1. Chapter 2 Atomic StructureBy
Mounia Elamrani
Blei / Odian’s General, Organic, and Biochemistry

2. Objectives Percent composition Dalton’s atomic Theory
Constant composition of matter and conservation of mass
Atomic mass
Atomic structure
Periodic table and properties of elements
Electron configuration and the valence shells
Octet rule
Mounia Elamrani Chapter 2 – The Atomic Structure

3. 2.1 The early atomic theory
The concept of the atom had limited scientific usefulness until the discovery of two important laws in 18th and 19th centuries:
The Law of conservation of mass: No detectable gain or loss of mass occurs in chemical reactions. Mass is conserved.
The Law of constant composition: In a given chemical compound, the elements are always combined in the same proportions by mass.
Mounia Elamrani Chapter 2 – The Atomic Structure

4. 2.1 Percent composition
The usual form for describing the relative masses of the elements in a compound is a list of percentages by mass, called the percent composition
It is the number of grams of the element in 100 g of the compound and can be calculated using:
Mounia Elamrani Chapter 2 – The Atomic Structure

5. Example
Analysis of 4.800g of niacin, one of the B-complex vitamins, yields 2.810g of C, g of H, g of N, and 1.249g of O. Calculate the mass percent of each element in the compound.
Mounia Elamrani Chapter 2 – The Atomic Structure

6. 2.2 Dalton’s Atomic Theory
Matter consists of tiny particles called atoms.
In chemical reactions, the atoms rearrange but they do not themselves break apart.
In any sample of a pure element, all the atoms are identical in mass and other properties.
The atoms of different elements differ in mass and other properties.
In a given compound the constituent atoms are always present in the same fixed numerical ratio.
Mounia Elamrani Chapter 2 – The Atomic Structure

7. 2.3 Atomic Mass
It follows from Dalton’s Atomic Theory that atoms of an element have a constant, characteristic atomic mass or atomic weight
For example, for any sample of hydrogen fluoride:
F-to-H atom ratio: 1 to 1
F-to-H mass ratio: 19.0 to 1.00
This is only possible if each fluorine atom is 19.0 times heavier than each hydrogen atom
Mounia Elamrani Chapter 2 – The Atomic Structure

8. 2.3 Atomic Mass Units (amu, u)
Atoms have extremely small masses. The mass of the heaviest known atom is about 4 x g.
We define the atomic mass unit, amu, using Carbon-12 as the standard:
1 atom of carbon-12 = 12 u (exactly)
1 u = 1/12 of mass of 1 atom of carbon-12 (exactly)
This definition results in the assignment of approximately 1 u for the mass of hydrogen (the lightest atom)
Mounia Elamrani Chapter 2 – The Atomic Structure

9. 2.4 The structure of atoms
Experiments have been performed that show atoms are comprised of subatomic particles
There are three principal kinds of subatomic particles:
Proton (p) – carries a positive charge, found in the nucleus, mass≃1u
Electron (e) – carries a negative charge, found outside the nucleus, about 1/1800 the mass of a proton, mass≃0u
Neutron (n) – carries no charge, found in the nucleus, a bit heavier than a proton, mass≃1u
Mounia Elamrani Chapter 2 – The Atomic Structure

10. 2.4 Atomic number and mass number
The number of protons in the nucleus is called the atomic number (Z), and is unique for each element.
Z = number of protons
The mass number (A) of an atom is the total number of particles in the nucleus:
A = (number of protons) + (number of neutrons)
The charge is given by:
Charge = number of proton - number of electrons
Mounia Elamrani Chapter 2 – The Atomic Structure

11. Exercises What is the charge of an atom containing 9p, 10n, and 9e?
What is the charge of an ion containing 12p, 13n, and 14e?
Which element has 22p and 26n, and what is its approximate mass?
Mounia Elamrani Chapter 2 – The Atomic Structure

12. 2.5 Isotopes
It turns out that most elements in nature are uniform mixtures of two or more kinds of atoms with slightly different masses
Atoms of the same element with different masses are called isotopes
Isotopes have the same atomic number (Z) but different mass (A)
Mounia Elamrani Chapter 2 – The Atomic Structure

