No, the atomic mass or atomic weight of an element is not always roughly equal to twice the atomic number. The atomic mass is determined by the combined mass of protons and neutrons in the nucleus of an atom.
The atomic number of an element represents the number of protons in the nucleus. However, the number of neutrons can vary among different isotopes of the same element. Isotopes are atoms of the same element with different numbers of neutrons.
Since isotopes have different numbers of neutrons, their atomic masses will be different. The atomic mass of an element is usually represented as an average value, taking into account the abundance of each isotope in nature. This average atomic mass is often not a whole number and may not be approximately twice the atomic number.
For example, let's consider carbon. The atomic number of carbon is 6, which means it has six protons. However, carbon has several isotopes, including carbon-12, carbon-13, and carbon-14. The atomic masses of these isotopes are approximately 12, 13, and 14, respectively. The average atomic mass of carbon, considering the natural abundance of its isotopes, is around 12.01, which is not approximately twice the atomic number.
Therefore, while there may be a rough correlation between the atomic mass and the atomic number for some elements, it is not a consistent rule. The atomic mass is determined by the specific isotopes and their abundance in a given element.