Nucleus: The nucleus is the central core of the atom and contains positively charged protons and uncharged neutrons. The nucleus is very dense and contains most of the atom's mass.

Electrons: Electrons are negatively charged particles that orbit the nucleus in fixed paths called shells. Each shell can hold a specific number of electrons, and the number of shells increases as you move away from the nucleus.

Electron Shells: The electron shells are labeled with the letters K, L, M, N, and so on. The K shell is the innermost shell and can hold up to 2 electrons. The L shell can hold up to 8 electrons, the M shell can hold up to 18 electrons, and so on.

Electron Subshells: The electron shells are further divided into subshells, which are labeled with the letters s, p, d, and f. The s subshell can hold up to 2 electrons, the p subshell can hold up to 6 electrons, the d subshell can hold up to 10 electrons, and the f subshell can hold up to 14 electrons.

Electron Configuration: The electron configuration of an atom describes the arrangement of electrons in the electron shells and subshells. The electron configuration is written using the shell letters and subshell letters, followed by the number of electrons in each subshell. For example, the electron configuration of helium is 1s2, which means that helium has 2 electrons in the K shell, both in the s subshell.

The Bohr model of the atom is a simplified representation of the atom that helps us to understand the basic structure of atoms and how electrons interact with the nucleus.