The theories of planetary formation indicate that Earth's original form had more hydrogen and helium and less of the current atmospheric nitrogen and oxygen. Water may have been added by comets that Earth swept from its solar orbit.
yes
Was the earth atmosphere's composition always the same?
always the same
Earth and Venus are similar in size, density, and overall composition.
pure substance
No. A compound will always have the same composition.
The composition of pure substances, such as elements and compounds, are always the same. The composition of mixtures can vary.
Venus and the earth have the same surface composition and have an atmosphere which has a complex weather system.
I believe it is an element.
Matter with a definite composition is a pure substance, such as an element, molecule, molecular compound, or ionic compound.
The composition of air is practically the same at the level of earth.
The composition of the Earth's core is thought to be similar to that of many metallic asteroids. Both are believed to contain high concentrations of iron and nickel, which are key components of the Earth's core. This similarity suggests that the core may have formed from the same materials that contributed to the formation of these asteroids in the early solar system.