The discovery of atomic energy came from 19th century chemistry. Chemists identified the basic elements and weighed their mass. They found that all the elements seemed to be multiples of the weight of hydrogen, but not quite. Atomic energy explained these anomalies.

The atomic mass of an oxygen atom is 16 and the weight of a hydrogen atom is 1.008. Today chemists use Carbon 12 as the standard of measurement, although in the past they used Oxygen 16 which caused some problems.

Whe Mendeleev created the periodic table, he arranged all known elements according to their atomic weight. Every 8th element had the same chemical properties as the first one, such as the noble gasses. Mendeleev filled the entire table and left blank spaces for unknown elements. He assumed that there must be undiscovered elements in this blank spaces and that these unknown elements would continue the pattern of increases in atomic mass and chemical property.

Mendeleev was correct. They discovered more and more missing elements and filled the periodic table.

Later on, chemists discovered isotopes. Atomic weights varied in each element. Oxygen had three stable isotopes – 16, 17, 18. Isotopes posed a puzzle. All atomic masses were really multiples of the weight of hydrogen. Technically, they discovered the mass of photons in an atom. The number of photons determined the mass of an element.

With this knowledge, transmutation is possible. One really can convert mercury into gold, vindicating alchemism. Of course, transmuting mercury into gold is not worth the cost, and at any rate, it would devalue gold.

However, there was a problem. Each atom was within 1% of a multiple of the hydrogen element. The basic arithmetic did not add up. Even stranger, the range changed according to a pattern. Here’s the chart. The curve increases until it peaks near Iron and Nickle and decreases for all subsequent elements.

Take hydrogen and helium. Hydrogen weighs 1.0079 while helium weighs 4.0026. This is a little less than 1% under the weight of 4 hydrogen atoms (4.032). What happens to the 0.03 of matter? 1% of the mass is lost or gained during transmutation into another element.

And that is explained by nuclear binding energy. Einstein described how mass can be converted to energy. When elements are fused together or split apart, matter converts to energy and is released. Nuclear fusion releases binding energy.

More specifically, nuclear fusion releases energy with elements lighter than iron, but requires energy to fuse elements heavier than iron. Nuclear Fission is the exact opposite. Splitting elements heavier than iron, such as uranium, releases energy, while splitting lighter elements such as helium requires energy.

As it turns out, that missing 0.03 of mass is a lot of energy.