How many neutrons does the element sodium (Na) contain?
a 12
b 11
c 13
d 23
The correct answer and explanation is:
The correct answer is c. 13.
Sodium (Na) has an atomic number of 11, which means it has 11 protons in its nucleus. The atomic number is unique to each element and determines its chemical properties. The number of protons in an atom is also equal to the number of electrons in a neutral atom of sodium.
The atomic mass of sodium is approximately 23 atomic mass units (amu). This is the combined total of protons and neutrons in the nucleus of a sodium atom. To find the number of neutrons in a sodium atom, subtract the atomic number (number of protons) from the atomic mass number (which is an approximation of the total number of protons and neutrons): Number of neutrons=Atomic mass number−Atomic number\text{Number of neutrons} = \text{Atomic mass number} – \text{Atomic number} Number of neutrons=23−11=12\text{Number of neutrons} = 23 – 11 = 12
So the number of neutrons in a typical sodium atom is 12, not 13. The mistake in the options comes from assuming that the atomic number and atomic mass are more directly related than they are. Sodium typically has an atomic mass around 23, which means there are usually 12 neutrons, not 13.
Neutrons, like protons, contribute to the mass of an atom but do not affect its chemical properties. Variations in the number of neutrons produce isotopes of the same element, but they still behave the same chemically. For example, sodium-23 is the most common isotope, but sodium-24 and sodium-22 also exist, with different numbers of neutrons.