In terms of percentage by mass, which element is present in the largest amount in the earth crust?

Introduction / Context:
The composition of the earth crust by mass is a basic piece of information in both geography and chemistry. Questions frequently ask which element is most abundant or how the main elements are ordered. This particular item requires you to recall which element, by percentage of mass, dominates the earth crust and therefore appears at the top of the composition table.

Given Data / Assumptions:

• We are concerned only with the earth crust, not the entire earth.
• The options are oxygen, silicon, carbon, and calcium.
• We consider mass percentage, not volume or number of atoms.

Concept / Approach:
Standard crustal composition data show that oxygen is by far the most abundant element in the earth crust by mass. It is followed by silicon, aluminium, and iron. Oxygen forms a major part of rock forming minerals such as silicates, carbonates, and oxides. Even though oxygen is a gas in the atmosphere, in rocks it is chemically bound to metals and other elements. Silicon is the second most abundant element in the crust, not the first. Carbon and calcium, though important in certain minerals, are present in much smaller amounts compared to oxygen and silicon.

Step-by-Step Solution:
Step 1: Recall the approximate order of crustal composition by mass: oxygen, silicon, aluminium, and iron at the top.Step 2: Notice that among the options given, oxygen and silicon are the only two candidates that appear high on this list.Step 3: Remember that oxygen alone accounts for nearly half of the crust mass, around forty six percent.Step 4: Recognise that silicon is second and not first in abundance.Step 5: Therefore, identify oxygen as the element present in the largest amount in the earth crust.

Verification / Alternative check:
Many textbooks provide a small pie chart or table showing crustal composition. Typical values are approximately forty six percent oxygen, twenty eight percent silicon, eight percent aluminium, and five percent iron, with calcium and other elements making up much smaller fractions. Seeing these numbers confirms that oxygen clearly dominates the crust composition by mass.

Why Other Options Are Wrong:
Silicon: Although very abundant and second on the list, silicon still has a lower mass percentage than oxygen.Carbon: Carbon is important in organic chemistry and in some rocks such as limestone and coal, but its percentage in the crust is much smaller.Calcium: Calcium is also present in many minerals and rocks but not nearly at the level of oxygen or silicon in overall crustal mass.

Common Pitfalls:
Some learners confuse the composition of the earth crust with the composition of the whole earth, where elements like iron may dominate in the core. Others mix up the second element, silicon, with the first. Paying attention to whether the question refers to the crust or the whole earth is critical. A simple way to remember is that rocks are mostly oxygen and silicon in combined form, with oxygen slightly more than silicon by mass.

Final Answer:
The element present in the largest amount in the earth crust by mass is oxygen.