Structure of atom (igcse)

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chemistry notes for IGCSE students.

chemistry notes for IGCSE students.

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  • 1. Chapter-3 Atomic structure
    • Atoms consist of a massive positively charged small central part called nucleus.
    • The nucleus contains protons and neutrons.
    • The electrons revolve around the nucleus in definite circular paths. These circular paths are called shells or orbits.
    • Each orbit has fixed energy. Therefore , these orbits are also known as energy shells or energy levels.
  • 2.
    • These orbits are represented by the symbols K, L, M, N etc....or numbers 1,2,3,4 respectively.
  • 3.
      • As long as as an electron moves in the same orbit it would not lose or gain energy.
      • Electron can lose energy only when it jumps from an orbit of higher energy level to another orbit of lower energy .
      • An electron jump from a lower energy orbit to a higher energy orbit only when it gains energy from outside.
  • 4.  
  • 5.
    • In a neutral atom number electrons are equal to number of protons.
    • Atomic orbit is the circular path in which the electron moves around the nucleus.
  • 6. Nucleons
    • The particles present in nucleus are called Nucleons.
    • Protons and neutrons are nucleons.
  • 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
  • 8. Distribution of electrons in shells
    • 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.
    • The electrons first occupy the shell with the lowest energy . The order of filling the electrons in shells follows the sequence K,L,M....
  • 9.
    • 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.
    • The outermost shell of an atom cannot accommodate more than 8 electrons.
  • 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.
  • 11. Valence shell
    • The outer most shell of an atom is called its valence shell. The valence shell is also called valence orbit.
  • 12. Valence electrons
    • The electrons present in the outermost shell of the atom are known as valence electrons.
    • Valence electrons are important because
    • 1. The valence electrons decide the reactivity of an element.
    • 2. The valence electrons decide the manner in which an atom form a bond with another atom.
  • 13.
    • 3. The valence electrons in an atom decide the combining capacity ( or valency ) of the element.
    • Valency (modern definition)
    • Valency of an element may be defined as the number of electrons in an atom that actually take part in bond formation.
  • 14.
    • Bond formation means combining of two atoms to form a molecule.
    • With the exception of helium ,whose valency is zero, we can estimate the valency of an atom by the following rules
    • (1) When the number of valence electrons in an atom is less than 4
    • then,valency = number of valence electrons.
  • 15.
    • (2) When number of valence electrons in an atom is equal to or greater than 4, then
    • Valency= 8 – number of valence electrons.
  • 16. Chemical reactivity of an element
    • The chemical reactivity of an atom is explained on the following basis
    • 1. Octet rule
    • 2. Electronic configuration
  • 17. 1.Octet rule
    • The octet rule was proposed by G.N Lewis.
    • This rule states that : ‘ The atom of an element combines with another atom to have eight electrons in its outer most shell’.
  • 18. 2. Electronic configuration
    • Electronic configuration of an atom helps us to understand the chemical reactivity of the element.
    • When the outermost shell of an atom is completely filled (have 8 electrons) the element is unreactive.
    • An atom having ‘8’electrons in its outermost shell is chemically inert.
  • 19.
    • E.g.: Elements like Helium, Neon , Argon etc. which have completely filled outermost shells are known as noble gases or inert gases.
    • These gases do not take part in chemical reaction and do not combine with other elements. Valency of noble gases are ‘0’.
  • 20.
    • An atom having less than ‘8’ electron in its outermost shell is chemically reactive. (except helium)
    • Home work
    • 1. Which one is more reactive – an element with atomic number 10 or an element with atomic number 9 ? Why ?
  • 21. Atomic Number ( z) The number of protons present in the nucleus of an atom of an element is called its atomic number.
  • 22.
    • In a neutral atom number of protons is equal to number of electrons.
    • So, atomic number = number of protons = number of electrons.
    • Z = p =e
  • 23. Mass number (A)
    • 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.
    • Mass number = Number of protons + Number of neutrons.
  • 24. Method to write the atomic symbol using atomic number and mass number
    • 1 . First the atomic symbol of the element is written.
    • 2. The atomic number (Z ) is written as subscript on the left side of the atomic symbol.
    • 3. The mass number (A) is written as super script on the left side of the symbol.
  • 25.
    • For example , an element “X” is represented as :
    • A Mass number
    • X Atomic symbol
    • Z Atomic number
    • Example:- 24 23 4
    • Mg, Na , He
    • 12 11 2
  • 26. Home work
    • 1. Calculate the number of protons , neutrons and electrons in an atom whose atomic number is 19 and mass number is 39.
    • 2. Workout the number of protons ,neutrons and electrons in each of the following atoms.
    • 56 Fe, 93 Nb, 235 U, 133 Cs, 137 Ba
    • 26 41 92 55 56
  • 27. Isotopes
    • Isotopes are atoms of same element having same atomic number but different mass number.
    • Eg:1. The three isotopes of hydrogen are
    • 1. Protium ( 1 1 H )
    • 2. Deuterium or D ( 2 1 H)
    • 3. Tritium or T ( 3 1 T )
    • 2. Isotopes of carbon
    • 12 C, 13 C, 14 C
    • 6 6 6
    • Carbon-14 is radioactive.
  • 28.
    • Isotopes contain same number of protons and electrons but have different mass number because they have different number of neutrons..
    • The chemical properties of elements are goverened by the number and arrangement of the electrons. So isotopes have identical chemical properties.
  • 29. Characteristics of isotopes
    • 1. Isotopes have the same electronic configuration and the same number of valence electrons and hence same chemical properties.
    • 2. Isotopes of an element have different masses, so the physical properties which depend upon atomic masses will be different.
    • Many physical properties such as melting point, boiling point , density etc depend upon atomic mass.
  • 30. Fractional mass of elements
    • The fractional atomic masses of elements are due to the existence of their isotopes having different masses.
    • EG: Chlorine has two isotopes 35 17 Cl and 37 17 Cl with natural abundance of 75% and 25% respectively.
    • The mass of 100 atoms=35x75+ 37x 25
    • = 3550
    • Mass of one atom = 3550/100
    • =35.5
    • 35.5 is the relative atomic mass of chlorine.
  • 31. Homework
    • 1. Calculate the relative atomic mass magnesium. The isotopes of magnesium and their percentage abundance are given below:
    • 24 Mg - 78.6%
    • 25 Mg - 10.1%
    • 26 Mg - 11.3%
  • 32. Homework
    • 2. Calculate the relative atomic mass Lithium The isotopes of lithium and their percentage abundance are given below:
    • There is 8 atoms of 6 Li for every 100 atoms of 7 Li
    • Ans: The total mass of 108 atoms=
  • 33. First 20 elements
    • Atomic number Element
    • 1. Hydrogen
    • 2. Helium
    • 3. Lithium
    • 4. Beryllium
    • 5. Boron
    • 6. Carbon
    • 7. Nitrogen
    • 8. Oxygen
  • 34.
    • 9. Fluorine
    • 10 Neon
    • 11 Sodium
    • 12 Magnesium
    • 13 aluminium
    • 14. silicon
    • 15. Phosphorous
    • 16. Sulphur
    • 17. Chlorine
    • 18. Argon
    • 19 Potassium
    • 20 Calcium
  • 35.
    • Deduce the electronic configuration of first 20 elements from their positions in the periodic table.
  • 36. Relative atomic mass and Relative formula mass (RAM and RFM)