Chapter 2 Matter is Made of Atoms

1. Chapter 2 Matter is Made of Atoms 2.2 Electrons in Atoms

2. Objective Objectives: Relate the electron to modern atomic theory, Compare electron energy levels in an atom, Illustrate valence electrons by Lewis electron dot structures

3. Electrons in motion • Electrons are in motion in the outer part of an atom • When two atoms come near, these electrons interact • Electrons and their arrangement in the atom greatly affect an element’s properties and behavior

4. Electrons in motion • Niels Bohr (early 20th century)- Proposed e-‘s must have enough energy to keep them in constant motion around the nucleus (enables e-‘s to overcome the attraction of the positive nucleus) • Bohr’s model (1913)- Planetary model The well-defined orbits are not correct. Electron motion within an atom is irregular and an electron may be close to the nucleus at times.

5. Electromagnetic spectrum • Electromagnetic radiation (radiant energy) travels in the form of waves- have both electrical and magnetic properties • Electromagnetic waves can travel through empty space Ex: radiant energy from the sun travels to Earth

6. Electromagnetic spectrum • Electromagnetic waves travel through space at the speed of light, approx. 300 million meters/sec

7. Electromagnetic spectrum • Properties of waves: Frequency and wavelength Frequency-number of vibrations/ second (Hertz-Hz) Wavelength- distance between two corresponding points on two consecutive waves • Wavelength is determined by frequency Low frequency = long wavelength (low energy) High frequency = short wavelength (high energy)

8. Electromagnetic spectrum

9. Electromagnetic spectrum • All the forms of radiant energy are parts of a whole range of electromagnetic radiation • The electromagnetic spectrum includes gamma rays, X rays, ultraviolet (UV), Visible, Infrared (IR), microwaves, radio waves

10. Electromagnetic spectrum

11. Electromagnetic spectrum • Gamma Rays- highest frequency, shortest wavelength, most energetic waves, can pass through most substances • X rays- lower frequency than gamma, but still high energy waves, can pass through soft body tissue, but stopped by bone • Ultraviolet-slightly more energetic than visible, can cause sunburns, mostly absorbed by ozone

12. Electromagnetic spectrum • Visible- part of the spectrum to which our eyes are sensitive, Our eyes and brains interpret different frequencies as different colors ROY G BIV - Newton Red- low energy Violet- high energy

13. Electromagnetic spectrum • Infrared- less energy than visible, given off by most “warm objects” • Microwaves/ Radio waves- lowest frequency

14. Electrons and Light • EMISSION SPECTRUM: Spectrum of light released from excited atoms of an element • Electrons can have only certain amounts of energy • Electrons can move around the nucleus only at distances that correspond to those amounts of energy • When e- absorb energy, they move to a higher energy state • When they fall back to a lower energy state- they emit energy (Light)

15. Electrons and Light ENERGY LEVEL: Region of space in which e-‘s can move about the nucleus of an atom ELECTRON CLOUD: Space around the nucleus of an atom where the atom’s e-‘s are found Each energy level can hold a limited number of electrons http://www.youtube.com/watch?v=QI50GBUJ48s&safety_mode=true&persist_safety_mode=1&safe=active

16. # of electrons in each level • Lowest energy level is the smallest and closest to the nucleus, Holds two electrons max • Second energy level is larger because it is farther away, Holds eight electrons max • Max # of e-‘s: 2, 8, 8,18, 32, 50, 78, 91

17. # of electrons in each level • Bohr Model Examples: H-1 (1 p, 1 e, 0 n) Li-7 (3 p, 3 e, 4 n) 0-16 (8 p, 8 n, 8 e)

18. VALENCE ELECTRONS • The electrons in the outermost energy level • Valence electrons are important because they interact when atoms come near each other • Chemical and physical properties of an element are directly related to the number and arrangement of valence electrons (v.e.)

19. VALENCE ELECTRONS • Atoms in group 1 have one valence electron • Atoms in group 2 have two valence electrons • Atoms in group 13 have three valence electrons • Atoms in group 14 have four valence electrons • Atoms in group 15 have five valence electrons • Atoms in group 16 have six valence electrons • Atoms in group 17 have seven valence electrons • Atoms in group 18 have eight valence electrons

20. LEWIS DOT DIAGRAM • Illustrates valence electrons as dots around the chemical symbol of an element. • Each dot represents one valence electron (max # of v.e. is 8) • The symbol represents the core of the atom. Ex: . . Li  Be  B 

21. LEWIS DOT DIAGRAM • Li has 2 e- in the first energy level and 1 in the second, • Be has 2 e- in the first energy level and 2 in the second, • B has 2 e- in the first energy level and 3 in the second • More examples: C, N, O, F, Ne