The atomic number and mass number are important ideas in understanding atoms. They help us organize the periodic table, which is a chart of all known chemical elements.

Atomic Number

1. What Is It?
   The atomic number (we use the letter $Z$ to represent it) tells us how many protons are in the center of an atom. This number is key to figuring out what the element is.

2. Why Does It Matter?

   • The periodic table lists elements in order of atomic number, starting from the smallest. For instance, hydrogen has an atomic number of 1, and carbon has an atomic number of 6.
   • The atomic number also tells us how many electrons are in a neutral atom (an atom with no charge). This affects how the element reacts with others.

Mass Number

1. What Is It?
   The mass number (shown as $A$) is the total count of protons and neutrons in an atom’s center. You can find it with this simple formula:
   $A = Z + N$
   Here $N$ is the number of neutrons.

2. Why Does It Matter?

   • The mass number helps us tell apart different versions of the same element, known as isotopes. For example, carbon-12 has a mass number of 12, while carbon-14 has a mass number of 14. Even though both are carbon, they have different numbers of neutrons.
   • Isotopes can behave differently and have unique uses in areas like medicine and archaeology.

Summary

• The atomic number is important for identifying elements and where they go in the periodic table.
• The mass number helps us understand the different forms of an element.
• Together, these two numbers help us learn about how elements work and how they are organized on the periodic table.
• Right now, there are 118 known elements on the periodic table, each with different atomic and mass numbers. This shows the variety of chemical behavior we see based on these two key properties.