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WBCS Preliminary(Chemistry): Atomic Structure

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  • An atom is the smallest particle of the element that can exist independently and retain all its chemical properties.Atoms are made up of fundamental particles: electrons, protons and neutrons.
  • Dalton's Atomic Theory: John Dalton provided a simple theory of matter to provide theoretical justification to the laws of chemical combinations in 1805. The basic postulates of the theory are:
    • All substances are made up of tiny, indivisible particles called atoms.
    • Atoms of the same element are identical in shape, size, mass and other properties.
    • Each element is composed of its own kind of atoms. Atoms of different elements are different in all respects.
    • Atom is the smallest unit that takes part in chemical combinations.
    • Atoms combine with each other in simple whole number ratios to form compound atoms called molecules.
    • Atoms cannot be created, divided or destroyed during any chemical or physical change.
  • Representation of an Atom by a Symbol: Dalton was the first scientist to use the symbols for elements in a very specific sense. When he used a symbol for an element he also meant a definite quantity of that element, that is, one atom of that element. A symbol signifies a shorthand representation of an atom of an element. The symbol of any element is based on the English name or Latin name (written in English alphabets) and many of the symbols are the first one or two letters of the element’s name in English. The first letter of a symbol is always written as a capital letter (uppercase) and the second letter as a small letter (lowercase). Examples are: (i) hydrogen- H (ii) aluminium- Al and not AL (iii) cobalt- Co and not CO. Symbols of some elements are formed from the first letter of the name and a letter, appearing later in the name. Examples are: (i) chlorine, Cl, (ii) zinc, Zn etc.
  • Other symbols have been taken from the names of elements in Latin, German or Greek. For example, the symbol of iron is Fe from its Latin name ferrum, sodium is Na from natrium, potassium is K from kalium. Therefore, each element has a name and a unique chemical symbol.
  • Size of the Atom/ Elements: Atoms are very small, they are smaller than anything that we can imagine or compare with. One hydrogen atom, the smallest atom known, is approximately 5 x 10-8 mm in diameter. Atomic radius is measured in nanometres. 1 m = 109 nm.
  • Atomic Mass: The mass of a particular atom is taken as a standard unit and the masses of other atoms are related to this standard. Hydrogen being the lightest element and being the smallest atom was chosen and assumed to have a mass of 1. An atom of hydrogen was assigned an atomic mass equal to one atomic mass unit (a.m.u). The number does not signify the mass of an atom in grams. It is just a pure number. The masses of atoms of other elements were compared to that of hydrogen, in order to find their atomic mass relative to it. If one atom of sulphur weighs as much as 32 atoms of hydrogen, then the relative atomic mass of sulphur is 32 a.m.u. This way of defining the mass of one atom of hydrogen has its difficulties. While the mass of one atom of hydrogen is considered as 1 atomic mass unit, hydrogen gas in its natural state has 3 isotopes of atomic mass 1, 2 and 3 respectively. Thus average mass works out to be 1.008 a.m.u rather than 1 a.m.u. This in turn complicates the atomic masses of all other elements. Later on, an atom of oxygen was preferred as standard by taking its mass as 16 units. However, in 1961 for a universally accepted atomic mass unit, carbon-12 isotope was chosen as the standard reference for measuring atomic masses. One atomic mass unit is a mass unit equal to exactly onetwelfth (1/12th) the mass of one atom of carbon-12. The relative atomic masses of all elements have been found with respect to an atom of carbon-12. It is equal to 1.66 x 10-24 g.
  • Molecule: A molecule is in general a group of two or more atoms that are chemically bonded together, that is, tightly held together by attractive forces. A molecule can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance. Atoms of the same element or of different elements can join together to form molecules.
  • The molecules of an element are constituted by the same type of atoms. Molecules of many elements, such as argon (Ar), helium (He) etc. are made up of only one atom of that element. But this is not the case with most of the nonmetals. For example, a molecule of oxygen consists of two atoms of oxygen and hence it is known as a diatomic molecule, O2. If 3 atoms of oxygen unite into a molecule, instead of the usual 2, we get ozone. The number of atoms constituting a molecule is known as its atomicity.
  • Atoms of different elements join together in definite proportions to form molecules of compounds. Compounds composed of metals and nonmetals contain charged species. The charged species are known as ions. An ion is a charged particle and can be negatively or positively charged. A negatively charged ion is called an ‘anion’ and the positively charged ion, a ‘cation’. Take, for example, sodium chloride (NaCl). Its constituent particles are positively charged sodium ions (Na+) and negatively charged chloride ions (Cl–). Ions may consist of a single charged atom or a group of atoms that have a net charge on them. A group of atoms carrying a charge is known as a polyatomic ion.
  • Chemical Formulae: The chemical formula of a compound is a symbolic representation of its composition. The chemical formulae of different compounds can be written easily.
  • The combining power (or capacity) of an element is known as its valency. Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound. The valency of the atom of an element can be thought of as hands or arms of that atom.
  • The simplest compounds, which are made up of two different elements are called binary compounds. While writing the chemical formulae for compounds, we write the constituent elements and their valencies. Then we must crossover the valencies of the combining atoms.
  • The formulae of ionic compounds are simply the whole number ratio of the positive to negative ions in the structure .
  • Molecular Mass: The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. It is therefore the relative mass of a molecule expressed in atomic mass units (u).
  • The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound. Formula unit mass is calculated in the same manner as we calculate the molecular mass. The only difference is that we use the word formula unit for those substances whose constituent particles are ions. Scientists use the relative atomic mass scale to compare the masses of different atoms of elements. Atoms of carbon-12 isotopes are assigned a relative atomic mass of 12 and the relative masses of all other atoms are obtained by comparison with the mass of a carbon-12 atom.
