Atomic Structure

A single grain of sand contains roughly 50 quintillion atoms (5 x 10¹⁹). Each one is mostly empty space. If the nucleus were a marble on the 50-yard line, the nearest electron would be orbiting somewhere in the parking lot. Yet that tiny nucleus holds 99.97% of the atom's mass.

Every atom has the same three components. Protons carry positive charge and sit in the nucleus. Neutrons carry no charge and sit alongside them. Electrons carry negative charge and occupy a cloud of probability around the nucleus. The number of protons defines the element: 1 proton is hydrogen, 6 is carbon, 92 is uranium. Change the proton count and you change what the atom is entirely.

For energy, electrons do the heavy lifting. Chemical reactions (burning coal, running a battery) work by rearranging electrons between atoms. Electricity is electrons flowing through conductors. Solar cells work because photons knock electrons loose. Nuclear reactions, by contrast, rearrange the nucleus itself, which is why they release roughly a million times more energy per atom than chemical reactions.

Worked Example

A carbon atom has 6 protons, 6 neutrons, and 6 electrons.

• Where is the mass? Protons and neutrons each weigh about 1 atomic mass unit. Electrons weigh roughly 1/1,836 of that. So 12 units of mass sit in the nucleus; 0.003 units orbit outside it.

What fraction of the atom's mass is in the electrons?

About 0.03%. Essentially zero. The nucleus holds virtually all the mass, which is why nuclear reactions (which alter the nucleus) involve vastly more energy than chemical reactions (which only rearrange electrons).

Understanding atomic structure is the gateway to both chemistry (how atoms bond) and nuclear physics (how atoms split or fuse to release energy).

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