Atoms are the fundamental building blocks of all matter, composed of three primary subatomic particles: protons, neutrons, and electrons. Protons carry a positive charge, electrons carry a negative charge, and neutrons are electrically neutral. Protons and neutrons are tightly packed together in the dense center of the atom, called the nucleus, while electrons orbit in a cloud.
The identity of a chemical element is determined entirely by the number of protons found in its nucleus. This number is a unique identifier, known as the atomic number. Changing the number of protons changes the element and defines an atom’s chemical properties.
The Fixed Number of Protons in Sodium
Every atom of sodium (Na) contains exactly 11 protons in its nucleus. This number is known as the atomic number (Z), and it serves as the defining characteristic of the element sodium. The atomic number can be found on the periodic table, where sodium is listed as element number 11.
If an atom were to gain a proton, increasing the count to 12, it would become magnesium (Mg). Conversely, if a sodium atom were to lose a proton, it would become neon (Ne). Therefore, any atom identified as sodium must possess 11 protons, regardless of its mass or other properties.
This fixed proton count provides the direct answer to the question about the isotope of sodium with a mass number of 25. An isotope of sodium must, by definition, be sodium, meaning it must contain 11 protons. The number of protons is the constant factor that defines the element’s identity.
What the Mass Number Represents
The term “mass number” (A) refers to the total count of particles found in an atom’s nucleus. Since the nucleus consists of protons and neutrons, the mass number is the sum of these two particle types. Electrons are excluded because their mass is minuscule and contributes negligible weight to the overall atom.
The mass number of 25 for Sodium-25 signifies that the total number of protons and neutrons is 25. This differs from the atomic number, which only counts the protons. Since all sodium atoms have 11 protons, the number of neutrons can be determined by subtracting the atomic number from the mass number.
The calculation for Sodium-25 is straightforward: 25 (Mass Number) minus 11 (Atomic Number) equals 14. This means that a specific atom of Sodium-25 contains 14 neutrons in its nucleus. The variation in the mass number is due only to a difference in the neutron count, as the proton count remains fixed at 11 for sodium.
How Isotopes Relate to Atomic Structure
An isotope is defined as any of two or more atoms of the same element that have an identical number of protons but a differing number of neutrons. This difference in neutron count allows for variations within a single element while maintaining its identity. The varying neutron count is what causes the different mass numbers.
The most abundant, naturally occurring form of sodium is Sodium-23, which has 11 protons and 12 neutrons (23 – 11 = 12). Comparing this to Sodium-25, which has 11 protons and 14 neutrons, illustrates the nature of isotopes. Both are sodium because they contain 11 protons, but they possess different mass numbers due to the varying number of neutrons.