2. Historically, scientists have used their knowledge of atomic properties to develop and refine atomic models. Today, this knowledge is applied to various research techniques. UNIT 2 Chapter 3: Atomic Models and Properties of Atoms Scientists can now determine colour patterns of ancient bird feathers by identifying elements present in fossils of the birds. Chapter 3: Atomic Models and Properties of Atoms

3. UNIT 2 Section 3.1 3.1 Developing a Nuclear Model of the Atom Chapter 3: Atomic Models and Properties of Atoms Dalton’s model (1808) Thomson’s model (1904) Rutherford’s model (1911) Bohr’s model (1913) quantum mechanical model (present)

4. UNIT 2 Section 3.1 Reviewing the Atomic Models of Dalton and Thomson Chapter 3: Atomic Models and Properties of Atoms John Dalton’s model of the atom: marked the beginning of a new way of explaining matter matter was described as being composed of small, indivisible spheres, which Dalton called atoms Why did the discovery of subatomic particles like electrons require a new atomic model? Dalton envisioned atoms as hard, solid spheres.

5. UNIT 2 Section 3.1 Reviewing the Atomic Models of Dalton and Thomson Thomson’s “plum pudding” model of the atom. Chapter 3: Atomic Models and Properties of Atoms J.J. Thomson’s model of the atom: incorporated his discovery of the electron, using cathode ray tubes an atom is a positively charged spherical mass with negatively charged electrons embedded within Rutherford’s experimental observations required a new atomic model.

6. UNIT 2 Section 3.1 Rutherford’s Experiments with Alpha Particles Chapter 3: Atomic Models and Properties of Atoms Expectation (Thomson’s model): particles pass through or some slightly deflected Observation: some particles deflected at large angles Alpha particles were aimed at gold foil. The scattering of the alpha particles was monitored.

7. Rutherford’s model of the atom: UNIT 2 Section 3.1 Rutherford’s Atomic Model The nuclear, or planetary model, of the atom. Chapter 3: Atomic Models and Properties of Atoms large deflections of particles proposed to be due to presence of an electric field at the centre of the atom an atom has a positively charged nucleus at the centre with electrons in motion surrounding the nucleus

8. UNIT 2 Section 3.1 The Limitations of Rutherford’s Atomic Model Chapter 3: Atomic Models and Properties of Atoms Based on the understanding of physics at the time, for an electron in motion around a central core: radiation must be emitted, so it was expected that a continuous spectrum of light energy was being given off because of radiation, the electron would lose energy and its orbit would decrease until it spiraled into the nucleus, destroying the atom

9. UNIT 2 Section 3.1 Rethinking Atomic Structure Based on the Nature of Energy Chapter 3: Atomic Models and Properties of Atoms Light is one form of electromagnetic radiation, which travels through space as waves Electromagnetic waves: have frequency, wavelength, and amplitude interact with matter in discrete particles called photons

10. When atoms are excited due to absorption of energy, they emit light as they lose energy and return to a non-excited state. UNIT 2 Section 3.1 Atomic Spectra Each element has a characteristic line spectrum. Chapter 3: Atomic Models and Properties of Atoms Atoms of each element emit light of particular wavelengths called a line spectrum or emission spectrum.

11. UNIT 2 Section 3.1 The Bohr Model of the Hydrogen Atom Chapter 3: Atomic Models and Properties of Atoms are in circular orbits can only exist in certain “allowed” orbits or energy levels (energy of electrons is quantized) do not radiate energy while in one orbit can jump between orbits by gaining or losing a specific amount of energy Niels Bohr set out to explain the stability of the nuclear model of the atom. In this model, electrons

12. UNIT 2 Section 3.1 Bohr’s Atomic Model Explains the Line Spectrum of Hydrogen Chapter 3: Atomic Models and Properties of Atoms Calculated wavelengths of the possible energies of photons that could be emitted from an excited hydrogen atom (transitions from n = 6, 5, 4, and 3 to n = 2) corresponded with hydrogen’s visible line spectrum Limitations could only explain single-electron systems (H, He +, Li 2+)

