Structure of atom (igcse)


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chemistry notes for IGCSE students.

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Structure of atom (igcse)

  1. 1. Chapter-3 Atomic structure <ul><li>Atoms consist of a massive positively charged small central part called nucleus. </li></ul><ul><li>The nucleus contains protons and neutrons. </li></ul><ul><li>The electrons revolve around the nucleus in definite circular paths. These circular paths are called shells or orbits. </li></ul><ul><li>Each orbit has fixed energy. Therefore , these orbits are also known as energy shells or energy levels. </li></ul>
  2. 2. <ul><li>These orbits are represented by the symbols K, L, M, N etc....or numbers 1,2,3,4 respectively. </li></ul>
  3. 3. <ul><ul><li>As long as as an electron moves in the same orbit it would not lose or gain energy. </li></ul></ul><ul><ul><li>Electron can lose energy only when it jumps from an orbit of higher energy level to another orbit of lower energy . </li></ul></ul><ul><ul><li>An electron jump from a lower energy orbit to a higher energy orbit only when it gains energy from outside. </li></ul></ul>
  4. 5. <ul><li>In a neutral atom number electrons are equal to number of protons. </li></ul><ul><li>Atomic orbit is the circular path in which the electron moves around the nucleus. </li></ul>
  5. 6. Nucleons <ul><li>The particles present in nucleus are called Nucleons. </li></ul><ul><li>Protons and neutrons are nucleons. </li></ul>
  6. 7. Comparison of properties ofconstituents of atom No Properties electron proton neutron 1. Symbol e p n 2. Relative charge -1 +1 0 3 Relative mass 0 1 1 4. Position Outside the nucleus In the nucleus In the nucleus 5. Discovered by J.J Thomson E. Goldstein James Chadwick
  7. 8. Distribution of electrons in shells <ul><li>The systematic distribution of electrons in various energy states (K,L,M,N-shells) of the atom of an element is called its electronic configuration. </li></ul><ul><li>The electrons first occupy the shell with the lowest energy . The order of filling the electrons in shells follows the sequence K,L,M.... </li></ul>
  8. 9. <ul><li>The maximum number of electrons that can be accommodated in any shell is given by 2n 2 formula where 'n' is the number of the shell. </li></ul><ul><li>The outermost shell of an atom cannot accommodate more than 8 electrons. </li></ul>
  9. 10. The shell closest to the nucleus has n=1 . Hence the maximum number of electrons in different shells are as follows; First orbit or K shell =2x1 2 = 2 Second orbit or L shell =2x 2 2 =8 Third orbit or M shell = 2x3 2 =18 and so on.
  10. 11. Valence shell <ul><li>The outer most shell of an atom is called its valence shell. The valence shell is also called valence orbit. </li></ul>
  11. 12. Valence electrons <ul><li>The electrons present in the outermost shell of the atom are known as valence electrons. </li></ul><ul><li>Valence electrons are important because </li></ul><ul><li>1. The valence electrons decide the reactivity of an element. </li></ul><ul><li>2. The valence electrons decide the manner in which an atom form a bond with another atom. </li></ul>
  12. 13. <ul><li>3. The valence electrons in an atom decide the combining capacity ( or valency ) of the element. </li></ul><ul><li>Valency (modern definition) </li></ul><ul><li>Valency of an element may be defined as the number of electrons in an atom that actually take part in bond formation. </li></ul>
  13. 14. <ul><li>Bond formation means combining of two atoms to form a molecule. </li></ul><ul><li>With the exception of helium ,whose valency is zero, we can estimate the valency of an atom by the following rules </li></ul><ul><li>(1) When the number of valence electrons in an atom is less than 4 </li></ul><ul><li>then,valency = number of valence electrons. </li></ul>
  14. 15. <ul><li>(2) When number of valence electrons in an atom is equal to or greater than 4, then </li></ul><ul><li>Valency= 8 – number of valence electrons. </li></ul>
  15. 16. Chemical reactivity of an element <ul><li>The chemical reactivity of an atom is explained on the following basis </li></ul><ul><li>1. Octet rule </li></ul><ul><li>2. Electronic configuration </li></ul>
  16. 17. 1.Octet rule <ul><li>The octet rule was proposed by G.N Lewis. </li></ul><ul><li>This rule states that : ‘ The atom of an element combines with another atom to have eight electrons in its outer most shell’. </li></ul>
  17. 18. 2. Electronic configuration <ul><li>Electronic configuration of an atom helps us to understand the chemical reactivity of the element. </li></ul><ul><li>When the outermost shell of an atom is completely filled (have 8 electrons) the element is unreactive. </li></ul><ul><li>An atom having ‘8’electrons in its outermost shell is chemically inert. </li></ul>
  18. 19. <ul><li>E.g.: Elements like Helium, Neon , Argon etc. which have completely filled outermost shells are known as noble gases or inert gases. </li></ul><ul><li>These gases do not take part in chemical reaction and do not combine with other elements. Valency of noble gases are ‘0’. </li></ul>
