Atomic Number vs Atomic Weight
Atoms are characterized by their atomic numbers. In the periodic table, atoms are arranged according to their atomic number. Atomic number provides a variety of information regarding the atom and its nature. Atomic weight is also important to get some idea about the elements.
What is Atomic Number?
Atoms are mainly composed of protons, neutrons and electrons. Nucleus of the atom contains protons and neutrons. In addition, there are electrons circling around the nucleus in orbitals. Atomic number of an element is the number of protons it has in the nucleus. The symbol for denoting the atomic number is Z. When the atom is neutral, it has the same number of electrons as protons. Thus, atomic number is equal to the number of electrons in this instance. However, it is always reliable to get the number of protons as the atomic number. The elements in the periodic table are arranged according to the increasing atomic number. This arrangement has automatically arranged them in increased atomic weight most of the time. Every element has separate atomic number, and no element has the same atomic number. Therefore, atomic number is a good way of distinguishing different elements. By looking at the atomic number itself, a lot of information about the element can be withdrawn. For example, it tells the group and the period where the element belongs to in the periodic table. Further, it gives information about oxidation states, charge of the ion, bonding behavior, nucleus charge, etc.
What is Atomic Weight?
Most of the atoms in the periodic table have two or more isotopes. Isotopes differ from each other by having a different number of neutrons, even though they have the same proton and electron amount. Since their neutron amount is different, each isotope has a different atomic mass. Atomic weight is the average weight calculated considering all the masses of isotopes. Each isotope is present in the environment, in different percentages. When calculating the atomic weight, both isotope mass and their relative abundances are taken into consideration. Moreover, the masses of atoms are extremely small, so we cannot express them in normal mass units like grams or kilograms. For our purposes, we are using another unit call atomic mass unit (amu) to measure the atomic weight.
IUPAC defines the atomic weight as follows:
“An atomic weight (relative atomic mass) of an element from a specified source is the ratio of the average mass per atom of the element to 1/12 of the mass of an atom of 12C.”
The weights given in the periodic table are calculated like this, and they are given as relative atomic mass.
What is the difference between Atomic Number and Atomic Weight?
• Atomic number of an element is the number of protons it has in the nucleus. Atomic weight is the average weight calculated considering all the masses of isotopes.
• Number of protons and neutrons are largely contributing to the atomic weight. (This is because the mass of an electron is very small compared to a proton or a neutron).
• Elements in the periodic table are arranged according to the increasing atomic number, not the atomic weight, but often, atomic weight increase can also be seen between the consecutive elements when they are arranged according to their atomic numbers.