13. Today’s quantum mechanical model of the atom incorporates the wave properties of electrons. UNIT 2 Section 3.2 3.2 The Quantum Mechanical Model of the Atom An electron density diagram represents an atomic orbital. Chapter 3: Atomic Models and Properties of Atoms Wave functions, initially described by Erwin Schrodinger, represent a region in space around a nucleus where an electron will be found. This region of space is called an atomic orbital

14. 14 Many scientists contributed to the development of the quantum mechanical model of the atom. –Bohr –Planck –DeBroglie –Heisenberg –Schrodinger –Pauli

15. UNIT 2 Section 3.2 The Quantum Mechanical Model of the Atom The circle does not represent a real boundary. Chapter 3: Atomic Models and Properties of Atoms Atomic orbitals can be visualized as “fuzzy clouds” The higher the density of the “cloud,” the higher the probability of finding an electron at that point. The cloud has no definite boundary. The region where an electron will spend 90 percent of its time is depicted by drawing a circle.

16. Heisenberg The Heisenberg uncertainty principle states that we cannot know both the position and the momentum of an electron at the same time. –Therefore, we do not know the exact path of the electron in an orbital but there is a probability of finding one in a certain space. –Quantum mechanical model uses complex orbital shapes (electron clouds), volumes of space where electrons are likely to be found

17. UNIT 2 Section 3.1 Quantum Numbers Describe Orbitals Chapter 3: Atomic Models and Properties of Atoms Electrons in the quantum mechanical model of the atom are described using quantum numbers. Three quantum numbers describe the distribution of electrons in the atom and a fourth describes the behaviour of each electron. Symbols for the four quantum numbers: n lmlml msms

18. UNIT 2 Section 3.2 The Principle Quantum Number, n Chapter 3: Atomic Models and Properties of Atoms Is the first quantum number Describes the energy level, or shell, of an orbital (the distance from orbital to nucleus) All orbitals with the same n value are in the same shell The larger the n value, the larger the size of the shell, the higher the energy Values can range from n = 1 to n = ∞ n = 1first shell n = 2 second shell n = 3 third shell n = 4fourth shell

19. UNIT 2 Section 3.2 The Orbital-Shape Quantum Number, l Chapter 3: Atomic Models and Properties of Atoms Is the second quantum number, also known as the angular momentum number Describes the shape of an orbital Refers to energy sublevels, or subshells (same n different l) Values depend on the value of n. They are positive integers from 0 to (n – 1) Each value is identified by a letter l = 0 s l = 1 p l = 2 d l = 3 f An energy sublevel is identified by combining n with the orbital letter. For example, n = 2, l = 1: 2p sublevel

20. UNIT 2 Section 3.2 The Magnetic Quantum Number, m l s, p, and d orbitals have characteristic shapes. Chapter 3: Atomic Models and Properties of Atoms Is the third quantum number Indicates the orientation of the orbital in space For a given l there are (2l +1) values for m l The total number of orbitals for an energy level is n 2

21. UNIT 2 Section 3.2 The Spin Quantum Number, m s Chapter 3: Atomic Models and Properties of Atoms Is the fourth quantum number Describes the direction the electron is spinning in a magnetic field – clockwise or counterclockwise Two possible values: +½ or –½ - only 2 electrons possible in each subshell To summarize:

22. UNIT 2 Section 3.2 Identifying Electrons Using Sets of Quantum Numbers Chapter 3: Atomic Models and Properties of Atoms According to the Pauli exclusion principle: an orbital can have a maximum of two electrons two electrons in an orbital must have opposite spins No two electrons of an atom have the same set of four quantum numbers.

23. What is the set of quantum numbers for an electron in a 2s orbital? UNIT 2 Section 3.2 Answer on the next slide Chapter 3: Atomic Models and Properties of Atoms

24. n =2, l = 0, m l = 0, m s = +½ Section 3.2 UNIT 2 Chapter 3: Atomic Models and Properties of Atoms L EARNING C HECK