Labeling an atom means writing a shorthand that tells you exactly what’s inside it: how many protons, neutrons, and electrons it has. The most common method places numbers around the element’s chemical symbol in specific positions, each representing a different piece of information. Once you understand what goes where, you can read or write the label for any atom, isotope, or ion.
The Three Numbers That Define an Atom
Every atom label builds on three values: the atomic number, the mass number, and (when relevant) the charge. The atomic number is the number of protons in the nucleus. It defines which element you’re dealing with, and it never changes for a given element. Carbon always has 6 protons, iron always has 26. You can find any element’s atomic number on the periodic table.
The mass number is the total count of protons plus neutrons in the nucleus. Unlike the atomic number, this can vary between atoms of the same element, which is how isotopes arise. A carbon atom with 6 neutrons has a mass number of 12, while one with 8 neutrons has a mass number of 14. Both are still carbon.
From these two numbers, you can calculate everything else:
- Protons = atomic number
- Electrons = atomic number (in a neutral atom)
- Neutrons = mass number minus atomic number
Standard Nuclear Notation
The formal way to label an atom is called AZX notation (sometimes written AZE). It places two numbers to the left of the element symbol: the mass number as a superscript on top, and the atomic number as a subscript below. For a carbon-12 atom, this looks like:
126C
The top number (12) tells you there are 12 total protons and neutrons. The bottom number (6) tells you there are 6 protons. Subtract one from the other and you know there are 6 neutrons.
In practice, the atomic number is often dropped because the element symbol already tells you what it is. If you see the symbol C, you already know the atomic number is 6. So many textbooks and scientific papers simply write 14C to mean carbon-14, with only the mass number as a superscript. Both formats are correct.
Hyphen Notation
Outside of formal equations, a simpler system is even more common. Hyphen notation writes the element name or symbol followed by a hyphen and the mass number: carbon-14 or C-14. This is the format you’ll see most often in news articles, medical contexts, and introductory chemistry courses. It conveys the same information as nuclear notation but is easier to type and read in plain text.
One small addition to know: if the letter “m” appears right after the mass number (as in technetium-99m), it indicates a metastable state, meaning the nucleus is in a higher-energy arrangement than its ground state. This comes up mainly in medical imaging.
Mass Number vs. Atomic Mass
A common point of confusion is the difference between the mass number used in atom labels and the atomic mass printed on the periodic table. The mass number is always a whole number because it’s a simple count of protons and neutrons. Carbon-12 has a mass number of exactly 12.
The atomic mass on the periodic table, though, is a weighted average of all naturally occurring isotopes of that element. That’s why carbon’s atomic mass is listed as approximately 12.01 rather than a clean 12. When you’re labeling a specific atom or isotope, you use the mass number (the whole number), not the decimal value from the periodic table.
Labeling Ions With Charge
A neutral atom has equal numbers of protons and electrons, so it carries no net charge. When an atom gains or loses electrons, it becomes an ion, and the label needs to reflect that. The charge goes as a superscript to the right of the element symbol, with the number before the sign. A sodium atom that has lost one electron is written Na+ (or Na1+), and a barium atom missing two electrons is Ba2+. An atom that gained electrons gets a minus sign: fluorine with one extra electron is F−, and a phosphate group with three extra electrons is PO43−.
The international chemistry authority IUPAC established this number-before-sign convention to match how we say it out loud: “two positive” rather than “positive two.” You’ll occasionally see older textbooks reverse the order, but the current standard puts the number first.
Lewis Dot Structures
Lewis dot structures label an atom’s valence electrons, the outermost electrons that participate in chemical bonding. You write the element symbol and place dots around it, one for each valence electron, distributed on four sides (top, bottom, left, right). Each side can hold up to two dots, representing a pair of electrons.
Carbon has 4 valence electrons, so its Lewis structure shows one dot on each of the four sides. Oxygen has 6 valence electrons, so two sides get paired dots and two sides get single dots. You can arrange the singles and pairs on whichever sides you prefer, as long as no side has more than two dots. This notation is especially useful for predicting how atoms will bond with each other, since unpaired dots represent electrons available for sharing.
Bohr Model Diagrams
When a homework problem asks you to “label an atom,” it may want a Bohr model diagram rather than written notation. In this model, you draw a central circle representing the nucleus and write the number of protons and neutrons inside it (sometimes abbreviated as “6p, 6n” for carbon-12). Concentric rings around the nucleus represent electron shells, and you place dots or small circles on each ring to show how many electrons occupy that shell.
The first shell (closest to the nucleus, called the K shell or n=1) holds a maximum of 2 electrons. The second shell (L shell, n=2) holds up to 8. The third shell (M shell, n=3) also holds up to 8 in simplified models. You fill from the inside out. So for carbon with 6 electrons, you’d place 2 electrons on the first ring and 4 on the second.
For ions, the diagram works the same way, but the electron count changes. A sodium atom (11 protons) normally has 11 electrons across three shells: 2, 8, 1. When it becomes Na+, it loses that outermost electron, leaving only two filled shells with 2 and 8 electrons.
Electron Configuration Notation
A more detailed way to label an atom’s electrons is electron configuration notation, which specifies exactly which subshells the electrons occupy. Each entry has three parts: the shell number, a letter indicating the subshell type (s, p, d, or f), and a superscript showing how many electrons are in that subshell.
Oxygen, with 8 electrons, fills its subshells in order: 1s2 2s2 2p4. That reads as “two electrons in the 1s subshell, two in the 2s, and four in the 2p.” The s subshells hold a maximum of 2 electrons, p subshells hold 6, d subshells hold 10, and f subshells hold 14. Electrons fill lower-energy subshells first, following a specific sequence that the periodic table is actually organized around. Reading the table left to right, top to bottom, mirrors the filling order.
This notation gives you more information than a Bohr diagram because it distinguishes between subshells within the same energy level. It’s the standard way to label atoms in college-level chemistry and beyond.