Proton number

The proton number is the number of protons in the nucleus of an atom. It defines the element and equals the number of electrons in a neutral atom. It is also called the atomic number. Malay: nombor proton · Chinese: 质子数.

The proton number is the single most important number for identifying an atom, because no two elements share the same proton number. In the Periodic Table the elements are arranged in order of increasing proton number, so the proton number also tells you where an element sits.

Example. Sodium has a proton number of 11: every sodium atom has 11 protons, and a neutral sodium atom therefore has 11 electrons. Carbon has proton number 6, and oxygen has proton number 8. If you are told an atom has 17 protons, it must be chlorine, and its neutral atom has 17 electrons arranged as 2.8.7.

Confusion to avoid. Do not confuse the proton number with the nucleon number. The proton number counts only the protons; the nucleon number counts protons and neutrons together. In the notation for an atom, the proton number is the smaller (bottom) figure and the nucleon number is the larger (top) figure. Subtracting the proton number from the nucleon number gives the number of neutrons.

The proton number is written as the lower figure in the standard atomic notation, with the nucleon number above it. Because it is the same for every atom of an element, you can read it straight from the Periodic Table instead of counting particles, which is why it is usually the first thing to write down when you analyse an atom. It never changes during a chemical reaction, even when the atom becomes an ion.

In the exam you use the proton number to name an element, to find the number of electrons in a neutral atom, and to write the electron arrangement. Because it controls the electron arrangement, and therefore the group and period, the proton number is the thread that ties the whole Matter and the Atomic Structure chapter to the Periodic Table.