Atomic Structure

1. Atomic Structure Modern Atomic Theory

2. Modern Atomic Theory • The atom consists of positive protons, negative electrons, and neutral neutrons. • Protons and neutrons are located in the nucleus of the atom, which is small and massive. • Electrons are located outside of the nucleus, which creates the volume of the atom.

3. The Modern View of Atomic Structure

4. Subatomic particles Actual mass (g) Relative mass (amu) Name Symbol Charge Electron e- 9.11 x 10-28 -1 1/1840 ≈ 0 Proton p+ 1 1.67 x 10-24 +1 0 Neutron n0 1 1.67 x 10-24 amu = atomic mass unit

5. Atomic Number • Henry Moseley discovered that each element has a unique positive charge. • Atomic Number or Z • Number of Protons • For neutral atoms # protons = # of electrons • How many protons and electrons are in Si? • 14 p and 14 e • What element contains 8 protons? • oxygen

6. Ions • Most atoms are not neutral. • An ion forms when an atom loses or gains electrons. • Lose electrons = positive charge • Gain electrons = negative charge • Ion charge is written as a superscript. • Charge = # protons - # electrons • What is the element with 16 protons and 18 electrons? • S-2 • What is the element with 20 protons and 18 electrons? • Ca+2

7. Atomic Mass • Atomic mass or A is the number of protons and neutrons in an atom. • Atomic Mass Unit or amu • based on carbon-12, exactly 12 amu • We always round atomic mass to the nearest whole number. • Why? • Atomic mass is the weighted average of all of isotopes for that element based on abundance.

8. Mass number X A Z Element symbol Atomic number Isotopic Notation • We write symbols for elements as follows:

9. 1 1 2 1 3 1 H H H Isotopes • Atoms with the same number of protons different number of neutrons, therefore different atomic masses.

10. Isotopes

11. Naming Isotopes • We can also put the mass number after the name of the element. • carbon- 12 • carbon -14 • uranium-235

12. Calculating averages • Multiply the atomic mass of each isotope by it’s abundance (expressed as a decimal), then add the results. • Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of 62.93 amu and the rest has a mass of 64.93 amu. • 62.93 amu x .691 = 43.5 amu 64.93 amu x .309 = 20.1 amu Total = 43.5 + 20.1 = 63.6 amu

13. Atomic Mass 23 amu • What is the atomic mass of Na? • What is the atomic number of Na? • Write this in isotopic notation: • To find the neutrons subtract the two numbers. 11 amu 23 Na 11 23 -11 = 12 amu

14. Symbols • Find the • number of protons • number of neutrons • number of electrons • Atomic number • Mass Number 35 p 45 n 80 Br 35 e 35 35 80 amu

15. Symbols • Find the • number of protons • number of neutrons • number of electrons • Atomic number • Mass Number 35 p 45 n 80 -1 Br 36 e 35 35 80 amu

16. Symbols • if an element has an atomic number of 34 and a mass number of 78 what is the • number of protons • number of neutrons • number of electrons • Complete symbol 34 p 44 n 34 e 78 Se 34

17. Symbols • if an element has 91 protons and 140 neutrons what is the • Atomic number • Mass number • number of electrons • Complete symbol 91 231 amu 91 231 Pa 91

18. Mass number H 1 1 Element symbol Atomic number Summary Element charge +1

19. Symbols • Find the • number of protons • number of neutrons • number of electrons • Atomic number • Mass Number • Is this an ion? • Is this an isotope? 56 p 79 n 135 +2 Ba 54 e 56 56 135 amu Yes Yes