  • Mole Concept: Since it is not possible to calculate the weight of particles individually, a collection of such particles called mole is taken for all practical purposes. It was discovered that the number of atoms present in 12g of carbon of 12C isotope is 6.023 x 1023atoms. This is referred to as Avogadro number after the discoverer Avogadro. A mole of a gas is the amount of a substance containing 6.023 x 1023 particles. It is a basic unit of the amount or quantity of a substance. The substance may be atoms, molecules, ions or group of ions.
  • Mass of 1 mole of a substance is called its molar mass. One mole of any gas at STP will have a volume of 22.4 L. This is called molar volume.
  • Credit for the discovery of electron and proton goes to J.J. Thomson and E.Goldstein, respectively. J.J. Thomson proposed that electrons are embedded in a positive sphere.
  • Rutherford’s alpha-particle scattering experiment led to the discovery of the atomic nucleus. Rutherford’s model of the atom proposed that a very tiny nucleus is present inside the atom and electrons revolve around this nucleus. The stability of the atom could not be explained by this model.
  • Neils Bohr’s model of the atom was more successful. He proposed that electrons are distributed in different shells with discrete energy around the nucleus. If the atomic shells are complete, then the atom will be stable and less reactive.
  • J. Chadwick discovered presence of neutrons in the nucleus of an atom. So, the three sub-atomic particles of an atom are: (i) electrons, (ii) protons and (iii) neutrons. Electrons are negatively charged, protons are positively charged and neutrons have no charges.
  • The discovery of the electron, proton and neutron was the starting point of new avenues of research in science, which gave physicists an insight into the structure and nature of the atoms of matter. An atom is made up of three elementary particles, namely electrons, protons and neutrons. Electrons have a negative charge, protons have a positive charge and neutrons have no charge. Neutrons are neutral. Due to the presence of equal number of negative electrons and positive protons the atom as a whole is electrically neutral. Based on the above findings, one can say that the atom has two major divisions.
    • The first is the centre of an atom, called its nucleus. The protons and neutrons are located in the small nucleus at the centre of the atom. Due to the presence of protons the nucleus is positively charged.
    • The second are electrons, which revolve around the nucleus in different shells (or orbits). Shells of an atom are designated as K,L,M,N,….The space around the nucleus in which the electrons revolve, determines the size of the atom.
  • The maximum number of electrons present in a shell is given by the formula 2n2, where ‘n’ is the orbit number or energy level index, 1,2,3,…Hence the maximum number of electrons in different shells are as follows: first orbit or K-shell will be = 2 .12 = 2, second orbit or L-shell will be = 2 .22 = 8, third orbit or M-shell will be = 2 .32 = 18, fourth orbit or N-shell will be = 2 .42= 32, and so on. The maximum number of electrons that can be accommodated in the outermost orbit is 8. Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a step-wise manner.
  • Valency: The electrons present in the outermost shell of an atom are known as the valence electrons. It is the decisive shell during a chemical reaction. The electrons of only this outermost shell are involved during chemical combinations; electrons are either given out from the outermost shell, or accepted into the outermost shell, or shared with the electrons in the outermost shell of another element. Elements having same number of valence electrons in their atoms possess similar chemical properties. The number of the valence shell in an atom determines its position in the Periodic Table i.e. the period to which the element belongs. Elements having 1, 2 or 3 electrons in the valence shell are metals. Exception is H and He. Elements having 4 to 7 electrons in their valence shell are non-metals. Valency is the combining capacity of an element. It is the number of electrons in an atom that actually take part in bond formation. For example, carbon atom with an atomic number 6 has 4 valence electrons.
  • Calculation of Valency: The number of valence electrons is the valency of the element. The valency of an element can also be calculated by finding the number of electrons required to complete octet (8). If the outermost shell of an atom is completely filled, its valency = 0. The outermost shells of the noble gases helium, neon, argon, krypton etc. are completely filled. Hence their valency is zero. Such elements are very un-reactive and inert by nature.
  • Atomic Number: The nuclei of atoms is made up of protons and neutrons. These two components of the nucleus are referred to as nucleons. The electrons occupy the space outside the nucleus. Since an atom is electrically neutral, the number of protons in the nucleus is exactly equal to the number of electrons. This number is the atomic number given by the symbol Z.
  • Mass Number: The total number of protons and neutrons present in one atom of an element is known as its mass number. Mass number = number of protons + number of neutrons.
  • Isotopes: Isotopes are atoms of the same element, which have different mass numbers. It is interesting to note that atoms of a given atomic number can have different number of neutrons. For example, take the case of hydrogen atom, it has three atomic species, namely protium (11 H), deuterium ((21 H or D) and tritium ((31 H or T). The atomic number of each one is 1, but the mass number is 1, 2 and 3, respectively. All isotopes of an element have the same number of valence electrons thus have identical chemical properties. The physical properties of the isotopes are different due to the difference in the number of neutrons in their nuclei. The densities, melting points and boiling points etc., are slightly different.
  • Isobars: Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars. These have different number of protons but equal sum of number of protons and neutrons.
  • Isotones: The atoms of different elements, which have the same number of neutrons but different atomic numbers, are called isotones.
  • Radioactivity: Radioactivity is a nuclear phenomenon. It is the spontaneous emission of radiation from the nucleus. In 1899, the study of radioactivity was taken up by Ernest Rutherford. He placed a little radium at the bottom of a small lead box and subjected the rays that emerged from it to the action of a very strong magnetic field at right angles to their direction. He found that the rays separated into three distinct constituents. Rutherford called the three types of radiation alpha (α), beta (β) and gamma (g) rays. The α-rays were deflected in a direction opposite to that of β-rays and α-rays carried a positive charge, β-rays carried a negative charge and those which passed undeviated were neutral or uncharged were g-rays.

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