The atom is the foundational unit of all matter. At the center of this unit is the atomic nucleus, a dense, positively charged core. This core contains almost all of the atom’s mass but occupies only an extremely small fraction of its total volume. The electrical charge of this central region is a primary factor in determining how atoms interact.
The Nucleus’s Building Blocks
The atomic nucleus is composed of two primary types of subatomic particles, collectively called nucleons: the proton and the neutron. These particles are packed tightly together in the atom’s core. The nucleus is incredibly small compared to the entire atom, estimated to be about 100,000 times smaller in diameter.
Despite its minute size, the nucleus holds more than 99.9% of the atom’s total mass. Protons and neutrons possess nearly identical masses, which are significantly greater than the mass of an electron. This mass concentration gives the nucleus its high density. Strong forces hold these particles together, overcoming the natural repulsion between the positively charged protons.
The Source of the Positive Charge
The nucleus possesses a positive electrical charge because of the specific charges carried by its constituent particles. The proton carries a single unit of positive electrical charge, denoted as +1. The neutron is electrically neutral, meaning it carries a charge of zero.
Since only the protons contribute an electrical charge, the overall charge of the nucleus must be positive. Neutrons contribute mass and help stabilize the nucleus, but they do not affect its charge. Therefore, the magnitude of the nuclear charge is determined solely by the count of protons. This positive charge generates the electrostatic attraction that holds negatively charged electrons in orbit.
Quantifying the Nuclear Charge
The total electrical charge of any atomic nucleus is determined by its atomic number (Z). The atomic number is defined as the count of protons found within the nucleus. Because each proton contributes one unit of positive charge, the nuclear charge is always equal to \(+Z\).
This number is a fundamental property that defines the chemical identity of the element. For example, a Helium atom has two protons (Z=2), meaning its nucleus carries a charge of +2. In a neutral atom, the number of negatively charged electrons orbiting the nucleus exactly matches the positive nuclear charge, resulting in a net charge of zero.