  19. 20. <ul><li>An atom having less than ‘8’ electron in its outermost shell is chemically reactive. (except helium) </li></ul><ul><li>Home work </li></ul><ul><li>1. Which one is more reactive – an element with atomic number 10 or an element with atomic number 9 ? Why ? </li></ul>
  20. 21. Atomic Number ( z) The number of protons present in the nucleus of an atom of an element is called its atomic number.
  21. 22. <ul><li>In a neutral atom number of protons is equal to number of electrons. </li></ul><ul><li>So, atomic number = number of protons = number of electrons. </li></ul><ul><li>Z = p =e </li></ul>
  22. 23. Mass number (A) <ul><li>The sum of the number of protons and the number of neutrons present in the nucleus of one atom of an element is called its mass number. </li></ul><ul><li>Mass number = Number of protons + Number of neutrons. </li></ul>
  23. 24. Method to write the atomic symbol using atomic number and mass number <ul><li>1 . First the atomic symbol of the element is written. </li></ul><ul><li>2. The atomic number (Z ) is written as subscript on the left side of the atomic symbol. </li></ul><ul><li>3. The mass number (A) is written as super script on the left side of the symbol. </li></ul>
  24. 25. <ul><li>For example , an element “X” is represented as : </li></ul><ul><li>A Mass number </li></ul><ul><li>X Atomic symbol </li></ul><ul><li>Z Atomic number </li></ul><ul><li>Example:- 24 23 4 </li></ul><ul><li>Mg, Na , He </li></ul><ul><li>12 11 2 </li></ul>
  25. 26. Home work <ul><li>1. Calculate the number of protons , neutrons and electrons in an atom whose atomic number is 19 and mass number is 39. </li></ul><ul><li>2. Workout the number of protons ,neutrons and electrons in each of the following atoms. </li></ul><ul><li>56 Fe, 93 Nb, 235 U, 133 Cs, 137 Ba </li></ul><ul><li>26 41 92 55 56 </li></ul>
  26. 27. Isotopes <ul><li>Isotopes are atoms of same element having same atomic number but different mass number. </li></ul><ul><li>Eg:1. The three isotopes of hydrogen are </li></ul><ul><li>1. Protium ( 1 1 H ) </li></ul><ul><li>2. Deuterium or D ( 2 1 H) </li></ul><ul><li>3. Tritium or T ( 3 1 T ) </li></ul><ul><li>2. Isotopes of carbon </li></ul><ul><li>12 C, 13 C, 14 C </li></ul><ul><li>6 6 6 </li></ul><ul><li>Carbon-14 is radioactive. </li></ul>
  27. 28. <ul><li>Isotopes contain same number of protons and electrons but have different mass number because they have different number of neutrons.. </li></ul><ul><li>The chemical properties of elements are goverened by the number and arrangement of the electrons. So isotopes have identical chemical properties. </li></ul>
  28. 29. Characteristics of isotopes <ul><li>1. Isotopes have the same electronic configuration and the same number of valence electrons and hence same chemical properties. </li></ul><ul><li>2. Isotopes of an element have different masses, so the physical properties which depend upon atomic masses will be different. </li></ul><ul><li>Many physical properties such as melting point, boiling point , density etc depend upon atomic mass. </li></ul>
  29. 30. Fractional mass of elements <ul><li>The fractional atomic masses of elements are due to the existence of their isotopes having different masses. </li></ul><ul><li>EG: Chlorine has two isotopes 35 17 Cl and 37 17 Cl with natural abundance of 75% and 25% respectively. </li></ul><ul><li>The mass of 100 atoms=35x75+ 37x 25 </li></ul><ul><li>= 3550 </li></ul><ul><li>Mass of one atom = 3550/100 </li></ul><ul><li>=35.5 </li></ul><ul><li>35.5 is the relative atomic mass of chlorine. </li></ul>
  30. 31. Homework <ul><li>1. Calculate the relative atomic mass magnesium. The isotopes of magnesium and their percentage abundance are given below: </li></ul><ul><li>24 Mg - 78.6% </li></ul><ul><li>25 Mg - 10.1% </li></ul><ul><li>26 Mg - 11.3% </li></ul>
  31. 32. Homework <ul><li>2. Calculate the relative atomic mass Lithium The isotopes of lithium and their percentage abundance are given below: </li></ul><ul><li>There is 8 atoms of 6 Li for every 100 atoms of 7 Li </li></ul><ul><li>Ans: The total mass of 108 atoms= </li></ul>
  32. 33. First 20 elements <ul><li>Atomic number Element </li></ul><ul><li>1. Hydrogen </li></ul><ul><li>2. Helium </li></ul><ul><li>3. Lithium </li></ul><ul><li>4. Beryllium </li></ul><ul><li>5. Boron </li></ul><ul><li>6. Carbon </li></ul><ul><li>7. Nitrogen </li></ul><ul><li>8. Oxygen </li></ul>
  33. 34. <ul><li>9. Fluorine </li></ul><ul><li>10 Neon </li></ul><ul><li>11 Sodium </li></ul><ul><li>12 Magnesium </li></ul><ul><li>13 aluminium </li></ul><ul><li>14. silicon </li></ul><ul><li>15. Phosphorous </li></ul><ul><li>16. Sulphur </li></ul><ul><li>17. Chlorine </li></ul><ul><li>18. Argon </li></ul><ul><li>19 Potassium </li></ul><ul><li>20 Calcium </li></ul>
  34. 35. <ul><li>Deduce the electronic configuration of first 20 elements from their positions in the periodic table. </li></ul>
  35. 36. Relative atomic mass and Relative formula mass (RAM and RFM)