that scientists realized that the atomic number was actually the key to understanding the properties of elements. They discovered this through experiments and observations, like measuring the wavelengths of light emitted by different elements.

Today, we know the atomic numbers of all the known elements, from hydrogen with an atomic number of 1 to oganesson with an atomic number of 118. Scientists have even created new elements by bombarding atoms with ions. These new elements have atomic numbers that haven't been observed before.

So, the atomic number is a fundamental concept in chemistry and physics. It helps us understand the building blocks of matter and how different elements interact with each other.