Atomic Structure in Chemistry, Summaries of Chemistry

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electrons revolve in fixed orbits or shells round the nucleus at a very high speed”, with each orbit associated with a fixed amount of energy. The electrons present in these shells neither lose nor gain energy until some external force is applied on it. Thus they maintain their position. As a result the inward force exerted by the nucleus is counterbalanced by the outward force of the moving electrons, thus preventing the electrons from falling into the nucleus and making the atom structurally stable. "ff NUCLEUS. LOWEST ENERGY HIGHEST ENERGY Fig. 1.9 Showing shells or orbits in an atom each with a fixed amount of energy. ATOMIC NUMBER (Z) The number of protons present in the nucleus of the atom of an element is called its atomic number, which is denoted by the letter Z. Since an electrically neutral atom has an equal number of protons and electrons, in such an atom : Atomic number (Z) iT] Number of protons Number of electrons. We have noted already that the atom of an element has a characteristic number of protons in its nucleus, which distinguishes it from the atoms of the other elements. The atomic number of an element never varies and it is a fixed value for a particular element. Velocity of bulcas SM arte Phe speed af an electron ahem Muth the spocilef bet MASS NUMBER (A) The sum of the number of protons and the number of neutrons present in the nucleus of the atom of an element is called the mass number of that element. It is denoted by the letter A. Mass number (A) = Number of protons + Number of neutrons Thus, if the atomic number and the mass number of an element are known, one can easily calculate the number of neutrons present in the nucleus of that element. Number of neutrons = Mass number (A) — Atomic number (Z) For example, take the sodium atom. Its atomic number is 11, and its mass number is 23. :. Number of neutrons = A -Z 23 - = 12. Symbolic representation of an element with its atomic number and mass number Suppose there is an element X with mass number A and atomic number Z. To symbolise this element its mass number should be written as superscript (on top) and its atomic number as subscript (at the bottom), ie. aX or x". Example : | 4 nit ISOTOPES You have learnt that each element has its unique atomic number, i.e. each atom of an element has the same number of protons in its nucleus. But the mass number of all the atoms of an clement may not be the same. In other words, the nuclei of all the atoms of an element (a5) have the same number of protons but some of them have a different number of neutrons. Therefore we can say that there are some atoms of an element that differ from the other atoms of that element with respect to mass number, not atomic number. Such atoms are called the isotopes of that element. Most elements found naturally have two or more isotopes. In fact most elements exist in nature as differing mixtures of their isotopes. | Isotopes are the atoms of the same element with the same atomic number but a different mass number due to the difference in the number of neutrons. . a Example : ilydrogen has three isotopes. Protium (ordinary hydrogen) — It has one (1H) proton and one electron butno neutron. Deuterium (heavy hydrogen) — It has one GH) proton, one neutron and one electron. Tritium (very heavy hydrogen) — It has one (H) proton, two neutrons and one electron. Probes 3H Tritlum 3H pez te 21, pistes], pr=1,e7=1, nad net mae Similarly, carbon has three isotopes, with their mass numbers being 12, 13 and 14 respectively. They are known as : Carbon-12 [!2C]; Carbon-13 [!3C] and Carbon-14 [/4C]. Oxygen too has three isotopes : Oxygen 16 ['80]; Oxygen 17 ['J0] and Oxygen 18 ['§0}. But chlorine has only two isotopes : Chlorine 35 (35CI] and Chlorine 37 (3C1). Properties of isotopes 1. The isotopes of an element have the same chemical properties, since there is no difference between their atomic numbers. 2. The isotopes of an element have the same electronic configuration, since they hold the same number of electrons. 3. The isotopes of an element differ from each other only in terms of physical properties, i.e. they have different mass, density, melting point, boiling point, erc. This is because of the difference in their respective mass numbers. ARRANGEMENT OF ELECTRONS AROUND THE NUCLEUS OF AN ATOM (ELECTRONIC CONFIGURATION) The distribution of electrons revolving in different orbits or shells round the nucleus of the atom of an element is called the electronic configuration of that element. —— of which Gi) The lithium atom has 3 electror occupy the first shell and the 3 occupies the > a The neon atom has 10 electrons. The first shell second shell (un) takes up 2 electrons and the second shell takes up 8 electrons. Note that the second (II or L) shell cannot occupy more than § electrons as per the formula 2n[2, 8] (iv) The potassium atom has 19 electrons, The first shell takes up 2 electrons and the second shell takes up 8 electrons. That leaves 9 electrons. According to the formula 2n’, all the 9 remaining electrons should oceupy the third shell, which can hold up tol8 electrons, But, since the outermost shell of an atom cannot have more than 8 