An atom comprises three main types of particles: protons, neutrons, and electrons. The structure of an atom basically shows how these particles are arranged in an atom. The discovery of atoms defined the start of a new and detailed world of Science. An atom, no matter how small, includes a variety of essential concepts. In this article, we will study the basic structure of an atom.
1. Basic Constituents of an Atom An atom consists of three subatomic particles- electrons, protons, and neutrons.
The nucleus of the atom, which is located in the centre, contains protons and neutrons. The number of protons in an atom's nucleus is known as its atomic number, and it determines which element the atom belongs to. Carbon atoms, for example, are all atoms with six protons. Shells or energy levels of electrons orbit the nucleus. The number of protons in the nucleus is always equal to the number of electrons in the atom. The number of electrons in the outermost shell is known as the valence electrons, and it determines the atom's chemical properties.
1.1 Electrons Experiments with cathode ray tubes by J.J. Thomson, a physicist from the late 1800s, demonstrated that all atoms contain tiny negatively charged subatomic particles or electrons. Electrons are subatomic particles that have a negative charge and a very small mass compared to protons and neutrons. They are found outside the nucleus of an atom, orbiting in shells or energy levels. The number of electrons in an atom is always equal to the number of protons in the nucleus, giving the atom a neutral overall charge. Electrons are responsible for the various chemical properties of an atom, as they determine the number of electrons in the outermost shell or valence shell. The valence electrons in the outermost shell determines how easily an atom can gain or lose electrons in chemical reactions, which in turn determines its reactivity and chemical properties. Electrons are also responsible for electromagnetic force and electrical conductivity, as they are the particles that carry the electric current. The movement of electrons in a material is what generates electricity. The electrons in the outermost shell of an atom are also responsible for the absorption and emission of light, which is how atoms give off light and create spectra.
1.2 Protons Although Goldstein discovered a positively charged particle, Ernest Rutherford is credited with discovering protons. Rutherford is credited with discovering alpha particles, which are much larger charged particles (more than 1000 times larger than electrons) produced during Uranium's radioactive decay. Protons are subatomic particles that are found in the nucleus of an atom, along with neutrons. They have a positive charge and a mass that is slightly less than that of a neutron. 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 6 protons are carbon atoms. The number of protons determines the chemical properties of an atom, as the number of protons in the nucleus determines the number of electrons in the outermost shell of the atom or the valence electrons. The protons in the nucleus also determine the stability of an atom. The protons are held together by the strong nuclear force, but when the number of protons becomes too large, the repulsion between them can overcome the force holding them together, making the nucleus unstable and prone to nuclear decay.
1.3 Neutrons Chadwick (1932) used alpha particles to bombard a thin sheet of Beryllium (Be). He observed the emission of electrically neutral particles with a mass slightly greater than that of protons. He gave the particles the name neutrons. Neutrons are subatomic particles that are found in the nucleus of an atom, with protons. They have a neutral charge with a mass that is slightly greater than that of protons. Neutrons play an important role in determining the stability of an atom, as well as its properties. The number of neutrons can vary even within the same element. Isotopes are the atoms of the same element that differ in the number of neutrons. For example, C-12 and C-14 are isotopes of carbon, because they have the same number of protons but different numbers of neutrons. Neutrons have no electrical charge and are not directly involved in chemical reactions. They play a crucial role in the stability of heavy elements and the behavior of nuclear reactions.
2. Nucleus The nucleus is the central part of an atom that contains protons and neutrons. The protons and neutrons are held together in the nucleus by the strong nuclear force, that is responsible for holding the protons and neutrons together in the nucleus. The strong nuclear force is many times stronger than the electromagnetic force that holds electrons in orbit around the nucleus. The nucleus is incredibly small compared to the rest of the atom. It's typically only about 10-14 meters across, while the atom as a whole is around 10-10 meters. Despite being small, the nucleus contains almost all of the atom's mass, and it's also responsible for the atom's behavior in many cases, like in nuclear reactions or radioactive decay.
3. Basic Structure of an Atom By 1932, the combined perception of the ancient Greeks, Dalton, Thomson, Rutherford, Chadwick, and many others had resulted in an understanding of an atomic structure that resembled what we see today in popular depictions of the atom. Every atom is made up of a dense central core of matter known as the nucleus. There are one or more protons in that nucleus. Electrons orbit the dense, positively charged core. The identity of elements is determined by the number of protons in the nucleus.
If we add another proton to the atom’s nucleus and another electron to its cloud to balance out the charge, we will have a new element, because the number of protons in the nucleus has changed. A detailed study of the Atomic Model can be done here.
4. Electron Distribution in Various Shells The Bohr-Bury Scheme proposed arranging particles in different orbits. The rules for writing the number of particles in different orbitals are as follows:
The formula 2n2 gives the maximum number of electrons that can be accommodated in each shell, n=1, 2, 3, 4. K = 2 × (1)2 = 2 L = 2 × (2)2 = 8 M = 2 × (3)2 = 18 N = 2 × (4)2 = 32
The outermost orbit can hold up to eight electrons.
Following the stepwise filling of orbitals, electrons first fill the inner levels.
Key Takeaways
An atom consists of three sub-atomic particles called electrons, protons, and neutrons.
The nucleus is the central part of the atom.
Electrons are negatively charged particles and revolve around the nucleus.
Protons are positively charged particles and are present in the nucleus.
Neutrons are neutral particles that are present in the nucleus with protons.