The atomic number is very important for understanding the properties of elements on the periodic table.

It tells us how many protons are in an atom's center, called the nucleus.

As you move from left to right across the table, the atomic number goes up. This change affects several important traits:

1. Electron Arrangement: The atomic number also decides how the electrons are set up around the nucleus. For example, oxygen, which has an atomic number of 8, is arranged as $1s^2 2s^2 2p^4$. But neon, with an atomic number of 10, is arranged as $1s^2 2s^2 2p^6$.

This different electron arrangement makes them behave very differently. Oxygen is reactive, meaning it easily combines with other elements. Neon, on the other hand, is a noble gas and doesn’t react much at all.

2. Reactivity: As you go down a column in the periodic table, elements usually become more reactive. For example, in group 1 (which contains alkali metals), lithium (Li) with an atomic number of 3 doesn't react as much as cesium (Cs), which has an atomic number of 55.

3. Size of Atoms: The size of an atom generally gets bigger as you go down a column because more electron layers are added. But if you move from left to right across a row, the size gets smaller because the nucleus pulls the electrons in closer.

Knowing these trends helps us understand how different elements act!