HOW COME? Attraction of atomic subparticles

Since protons have a positive charge, and particles with the same charge repel, how do atoms hold together? - asks a reader.

In the teeny-tiny world of atoms, the old saying is really true: Opposites attract - so strongly, in fact, that the results can be explosive. When a negative charge builds up in the bottom of a thundercloud, the charge attracts positively charged particles in the earth below. And then an enormous spark - a lightning bolt - connects cloud and ground. Negatively charged electrons zigzag down to meet positively charged protons on their way up, with a crack and boom.

Atoms come in a range of sizes. But every atom has a center (nucleus), containing even tinier particles called protons, surrounded by a swarm of electrons. The smallest atom, hydrogen, has just one proton, balanced by one lone electron. But every atom bigger than simple hydrogen also has particles called neutrons in its center. A uranium atom, for example, has 92 electrons orbiting its nucleus, where 92 protons and 146 neutrons peacefully coexist.

The electromagnetic force holds entire atoms together. The overwhelming attraction of negatively charged electrons to positively charged protons keeps atoms bound into bundles, the building blocks of matter, from your body to your desk lamp. If it weren't for the fact that protons and electrons find each other so appealing, the entire universe would crumble to dust.

So what about the neutrons? Neutrons have no electrical charge - they're neutral, hence their name. Protons aren't electrically attracted to neutrons, and neither are electrons.

But neutrons aren't just fillers at the center of atoms. They actually play a crucial role in keeping multiple-proton atoms intact. Particles with the same electrical charge, like protons, repel, shoving each other away like the south ends of two bar magnets. Without neutrons playing a kind of buffer role, the nucleus of an atom would simply disintegrate.

How? Neutral neutrons and positive protons don't repel each other, but they do feel a kind of nonelectrical attraction. Enter the Strong Force. The strong nuclear force is one of the four fundamental forces in the universe, which include electromagnetism, the weak nuclear force, and gravity. The strong force is more than 1,000 times stronger than familiar gravity. But unlike gravity, the strong force operates only inside a nucleus.

The strong force attracts protons to neutrons, neutrons to neutrons, and protons to protons. Unlike the electromagnetic force, which increases with nearness and weakens over distance, the strong force intensifies as particles start to separate. Let protons and neutrons move apart the tiniest bit, and the strong force increases, like an elastic band pulling harder as it stretches.

Scientists say that the strong force does its work through carrier particles called gluons. Protons and neutrons are made of smaller particles called quarks. In the nucleus, the quarks exchange gluons like ping-pong balls, holding the atomic center - the entire universe of matter - together.