13. 2.5 Counting Particles in Isotopes
Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms.
12C C 14C
#P _______ _______ _______
#N _______ _______ _______
#E _______ _______ _______
Mounia Elamrani Chapter 2 – The Atomic Structure

14. 2.5 Calculating an element’s average atomic mass
We average the masses of isotopes using their masses and relative abundances to give the average atomic mass of an element:
Naturally occurring chlorine is a mixture of two isotopes:
Contribution
Mass (u)
Abundance (%)
Isotope
* = u
75.77
Cl-35
* = u
24.23
Cl-37
(Rounded) Total = u
Mounia Elamrani Chapter 2 – The Atomic Structure

15. Exercises
What is the symbolic notation for the isotopes of oxygen that contain 7 and 8n, respectively?
Calculate the number of protons and neutrons in the two isotopes of nitrogen
Magnesium consists of three isotopes of masses 24.0amu, 25.0amu, and 26.0amu with abundances of 78.70%, 10.13%, and 11.17% respectively. Calculate the average atomic mass of Mg.
Mounia Elamrani Chapter 2 – The Atomic Structure

16. 2.6 The Periodic Table
It Arranges elements by increasing atomic number
Elements are arranged in numbered rows called periods
The vertical columns are called groups or families (group labels vary)
Elements with similar physical and chemical properties, are placed in vertical columns, or groups.
Li, Na, K: are soft, very reactive metals.
He, Ne, Ar: are non reactive gases.
Mounia Elamrani Chapter 2 – The Atomic Structure

17. 2.6 Some important classifications
A groups = representative elements or main group elements
I A= alkali metals
II A = alkaline earth metals
VI A = chalcogens
VII A = halogens
VIII = noble gases
B groups = transition elements
Inner transition elements = elements and
58 – 71 = lanthanide elements
90 – 103 = actinide elements
Mounia Elamrani Chapter 2 – The Atomic Structure

18. 2.6 Modern Periodic Table
Note: Placement of elements 58 – 71 and 90 – 103 saves space
Mounia Elamrani Chapter 2 – The Atomic Structure

19. 2.6 Metals, Nonmetals, and Metalloids…
Tend to shine (have metallic luster)
Can be hammered or rolled into thin sheets (malleable) and can be drawn into wire (ductile)
Are solids at room temperature (except Hg) and conduct electricity
Mounia Elamrani Chapter 2 – The Atomic Structure

20. 2.6 Metals, Nonmetals, and Metalloids…
Lack the properties of metals
At room T can be solid, liquid, or gas
React with metals to form (ionic) compounds
Metalloids
Have properties between metals and nonmetals (semiconductors)
Mounia Elamrani Chapter 2 – The Atomic Structure

21. 2.6 Metals, Nonmetals, and Metalloids…
Mounia Elamrani Chapter 2 – The Atomic Structure

22. 2.7 Electron Organization Within The Atom
Some early scientists thought that electrons were orbiting around the nucleus
Later scientist discovered, through the emission of light by the elements, that electrons have energy states
It all started with atomic emission spectra...
Mounia Elamrani Chapter 2 – The Atomic Structure

23. Continuous Spectrum vs. Line Spectrum
Visible Light Spectrum Rb Na K
Hydrogen Line Spectrum
Mounia Elamrani Chapter 2 – The Atomic Structure

24. 2.7 Electromagnetic Radiation and Energy
Radiation carries energy through space
Light is a form of electromagnetic radiation
Electromagnetic radiation is characterized by its wave nature
The frequency , ν (in Hz or s-1), of a radiation is the number of “waves”, or complete vibrations, in one second
Each color of the rainbow has a specific frequency
Visible light range: 8x104 to 4x104 Hz
The higher the energy of the radiation, the higher its frequency, ν
Mounia Elamrani Chapter 2 – The Atomic Structure

25. 2.7 Electromagnetic Radiation and Energy
Mounia Elamrani Chapter 2 – The Atomic Structure

26. 2.7 Atomic Energy States Excited State Emission Absorption
Planck said that energy can only be absorbed or released from atoms in fixed amounts called quanta
Atoms absorb energy first, then they release it in the form of a radiation (or color if in the visible range)
When energy is absorbed, the e- “jumps” to the excited state
When energy is released, or emitted, the e- “drops” to the ground state
Excited State
Absorption
Emission
Ground State
Mounia Elamrani Chapter 2 – The Atomic Structure

