An atom is the smallest unit of matter that still retains its macro properties. But it’s also a quantum entity. It interacts with all four fundamental forces of nature. It connects our macro world to the quantum world.
Ernest Rutherford found that the atom has a very small nucleus, 10^-15 m across, while most of it is empty with electrons orbiting about 100,000 times further away in a cloud.
But if electrons were orbiting around a nucleus, they must be undergoing a constant acceleration. And accelerating charges emit electromagnetic waves, or light. If this were true, they would quickly radiate away all their energy and spiral inwards, crashing into the nucleus. It would be impossible for atoms to be stable.
To resolve this conundrum, French physicist Louis de Broglie proposed that electrons behave like waves too. These waves can only exist at specific, discrete energy levels. Electrons "orbit" the nucleus in stable patterns where the wave fits perfectly in the orbit. These energy levels are like rungs on a ladder; electrons can jump between them but can’t exist in between.
When an electron drops from a higher energy level to a lower one, it releases energy in the form of a photon. And when electrons absorb a photon, it leaps them to a higher energy state. These electron energy transitions in any atom are unique for every element determined by the number of protons in the nucleus. They act as atomic fingerprints, producing distinct spectral lines that scientists can use to identify them.
The photon is the carrier of the electromagnetic force, enabling interactions between charged particles. it's the second strongest fundamental force, and determines the unique chemical and physical properties of each element.
But if protons inside the nucleus are all positively charged, they should repel each other due to the electromagnetic repulsion. But at distances smaller than the radius of a nucleus, the strong force is 137 times stronger and overwhelms the electromagnetic repulsion between protons.
But protons and neutrons are made up of even smaller particles called quarks. Inside them, you’ll find two types: up quarks and down quarks. Up quarks carry a positive electric charge of +2/3, and down quarks a negative charge of -1/3, but they can only combine to form particles with an integer charge. Protons are made of two up quarks and one down quark, combining for +1 charge, while neutrons consist of one up quark and two down quarks, making them neutral.
They don’t just have an electric charge, they also carry a "color charge." This is not like optical colors, but a metaphor to describe a kind of charge. There are three color and anticolor charges: red, green, and blue. For a particle to exist, its combination of color charges must be neutral or "colorless" either by containing a combination red, green and a blue charged quarks, or containing a color and anti-color charged quarks. Inside a proton or neutron, the three quarks are constantly exchanging gluons, which are the carriers of the strong force. The constant exchange of colors creates the strong force.