Nitrogen is a large fish in a very small pond!

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

30 August 2020

Some readers got confused by last week’s article where I estimated the number of atoms that make up the Earth. I listed the five most abundant elements in the planet, but several people thought I was referring to the atmosphere. Hence, they wrote in to tell me that nitrogen is the most common element at 79 per cent.

Well, the Earth can be divided into three major parts: the atmosphere, the crust and the core. The atmosphere is the lightest of these and the core is the most massive. So, even though nitrogen is the most abundant in the air, its mass is insignificant when considering the total planet.

One reader asked if it is possible to find out the total mass of the atmosphere over Kenya. The answer is yes; and I showed how to work it out in 2010 (ten years ago).

Here is a quick recapitulation: we start by measuring the air pressure on the ground and noting that pressure is force (or weight) per unit area.  Then we multiply the pressure by the area to get the weight. Finally, we calculate mass by dividing the weight by the acceleration due to gravity.

The average atmospheric pressure on the ground is about 100,000 newtons per square metre (notice the small letter “n” in newtons). The area of Kenya is about 580,000 square kilometres; or about 580 billion square metres. Multiplying the two yields a total force (weight) of 58,000 trillion newtons. This the number 58 followed by 15 zeroes.

Finally, we divide this force by the average acceleration due to gravity of 10 newtons per kilogram and we get that the total mass of all the air in Kenya is about 5,800 trillion kilograms.

In the 2010 article, I worked out the mass of the atmosphere over the entire planet and the result was 500 billion, billion kilograms; that is the number 5 followed by 20 zeroes. Now that’s a very large mass, but it pales into insignificance when compared to that of the plane – 6 with 24 zeroes.

If we divide the mass of the atmosphere by that of the planet, the answer comes to 0.0001. That is, the atmosphere is just one ten-thousandth of the earth’s mass. In other words, it is 0.01 per cent.

Clearly then, nitrogen makes 79 per cent of the section that is 0.01 per cent of the planet. Truly, a big fish in a small pond!