electrons, the third shell takes up only 8 electrons and the fourth shell takes 1 the electronic configuration of potassium is K M N 2, 8 8 1 electron. Therefore, He u 0) i) co) Fig. 1.11 Distribution of electrons in successive shells in He, Li, Ne and K. VALENCE SHELL, VALENCE ELECTRONS AND VALENCY Valence shell : The outermost shell of an atom is known as its valence shell or valence orbit. Valence electrons : The electrons present in the valence shell of an atom are called valence electrons. The number of valence electrons varies from | to 8 for the atoms of the different elements. The valence electrons of an atom determine the valency of that element. tiv) Valency : Valency is the combining capacity of an atom with the atoms of other elements so as to form the molecules of compounds. Valency can, therefore be defined in terms of loss or gain of electrons as well. Why do atoms combine to form the molecules of compounds ? Atoms tend to combine to attain the electronic configuration of the nearest inert gas. You know that the maximum number of electrons present in the outermost shell of an atom is 8, except in the case of hydrogen and helium. If an atom has 8 electrons in its valence shell it acquires a chemically stable structure and thus becomes inert. Example + \\\ the inert (noble) gases. except heliur have § electrons int respective valence ondition ts called verter Heliuni fas 2 electrons onl thas only one orbit, which ] ® 1} = 2) cannot This condition is callec umd ¢ are condivions of che: all iner combing with t Ws ob wy t Therefore anything less than 8 electrons in its outermost shell (except in helium) makes an atom chemically unstable and reactive. However, most of the atoms known to us have electrons less than 8 in their respective valence shells. Therefore they tend to combine with other atoms to complete their octet and attain stability. During chemical combination one of the atoms donates one or more electrons, which are accepted by another atom to form a chemical bond between the two. Sometimes instead of gaining or losing electrons, atoms share their electrons too. fo) Diagrams of the atoms of the first twenty elements Elec. Config. 2,3 Elec, Config. 2,4 [Hydrogen }H Helium ‘He Lithium 7Le Beryllium °Be Elec. Config. 1 Elec. Config. 2 Elec. Config. a1 Elec. Contig. 2,2 Boron '\B Carbon %C Oxygen ':0 Elec, Contig. 2,5 Elec. Contig. 2.6 Fluorine "}F Elec. Config. 2.7 Neon “Ne Elec. Contig. 2.8 Sodium “Na Elec. Config. 2,8,1 Magnesium “* Elec. Contig. 2,82 ini 27, Aluminium “Al Elec. Contig. 2,8,3 Silicon Si Elec, Config. 2,8,4 Phosphorus $:P Elec. Config. 2.8.5 Sulphur 22S Elec. Config, 2,86 Chlorine *Cl Elec. Contig. 2.8.7 Argon Ar Elec. Contig. 2.8.8 Potassium “SK Elec. Contig. 2,8,8.1 Calcium “Ca Elec. Config. 2.8,8,2 A ‘chemical bond’ is the binding force between the two.or more atoms of a molecule. | Therefore the valency of an clement can be defined as the number of electrons lost, gained or shared by its atom during chemical combination. The loss or gain of electrons by atoms leads to the transformation of atoms into ions. | An ion is a charged particle formed by the loss or the gain of electrons by an atom. Ions are of two types : cations and anions. A positively charged ion is called a cation and a negatively charged ion is called an anion. Therefore such valency of an atom is called _electropositive or electronegative valency _ [collectively they are called ionic Positive valency : During chemical combination, when an atom donates electrons, to acquire a stable electronic configuration and thus becomes a positively charged ion, it is said to have positive (electropositive) valency. All the metals (and hydrogen) have positive valency, since they have electrons numbering from | to 3 in their valence shells. During chemical combination they donate these electrons, This results in a greater number of protons and a lesser number of electrons left with them. Therefore these atoms become positively charged, forming cations. Depending upon the number of electrons donated, these elements are monovalent (Na*) or bivalent (Mg?*) or trivalent (AI**). FORMATION OF ELECTROPOSITIVE IONS | electron, Nas Example 1: No Unstable sodium atom Stable sodium ion No, of electrons = 11 No, of electrons = 10 Mectront 2¢K), 8 (Lb). 1 (Mo configuration Electronic configuration 2 (K), 8 CL) (nearest to neon) Example 2: Mg —=2) or trivalent (N*). [Hydrogen is an exception because, though it is a non-metal, it has greater tendency to lose electrons]. Note : Carbon is a non-metal with 4 valence electrons. It neither donates nor accepts electrons. It shares its electrons. FORMATION OF ELECTRONEGATIVE IONS +1 electron, - _+l electron, oy Example 1: C) Unstable chlorine atom Stable chlaride ion No, of electrons = 17 No. of electrons = 18 Electronic configuration Electronic configuration 2 (K), 8 (L), 7 (MD) 2 (K), 8 (L), 8 (M) {nearest to neon) r the 4 he In the case of monovalency - sign, not + 1or-1 Example 2; 9 —t2lecttons , ¢: Stable oxide ion No. of electrons = 10 Unstable oxygen atom No, of electrons = 8 Electronic configuration 2 (K), 8 (L) (nearest to neon) Electronic configuration 2 (K), 6 (L) VARIABLE VALENCY Some elements exhibit more than one valency. They are said to have variable