Atomic Structure

Protons

• Protons are positively charged subatomic particles found within the atomic nucleus. Each proton carries a relative electrical charge of $+1$ and has a relative atomic mass of $1$ atomic mass unit. The number of protons uniquely identifies an element.

Neutrons

• Neutrons are neutral subatomic particles, meaning they carry no electrical charge, and are also located within the atomic nucleus. Like protons, each neutron has a relative atomic mass of $1$ atomic mass unit. Their primary role is to contribute to the atom's mass and nuclear stability.

Electrons

• Electrons are negatively charged subatomic particles that orbit the nucleus in specific energy levels or shells. Each electron carries a relative electrical charge of $-1$, but its relative mass is approximately $1/2000$th that of a proton or neutron, making it effectively negligible in mass calculations. The vast majority of an atom's volume is occupied by the electron cloud, which is mostly empty space.

Atomic Number (Z)

• The atomic number, symbolized as $Z$, is defined as the total number of protons present in the nucleus of an atom. This number is the fundamental identifier of an element, meaning that all atoms with the same atomic number belong to the same chemical element. Elements are arranged in the periodic table according to their increasing atomic number.

Mass Number (A)

• The mass number, symbolized as $A$ (also known as the nucleon number), represents the total count of nucleons (protons and neutrons) within an atom's nucleus. It provides an approximate measure of the atom's relative atomic mass, as protons and neutrons account for almost all of an atom's mass. The number of neutrons can be calculated by subtracting the atomic number from the mass number ($A - Z$).

Standard Nuclear Notation

• Atoms are conventionally represented using nuclear notation, which provides a concise summary of their composition. This notation takes the form $^A_Z X$, where $X$ is the chemical symbol of the element, $A$ is the mass number (written as a superscript), and $Z$ is the atomic number (written as a subscript). This symbolic representation allows for quick identification of an atom's proton and nucleon count.

• For example, the notation $^{12}_6 C$ indicates a carbon atom with a mass number of 12 and an atomic number of 6. This immediately tells us it has 6 protons and a total of 12 protons plus neutrons. Similarly, $^{23}_{11} Na$ represents a sodium atom with 11 protons and a total of 23 nucleons.

To determine the number of subatomic particles in a neutral atom from its nuclear notation ($^A_Z X$), a systematic approach is used. The atomic number ($Z$) directly gives the number of protons, which in turn equals the number of electrons for a neutral atom. The number of neutrons is found by subtracting the atomic number from the mass number ($A - Z$).

For instance, in an atom represented as $^{35}_{17} Cl$, the atomic number $Z=17$ means there are 17 protons and 17 electrons. The mass number $A=35$ means the number of neutrons is $35 - 17 = 18$. This method allows for a complete breakdown of an atom's fundamental composition.

Atomic Number vs. Mass Number

• It is crucial to distinguish between the atomic number and the mass number as they convey different, though related, information. The atomic number ($Z$) is the unique identifier of an element, defining its chemical identity and properties, whereas the mass number ($A$) indicates the total count of protons and neutrons, primarily reflecting the atom's mass. Two atoms with the same atomic number but different mass numbers are isotopes of the same element.

Negligible Electron Mass

• A common misconception is that electrons contribute significantly to an atom's overall mass. However, the mass of electrons is considered negligible when calculating the mass number because their individual mass is approximately $1/2000$th that of a proton or neutron. Therefore, the mass number effectively represents only the sum of protons and neutrons in the nucleus.

Mass Number is Not Actual Mass

• While the mass number provides a convenient approximation of an atom's relative mass, it is important to remember it is a count of nucleons, not a precise mass measurement. The actual atomic mass is slightly different due to the mass defect (energy released during nucleus formation) and the small, non-zero mass of electrons. For most introductory purposes, however, the mass number serves as a practical indicator of relative atomic mass.