The atomic number of sodium is the highest among the elements in its group due to its position on the periodic table. Sodium is located in Group 1, also known as the alkali metals, and has an atomic number of 11. This means that sodium has 11 protons in its nucleus, which is the highest number for any element in its group.

The atomic number is determined by the number of protons in an atom's nucleus. As you move down a group on the periodic table, the number of protons increases because each element has one more proton than the one above it. Sodium is the third element in Group 1, following hydrogen and lithium, and thus has the highest atomic number in its group.

According to the periodic table, the atomic number of sodium is 11, which is significantly higher than the atomic numbers of hydrogen (1) and lithium (3). This increase in atomic number is due to the addition of more protons in the nucleus as you move down the group. The periodic table is organized in such a way that elements with similar chemical properties are grouped together, and the atomic number increases sequentially from left to right and from top to bottom.

Data from the periodic table confirms that sodium has an atomic number of 11, which is the highest among the alkali metals. This information is consistent with the periodic table's arrangement and the principles of atomic structure. The atomic number of sodium is a fundamental property that defines its position in the periodic table and influences its chemical behavior.