Structure of the Atom

Structure of the Atom

Questions and Answers

  • Which of the following pairs has almost similar masses?
      - a. Proton and electron
      - b. Neutron and electron
      - c. Electron and hydrogen (protium)
      - d. Neutron and hydrogen (protium)

  • Aluminium has 13 protons and 14 neutrons. What is its mass number?
      - Mass number = Number of protons + Number of neutrons = 13 + 14 = 27

  • Silicon’s mass number is 28 and its atomic number is 14. What is its proton and neutron number?
      - Protons = Atomic number = 14
      - Neutrons = Mass number - Atomic number = 28 - 14 = 14

  • Chlorine has 18 neutrons and it has a mass number of 35. What is its proton number?
      - Protons = Mass number - Neutrons = 35 - 18 = 17

  • Write the nuclear symbol of aluminium, silicon, and chlorine.
      - Aluminium: 27<em>13extAl^{27}<em>{13} ext{Al}   - Silicon: 28</em>14extSi^{28}</em>{14} ext{Si}
      - Chlorine: 1735extCl^{35}_{17} ext{Cl}

Atomic Mass and Isotope

Learning Outcomes
  • Explain the terms atomic mass and isotope.
  • Calculate the atomic masses of elements that have isotopes.
Discussion Questions
  1. In the drawbacks of Thomson’s Atomic Theory, we have seen that atoms of the same element are not always alike. What was the reason for this?
  2. How is it possible to get the atomic mass of an element considering that the mass of an atom is concentrated in the nucleus?
  3. Do you think the atomic mass of an element will always be the same? Why?
  4. What is the difference between mass number and atomic mass of an element?
Isotopes Defined
  • All atoms of the same element have the same number of protons, but some may have different numbers of neutrons. For example, most carbon atoms have six neutrons, but some have seven or eight. Atoms of the same element that differ in their numbers of neutrons are called isotopes.
  • Many isotopes occur naturally, and usually, one or two isotopes of an element are the most stable and common. Isotopes generally have the same physical and chemical properties because they have the same numbers of protons and electrons.
Example: Hydrogen Isotopes
  • Types of Hydrogen Isotopes:
      - Most hydrogen atoms: Proton = 1, Electron = 1, Neutron = 0 (Protium).
      - Hydrogen atoms with one neutron: Deuterium.
      - Hydrogen atoms with two neutrons: Tritium (also known as heavy hydrogen).
Stability of Isotopes
  • Atoms require a certain proportion of neutrons to protons to maintain a stable nucleus. An imbalance leads to an unstable or radioactive nucleus, which undergoes radioactive decay (the process where isotopes release particles).
      - Example of Radioisotope: Carbon-14 is a radioisotope; in contrast, Carbon-12 and Carbon-13 are stable isotopes.
Dalton’s Atomic Theory Revisited
  • Dalton proposed that atoms of a given element are identical, but the existence of isotopes shows that atoms of the same element can have varying masses.
  • Elements in nature exist as constant uniform mixtures of their naturally occurring isotopes, which are present in the same relative abundance.
Example Calculation
  • Lithium Isotopes
      - For Lithium with 3 neutrons:
        - Atomic number = 3 (number of protons)
        - Mass number = 3 (protons) + 3 (neutrons) = 6
      - For Lithium with 4 neutrons:
        - Atomic number = 3 (same as above)
        - Mass number = 3 (protons) + 4 (neutrons) = 7
Atomic Mass Measurement
  • Mass of atoms is so small that a device called a mass spectrometer is used to measure them.
  • Carbon-12 Definition: By definition, one atom of carbon-12 is assigned a mass of 12 atomic mass units (amu).
  • An atomic mass unit (amu) is the mass equal to one-twelfth the mass of an atom of carbon-12: 1 amu = 1.660539040imes10−24extg1.660539040 imes 10^{-24} ext{g}.