valency. Examples : \ron, copper. tin, lead, sulphur, phosphorus, etc, (a) In the case of the metals exhibiting variable valency, we represent the lower valency by adding the suffix ous to the name of the metal; to represent the higher valency the suffix ic is attached to the name of the metal. For example, (b) In the case of a non-metallic atom the number of the other types of atom attached to it determines its valency. Example : Phosphorus has valencies 3 and 5. With chlorine it forms two compounds, PCI, and PCl,. Therefore the molecule of phosphorus trichloride, which has three chlorine atoms in it, has the lower valency (3), and the molecule of phosphorus pentachloride, which has five chlorine atoms in it, | has the higher valency (5) for phosphorus atom. RADICALS Two or more non-metals that collectively accept or donate one or more electrons and become negatively or positively charged in the process are called radicals. Radicals behave as a single reactant, though they occur in | compound form. Table 1.5 : Representation of some radicals the metal iron has valencies +2 and +3. 3 ‘Name of radical R mitatio: Vaien For the lower valency (+2) we write a = —— a ad ferrous (Fe**) and for the higher valency — NUE 3 | A - 24: trate NO; 1 (+3) we write ferric (Fe). Note that the | Nitrite NO; 1 symbol remains the same but the name Bisulphate HSO; 1 changes. In the modern method the Bisulphite HSO; 1 variable valency of the element is | Bicarbonate HCO; } represented by Roman numbers. Thus orig a , ferrous ion is represented as Fe (II) and Sulphate Soe : ferric ion as Fe (III). Sulphite S03 : The advantage of the modern convention | Carbonate coy 2 is that neither the name of the element Dichromate eg0y 2 nor its symbol changes. perinephate POs 3 Table 1.4: Metals with variable valency Element Symbol Lower valency state Higher valency state Copper Cu Cut or Cu(1) [Cuprous] Cu** or Cu (I) [Cupric] Iron Fe Fe* or Fe (IL) [Ferrous] Fe>* or Fe (IID) [Ferric] Silver Ag Ag* or Ag (1) [Argentous] Ag** or Ag (II [Argentic] Lead Pb Pb** or Pb (IT) [Plumbous] Pb** or Pb (IV) [Plumbic] Tin Sn Sn?* or Sn (II) [Stannous] Sn** or Sn (IV) [Stannic] Mercury Hg Hg* or Hg (1) [Mercurous] Hg** or Hg (II) [Mercuric] 3. Name the following : (a) The sub-atomic particle with negative charge and negligible mass. (b) Protons and neutrons present in the nucleus. (c) The electrons present in the outermost shell. (d) Arrangement of electrons in the shells of an atom. (ec) The binding force between atoms ina molecule of a compound. : (f) The number of protons present in the nucleus of an atom. (g) The sum of the number of protons and neutrons of an atom. (h) Atoms of same element with same atomic number but a different mass number. (i) The smallest unit of an element which takes part in a chemical reaction. Multiple Choice Questions (a) The outermost shell of an atom is known as (i) valency (ii) valence electrons (iii) nucleus (iv) valence shell (b) The number of valence electrons present in magnesium is (i) two (ii) three (iii) four (iv) five (c) The sub atomic particle with negative charge is (i) proton (ii) neutron (iii) electron (iv) nucleon (d) Ifthe atomic number of an atom is 17 and mass number is 35 then number of neutron is (i) 35 (ii) 17 (iii) 18 (iv) 52 (e) The force of attraction that binds two atoms together, in a molecule, is called (i) valency (ii) a chemical bond (iii) valence shell (iv) nuclear energy (f) The number of electrons in an atom is equal to the number of (i) protons in a neutral atom (ii) neutrons in a neutral atom (iii) nucleons in a neutral atom (iv) none of the above 10. ll. 12. 13. 14. 15. 16. (g) The sum of number of protons and number of neutrons present in the nucleus of an atom is called its (i) mass number (iii) number of electrons (iv) all of the above Name three fundamental particles of an atom. Give the symbol with charge, on each particle. Define the following terms : (a) Valency shell (b)_ Radioactivity (c) Atomic mass (d) Nucleons (e) Electronic configuration Mention briefly the salient features of Dalton’s atomic theory (five points). (a) What are the two main features of Rutherford’s atomic model ? (b) State its one drawback. What are the observations of the experiment done by Rutherford in order to determine the structure of an atom ? State the mags number, the atomic number of neutrons and electronic configuration of the following atoms. Ne, AM, SSCl and BK Also, draw atomic diagrams for them. Answer the following questions : (ii) atomic number (a) What are isotopes ? How does the existence of isotopes contradict Dalton’s theory ? (b) What is variable valency ? Name two elements having variable valency and state their valencies. The atomic number and the mass number of sodium are 11 and 23 respectively. What information is conveyed by this statement. Draw the diagrams representing the atomic structures of the following : (a) Sodium atom = (b)_ Chlorine atom (c) Carbon atom (d) Oxygen atom. Explain the rule according to which electrons are filled in various energy levels. . The atom of an element is made up of 4 protons, 5 neutrons and 4 electrons. What is its atomic number and mass number ? In what respects do the three isotopes of hydrogen differ ? Give their structures.