Atomic Structure

Atom - The name atom originated from the Greek Word atomos, meaning "indivisible" or uncuttable, or indivisible, something that cannot be divided further. Atoms are the basic building blocks of matter that make up everyday objects. The atom is a basic unit of matter that consists of a dense central nucleus surrounded by negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons.

Some of the examples of atom are - neon, Ne; hydrogen, H; plutonium, Pu; protium, an isotope of hydrogen; a calcium ion, Ca2+.

Hydrogen is the smallest and lightest known atom.

Molecule- A molecule is the smallest particle in a chemical element or compound that has the chemical properties of that element or compound. Molecules are made up of atom s that are held together by chemical bonds. These bonds form as a result of the sharing or exchange of electron s among atoms.

Molecules are always in motion. In solids and liquids, they are packed tightly together. In a solid, the motion of the molecules can be likened to rapid vibration. In a liquid, the molecules can move freely among each other, in a sort of slithering fashion. In a gas , the density of molecules is generally less than in a liquid or solid of the same chemical compound, and they move even more freely than in a liquid.

Mole - The mole (abbreviation, mol) is the Standard International ( SI ) unit of material quantity. One mole is the number of atom s in precisely 12 thousandths of a kilogram (0.012 kg) of C-12, the most common naturally-occurring isotope of the element carbon. This number is equal to approximately 6.022169 x 10 23 , and is also called the Avogadro constant.

The mole is of interest primarily to chemists and physicists. Sometimes, larger or smaller units are defined by attaching power-of-10 prefix multiplier s. For example, a kilomole (1 kmol) is 10 3 mol, or approximately 6.022169 x 10 26 , and a millimole (1 mmol) is 10 -3 mol, or approximately 6.022169 x 10 20 .

Atomic Mass- Atomic mass or atomic weight is the average mass of atoms of an element, calculated using the relative abundance of isotopes in a naturally-occurring element.

Examples: The atomic mass of carbon is 12.011; the atomic mass of hydrogen is 1.0079

Molar Mass - Molar Mass is the mass of one mole of a substance (6.02 x 1023 formula units). The unit for molar mass (note it is the mass of a mole) is grams/mole.

Atomic Weight - Atomic Weight is the molar mass of an element.

Formula Weight - Formula Weight is the molar mass of an IONIC compound.

Molecular Weight - Molecular Weight is  the molar mass of a COVALENT compound.

Molecular Mass - Molecular Mass is the mass of a given molecule (NOT MOLES of molecules). The units are atomic mass units (amu).

1amu =1.660 538 782×10–27 kg

In Other Words - 1 amu is 1/12 the mass of a carbon-12 atom which has a mass of 12.0000.

Constituents of an Atom- The basic particles of an atom are an electron, a proton and a neutron.

Electron - An electron is a negatively charged subatomic particle. It can be either free (not attached to any atom), or bound to the nucleus of an atom. Electrons in atoms exist in spherical shells of various radii, representing energy levels. The larger the spherical shell, the higher the energy contained in the electron.

Proton - A proton is a subatomic particle found in the nucleus of every atom.  The particle has a positive electrical charge, equal and opposite to that of the electron. If isolated, a single proton would have a mass of only 1.673 x 10-27 kilogram, just slightly less than the mass of a neutron.

Neutron - A neutron is a subatomic particle contained in the atomic nucleus. It has no net electric charge, unlike the proton's positive electric charge. The number of neutrons in an atomic nucleus determines the isotope of that element.

Atomic number- The number of protons in the nucleus of an atom determines an element's atomic number. In other words, each element has a unique number that identifies how many protons are in one atom of that element. For example, all hydrogen atoms, and only hydrogen atoms, contain one proton and have an atomic number of 1. All carbon atoms, and only carbon atoms, contain six protons and have an atomic number of 6. Oxygen atoms contain 8 protons and have an atomic number of 8. The atomic number of an element never changes, meaning that the number of protons in the nucleus of every atom in an element is always the same.

