An atom is the smallest unit of a chemical element that retains the chemical properties of that element. It is composed of three main subatomic particles: protons, neutrons, and electrons.
The structure of an atom can be described as follows:
Protons: Protons are positively charged particles found in the nucleus of an atom. They have a relative mass of 1 and carry a positive electric charge of +1.
Neutrons: Neutrons are electrically neutral particles also located in the nucleus. They have a relative mass of 1, similar to protons, but carry no electric charge (they are neutral).
Electrons: Electrons are negatively charged particles that orbit the nucleus in specific energy levels or shells. They have a much smaller mass compared to protons and neutrons (about 1/1836th the mass of a proton) and carry a negative electric charge of -1.
The number of protons in an atom determines its atomic number and defines the element to which it belongs. For example, an atom with one proton is hydrogen, while an atom with six protons is carbon. The number of neutrons can vary within an element, giving rise to different isotopes of the same element, which have the same number of protons but different numbers of neutrons.
The electrons in an atom are distributed in different energy levels or shells, with each shell capable of accommodating a specific number of electrons. The innermost shell, closest to the nucleus, can hold a maximum of 2 electrons, while the second shell can hold up to 8 electrons. Subsequent shells have higher capacities but follow certain rules based on quantum mechanics.
The arrangement of electrons in an atom follows the Aufbau principle, which states that electrons fill the lowest energy levels first before occupying higher energy levels. This principle, along with other rules, determines the electron configuration of an atom.
In summary, an atom consists of a central nucleus containing protons and neutrons, surrounded by electrons occupying specific energy levels or shells. The combination of protons, neutrons, and electrons determines the atomic number, atomic mass, and chemical properties of the element.