An atom is the basic unit of matter. It is the smallest unit of an element that retains the properties of that element. Atoms are made up of three types of subatomic particles: protons, neutrons, and electrons. The protons and neutrons are located in the center of the atom, called the nucleus, while the electrons orbit the nucleus in shells or energy levels.
The number of protons in the nucleus of an atom is called the atomic number, and it determines the element to which the atom belongs. For example, all atoms with six protons in their nucleus are carbon atoms, while all atoms with eight protons in their nucleus are oxygen atoms. The number of neutrons in the nucleus can vary, but the number of protons determines the atomic number and thus the element.
The electrons in an atom are arranged in shells around the nucleus. The number of electrons in the outermost shell, or valence shell, determines the chemical properties of the atom. Atoms with full valence shells are generally more stable than atoms with incomplete valence shells, which tend to react with other atoms to fill their valence shells.
Atoms can also be combined to form molecules, which are the basic building blocks of compounds. The chemical properties of a compound are determined by the types and numbers of atoms that make it up, as well as their arrangement and bonding.
In summary, An atom is the basic unit of matter, made up of protons, neutrons, and electrons, the number of protons in the nucleus of an atom determines the element to which the atom belongs, and the arrangement of electrons in the energy levels of an atom determines its chemical properties.