How is the structure of the atom related to the organization of the modern periodic table?

1. The Period

Atoms have energy levels for its electrons. In most cases, those levels fill up from the ground up. Each energy level contains a fixed number of electrons. The number of electrons in a neutral atom of a certain element is also definite. As a result, the number of energy levels those electrons occupy in a ground state atom is certain for an element.

As its name suggests, the periodic table has multiple rows known as periods. Atoms of elements in the same row have the same number of occupied energy levels per atom. This is true only when the atom is neutral and in its ground state.

For example:

Sodium, Na, is in the third period. A neutral Na atom in its ground state has three occupied energy levels: 1, 2, and 3. Potassium, K, is in the fourth period. A neutral K atom in its ground state has four occupied energy levels: 1, 2, 3, and 4. 2. The Group Number

Among all occupied energy levels, the level with the highest energy is known as the valence shell. The number of electrons in that level in a neutral atom could predict chemical properties of the atom.

The periodic table is divided into columns. Each column is known as a group. Elements in each group has the same number of electrons in their valence shell. Again, this holds only if the atom is neutral and in its ground state. As a result, elements in each group have similar chemical properties.

For example, both Na and K are found in the leftmost column of the periodic table. Both are group 1 elements. The two elements have similar chemical properties:

Both Na and K are easy to cut in their metallic state under room temperatures. They are known to be malleable. Both Na and K react vigorously with water. The solution they produce will be basic. This is potentially why elements in group 1 are also known as alkali metals.