27. 2.8 The Quantum Mechanical Atom
Complex mathematical theory that was able to predict a variety of atomic properties
The detailed structure of the atom can explain the periodicity of the chemical properties of elements
Elements can be built by adding electrons and protons to the nuclei
There are rules guiding the way the electrons are organized
Mounia Elamrani Chapter 2 – The Atomic Structure

28. 2.8 Bohr Model First model of the electron structure
Gives levels where an electron is most likely to be found
Incorrect today, but a key in understanding the atom
Mounia Elamrani Chapter 2 – The Atomic Structure

29. Shells, Subshells, and Orbitals
Shells Contain electrons that are similar in energy and distance from nucleus:
Low energy electrons are closest to the nucleus
Identified by a whole number (1, 2, 3…) called the principal quantum number (n)…..
The first shell (n=1) is lowest in energy, the 2nd level is next in energy (n=2) and so on: 1<2<3<4
Subshells are locations within a shell: s<p<d<f
Atomic Orbitals define the region in space where electrons are most likely to be.
Mounia Elamrani Chapter 2 – The Atomic Structure

30. Number of Electrons Maximum number of electrons in any level = 2n2
Mounia Elamrani Chapter 2 – The Atomic Structure

31. Order of Electron Filling
All electrons in the same energy level have similar energy.
Shell electrons
Shell electrons
Shell electrons (8 first, later 10)
Order of filling for the first 20 electrons
Shell
2e 8e 8e 2e
Mounia Elamrani Chapter 2 – The Atomic Structure

32. Electron Configuration
Lists the shells containing electrons
Written in order of increasing energy
Element \ Shell
He 2 C F Ne Al Cl
Mounia Elamrani Chapter 2 – The Atomic Structure

33. Exercises A. The electron configuration for sulfur
1) 2,6 2) 8,2,6 3) 2, 8, 6
B. The element in period 3 with two electrons in the outermost energy level
1) Mg 2) Ca 3) Na
Mounia Elamrani Chapter 2 – The Atomic Structure

34. Valence Electrons
Valence or outer shell, or outermost electrons, are the electrons filling the last level of energy in a given atom
Indicate the number of valance electrons:
A. O 1) 4 2) 6 3) 8
B. Al 1) ) 3 3) 1
C. Cl 1) 2 2) 5 3) 7
Mounia Elamrani Chapter 2 – The Atomic Structure

35. Periodic Law
All the elements in a group have the same electron configuration in their outermost shells
Example: Group 2A
Be 2, 2
Mg 2, 8, 2
Ca 2, 8, 8, 2
Specify if each pair has chemical properties that are similar (1) or different (2):
A. Cl and Br B and
C and – 4 D. P and S
E. O and S
Mounia Elamrani Chapter 2 – The Atomic Structure

36. 9. Electron Configurations and the Periodic Table
- Note that the 3d orbital fills after the 4s orbital.
- Similarly, the 4f orbital fills after the 5d orbital. Si V
Mounia Elamrani Chapter 2 – The Atomic Structure

37. Electron Configurations and the Periodic Table
There is a shorthand way of writing electron configurations
Write the core electrons corresponding to the filled Noble gas in square brackets.
Write the valence electrons explicitly.
Example, P: 1s22s22p63s23p3
but Ne is 1s22s22p6
Therefore, P: [Ne]3s23p3.
Mounia Elamrani Chapter 2 – The Atomic Structure

38. Electron Configurations
There are a couple exceptions due to more stable electron configuration:
Cr group prefers to have a half filled nd orbital with 5 electrons
Cr: [Ar] 4s2 3d4 becomes
Cr: [Ar] 4s1 3d5
Cu group prefers to have a completely filled nd orbital with 10 electrons
Cu: [Ar] 4s2 3d9 becomes
Cu: [Ar] 4s1 3d10
The electron configurations (using noble gas abbreviations) are given for the Group 2A and Group 3A elements. The relationship between the group number and the number of valence shell electrons is illustrated.
Mounia Elamrani Chapter 2 – The Atomic Structure