Mass Number - The sum of the number of neutrons and protons in an atomic nucleus. Also called nucleon number.

Isotopes- Atoms of the same element can have different numbers of neutrons; the different possible versions of each element are called isotopes. For example, the most common isotope of hydrogen has no neutrons at all; there's also a hydrogen isotope called deuterium, with one neutron, and another, tritium, with two neutrons.

Isobars - Isobars are atoms (nuclides) of different chemical elements that have the same number of nucleons. Correspondingly, isobars differ in atomic number (or number of protons) but not in mass number. An example of a series of isobars would be 40S, 40Cl, 40Ar, 40K, and 40Ca. The nuclei of these nuclides all contain 40 nucleons, however they contain varying numbers of protons and neutrons.

The word "isobar" was coined by Alexander von Humboldt. It is derived from the Greek word isos, meaning "equal" and baros, meaning "weight".

Isotones- Two nuclides are isotones if they have the same neutron number N, but different proton number Z. For example, boron-12 and carbon-13 nuclei both contain 7 neutrons, and so are isotones. Similarly, 36S, 37Cl, 38Ar, 39K, and 40Ca nuclei are all isotones of 20 because they all contain 20 neutrons. Despite its similarity to the Greek for "same stretching", the term was formed by replacing the "p" in "isotope" with "n" for "neutron".

Isoelectronic - According to IUPAC Gold Book - Two or more molecular entities are described as isoelectronic if they have the same number of valence electrons and the same structure, i.e. number and connectivity of atoms, but differ in some of the elements involved. Thus: CO, N2 and NO+ are isoelectronic. CH2=C=O and CH2=N=N are isoelectronic. CH3COCH3 and CH3N=NCH3 have the same number of electrons, but have different structures, hence they are not described as isoelectronic.

Thomson’s Model of an Atom

J. J. Thomson considered that the structure of an atom is something like a raisin bread, so that his atomic model is sometimes called the raisin bread model. He assumed that the basic body of an atom is a spherical object containing N electrons confined in homogeneous jellylike but relatively massive positive charge distribution whose total charge cancels that of the N electrons. The schematic drawing of this model is shown in the following figure. Thomson's model is sometimes dubbed a plum pudding model.

Rutherford’s Atomic Model

British physicist Ernest Rutherford (1871-1937) has created a planetary model of atom exploring a dispersion of a-particles transiting through a thin metal foil. According to this model an atom looks like a tiny planetary system, in which the forces of an electrical attraction operate.
The center of an atom is a positively charged kern. Almost all mass of an atom is focused there. Negatively charged electrons rotate around the kern.


When white light passes through a glass prism it split into seven colors of light, these color bands of light are called Spectrum as shown in the figure below.

The seven colors of light are also termed in short as VIBGYOR. This phenomenon is similar to the formation of rainbow in the sky during rainy days. A rainbow in the sky is formed when Sun’s rays passes through the water droplets hanging in the sky resulting in the scattering of light into seven different colors.

Tiny droplets of water refract the white light from the Sun and create a spectrum of colors similar to what happens in a prism. Since the droplets are spheres, the light is reflected internally in the droplets and the spectrum or rainbow returns toward the direction of the light. That is why the Sun will always be behind you when you see a rainbow.

Zeeman’s Effect

The atomic energy levels, the transitions between these levels, and the associated spectral lines discussed to this point have implicitly assumed that there are no magnetic fields influencing the atom. If there are magnetic fields present, the atomic energy levels are split into a larger number of levels and the spectral lines are also split. This splitting is called the Zeeman Effect.

Stark’s Effect

The splitting of single spectral lines of an emission or absorption spectrum of a substance into several components when the substance is placed in an electric field. The effect occurs when several electron orbitals in the same shell, which normally have the same energy level, have different energies due to their different orientations in the electric field. The Stark effect is named after its discoverer, German physicist Johannes Stark (1874-1957).

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