Average Atomic Mass

  • Average Atomic Mass = [(% isotope 1)(mass of isotope 1)] ÷ 100 + [(% isotope 2)(mass of isotope 2)] ÷ 100 + …
Example Problem - Lithium Average Mass Calculation
  • Given isotopes: Li-7 (93%) and Li-6 (7%):
    Average mass = (7imes93)100+(6imes7)100=6.93extamu\frac{(7 imes 93)}{100} + \frac{(6 imes 7)}{100} = 6.93 ext{ amu}.
Example Problem - Boron Average Atomic Mass
  • Boron has two isotopes: B-10 (5 neutrons) = 10 amu and B-11 (6 neutrons) = 11 amu.
  • Average atomic mass = [(10 x 20) ÷ 100] + [(11 x 80) ÷ 100] = 10.8 amu.
Example Problem - Neon Average Atomic Mass
  • Neon has three isotopes: Ne-20 (10 neutrons = 19.99 amu), Ne-21 (11 neutrons = 20.99 amu), Ne-22 (12 neutrons = 21.99 amu).
  • Average atomic mass calculation:

    =[(0.9092)(19.99)]+[(0.003)(20.99)]+[(0.0885)(21.99)]=20.17extamu= [(0.9092)(19.99)] + [(0.003)(20.99)] + [(0.0885)(21.99)] = 20.17 ext{ amu}.
Questions on Isotopes
  1. In an element with isotopes, which of the subatomic particles differ, and which remain unchanged?
  2. How many protons, electrons, and neutrons are in each of the following atoms?
       - Co$^{60}{27}$, Ca$^{45}{20}$, Na-24, Sr-90
  3. Calculate the average atomic mass of copper: Cu-63 (69.15%) & Cu-65 (30.85%).
  4. Calculate the average atomic mass of chlorine: Cl-35 (75%) & Cl-37 (25%).

Main Energy Levels

Learning Outcomes
  • Describe main energy levels.
Discussion Questions
  1. Who discovered energy levels? What is the other name for energy levels?
  2. In the Bohr’s atomic model, why do electrons disperse themselves?
  3. How do electrons arrange themselves according to Bohr’s atomic model?
Bohr’s Model
  • Proposed in 1913 by Niels Bohr, where electrons revolve around the nucleus in specific orbits.
  • Principal Energy Levels (denoted by the principal quantum number n) include levels K (1), L (2), M (3), N (4), etc.
  • The energy of the orbit increases as it moves away from the nucleus: K < L < M < N or 1 < 2 < 3 < 4.
Questions on Main Energy Levels
  1. What is the main energy level of an electron?
  2. How many main energy levels does each of the first 20 elements have (H to Ca)?

Electronic Configuration on Main Shells

Learning Outcomes
  • Define electronic configuration.
  • Write the ground state electronic configuration of elements.
  • Draw diagrams showing the electronic configuration of the first 20 elements.
  • Write electronic configuration using noble gas as a core.
Helpful Analogy
  • Visualize an atom as a small nucleus enveloped by a larger space filled with electrons divided into main energy levels (shells).
Electrons in Shells
  • An atom can hold up to 2n22n^2 electrons, where n is the number of the level.
      - K shell: up to 2
      - L shell: up to 8
      - M shell: up to 18
      - N shell: up to 32
      - O shell: up to 50
Electron Configuration Example: Calcium (Atomic Number = 20)
  • Configuration: K: 2, L: 8, M: 8, N: 2 = 2, 8, 8, 2.
Drawing Electronic Configuration
  • Provide diagrams to show configurations for the first 20 elements.

Valence Electrons

Learning Outcomes
  • Describe valence electrons.
Activity on Valence Electrons
  • An atom X with atomic number 20 has electronic configuration K=2, L=8, M=8, N=2. Identify: 1. Valence electrons, 2. Penultimate shell electrons, 3. Antipenultimate shell electrons.
Role and Importance of Valence Electrons
  • Valence electrons are involved in chemical bonding and hence determine the element's chemical properties. The reaction of atoms occurs when electrons are lost, gained, or shared during chemical reactions.

Key Terms and Equations

  • Atom
  • Atomic Theory
  • Atomic Number (Z)
  • Mass Number (A)
  • Isotope
  • Atomic Mass Unit (amu)
  • Average Atomic Mass:
    ext{Average Atomic Mass} = rac{(% ext{isotope 1})( ext{mass of isotope 1}) + ( ext{isotope 2})( ext{mass of isotope 2}) + …}{100}
  • Formula for Mass Number:
    extMassNumber=extNumberofProtons+extNumberofNeutrons.ext{Mass Number} = ext{Number of Protons} + ext{Number of Neutrons}.