The number above an element’s symbol on the periodic table is its atomic number, which tells you exactly how many protons are packed into the nucleus of that atom. Hydrogen has an atomic number of 1 (one proton), helium is 2, lithium is 3, and so on all the way up to oganesson at 118. This single number defines what element an atom is.
What the Atomic Number Actually Counts
Every atom has a core called the nucleus, and inside that nucleus are protons and neutrons. The atomic number counts only the protons. It’s represented by the letter Z in scientific notation, and it never changes for a given element. Every carbon atom in the universe has exactly 6 protons. Every gold atom has exactly 79. If you changed the number of protons, you’d literally have a different element.
In a neutral atom (one that hasn’t gained or lost an electrical charge), the atomic number also tells you the number of electrons orbiting the nucleus. A nitrogen atom with atomic number 7 has 7 protons and 7 electrons. Those electrons are what drive an element’s chemical behavior, which is why the atomic number is so fundamental to how the periodic table is organized.
Why It Sits at the Top of the Tile
Most periodic tables display two numbers alongside each element’s symbol. The layout can vary slightly between different versions, but the most common arrangement places the atomic number above the symbol and the atomic mass below it. Some tables put the atomic number in the upper left corner and the atomic mass in the upper right. Regardless of placement, the atomic number is always a clean whole number, while the atomic mass is a decimal (like 12.011 for carbon or 26.982 for aluminum). That’s the fastest way to tell them apart.
The atomic number determines the order of every element on the table. Elements are arranged left to right, top to bottom, in ascending order of their atomic number. Hydrogen (1) comes first, then helium (2), lithium (3), beryllium (4), and so on. This sequence isn’t arbitrary. Elements with similar chemical properties fall into the same vertical columns precisely because of how electrons fill in around the nucleus as the proton count increases.
Atomic Number vs. Atomic Mass
The number below the symbol is the atomic mass, and it represents a different measurement entirely. While the atomic number counts only protons, the atomic mass reflects the combined weight of protons and neutrons in the nucleus. Protons and neutrons each contribute roughly one unit of mass, so an element’s mass number is always larger than (or equal to, in the case of hydrogen) its atomic number.
The reason the atomic mass appears as a decimal is that most elements exist as a mixture of isotopes. Isotopes are atoms of the same element (same number of protons) but with different numbers of neutrons. Carbon, for example, mostly exists as carbon-12 (6 protons, 6 neutrons) but a small fraction is carbon-13 (6 protons, 7 neutrons). The atomic mass listed on the periodic table is a weighted average across all naturally occurring isotopes, which is why carbon’s reads 12.011 rather than a round number.
How to Calculate Neutrons
You can figure out how many neutrons an atom has using a simple formula: subtract the atomic number from the mass number. For a specific isotope of tungsten written as tungsten-186, you’d take 186 (the mass number) minus 74 (the atomic number, meaning 74 protons) to get 112 neutrons. This works for any element as long as you know which isotope you’re looking at.
In nuclear notation, which you might see in a chemistry or physics class, the mass number is written as a superscript to the upper left of the element symbol, and the atomic number as a subscript below it. So copper-63 would appear with 63 above and to the left and 29 below and to the left of Cu. This is a different layout than the periodic table tile, but the atomic number means the same thing in both contexts: the count of protons.
Why Atomic Number Replaced Atomic Weight
The periodic table wasn’t always organized by atomic number. When Dmitri Mendeleev first published his table in 1869, he arranged the 63 known elements by atomic weight. This worked remarkably well, but it created a few stubborn problems. Three pairs of elements, including argon and potassium, had their chemical properties in the wrong order when sorted by weight alone.
In 1913, a Dutch physicist named Antonius van den Broek proposed that an element’s properties were determined not by its weight but by its position number in the table. Shortly after, the English physicist Henry Moseley confirmed this experimentally by measuring the X-ray signatures of elements from aluminum to gold. Each element produced a unique X-ray pattern that corresponded to a specific whole number. Moseley’s work established that the properties of elements are periodic functions of their atomic number, not their atomic weight. That principle still governs the periodic table today.