Write the full atomic symbol for the only atom that has its mass number equal to its atomic number.

When exploring the periodic table, one of the first concepts to understand is the atomic number, represented by the symbol 'Z'. The atomic number is fundamental because it defines what element an atom is. Specifically, the atomic number corresponds to the number of protons found in the nucleus of an atom. Since each element on the periodic table has a unique number of protons, the atomic number serves as an atom's identity card.

For example, hydrogen, with an atomic number of 1, has one proton in its nucleus. This is crucial for students to remember: the atomic number is always a whole number and never a decimal or fraction. The uniqueness of the atomic number means that no two elements can have the same value for 'Z'.

Moving beyond the atomic number, the mass number, represented as 'A', is the combined total of protons and neutrons within an atom's nucleus. It's important to note that electrons are not included in this count, as their mass is negligible compared to protons and neutrons. While the atomic number remains constant for an element, the mass number can vary because atoms of the same element can have different numbers of neutrons.

This variance leads to the existence of isotopes. Remember, the mass number is not listed on the periodic table because it can differ between isotopes. When you’re working with atomic symbols, you’ll typically see the mass number as the superscript placed before the element symbol, like in the example of hydrogen-1 where it is represented as '1H'. In this case, both the atomic number and the mass number are the same because hydrogen-1 has no neutrons, but that's a unique scenario.

As our journey into atomic structure continues, we encounter isotopes. Isotopes are different forms of the same element that share the same atomic number but have different mass numbers due to varying numbers of neutrons. This is a key concept in chemistry because isotopes of an element can exhibit slightly different physical properties, and some isotopes can be unstable, leading to radioactive decay.

For instance, while hydrogen-1, also known as protium, has no neutrons at all, its sibling isotope deuterium has one neutron, and another isotope, tritium, has two neutrons. They all have the atomic number 1, but different mass numbers: 1 for protium (1H), 2 for deuterium (2H), and 3 for tritium (3H). This concept of isotopes underscores the idea that an element's identity is tied to its protons, not its neutrons, allowing for fascinating variations within the same type of atom.