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Atomic Structure

Atomic Structure. REVIEW. The atom is the smallest unit of an element that has the properties of that element. According to Rutherford’s nuclear model of the atom, the nucleus is tiny, dense, and positively charged.

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Atomic Structure

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  1. Atomic Structure

  2. REVIEW • The atom is the smallest unit of an element that has the properties of that element. • According to Rutherford’s nuclear model of the atom, the nucleus is tiny, dense, and positively charged. • The rest of the atom, which is made of electrons, is very large compared to the nucleus and negatively charged.

  3. REVIEW Electron cloud Diffuse, negatively charged ~10-15 m ~10-10 m Nucleus Tiny, dense, positively charged

  4. Charge and Mass • Elementary charge (e) = 1.6x10-19 Coulombs • Coulomb (C) = SI unit of electric charge • Charge can be + or -. • For convenience’s sake, we write +1 instead of +1.6x10-19 C. • Also write -1 instead of -1.6x10-19 C. • Masses of atoms are measured in atomic mass units (amu). • 1 amu = 1/12 mass of carbon-12 atom. • 1 amu = 1.66x10-24 grams

  5. Subatomic Particles • Proton (p+) • Location: Nucleus • Charge: +1 • Mass: 1.007 amu • Neutron (no) • Location: Nucleus • Charge: neutral • Mass: 1.008 amu • Electron (e-) • Location: Electron cloud (outside nucleus) • Charge: -1 • Mass: 0.0005 amu

  6. Atomic Number (Z) • Atomic Number (Z) – number of p+ in the nucleus. • Atomic number of oxygen = 8 • All oxygen atoms have 8 p+. • No other type of atom has 8 p+. • Atomic number of uranium = 92 • All uranium atoms have 92 p+. • No other type of atom has 92 p+.

  7. Mass Number (A) • Mass Number (A) – combined number of p+ & no. • A = p+ + no • Number of no = A – Z • What is the mass number of an atom that has 8 p+ and 9 no? • ANSWER: 8 p+ + 9 no = 17 • How many no are in a titanium atom with a mass number of 48 (Z = 22 for titanium)? • ANSWER: A – Z = (48 – 22) = 26 no

  8. = protons = neutrons Z = A = Z = A = Z = A = Z = A = Atomic and Mass Numbers • What are the atomic and mass numbers of the following atoms? 1 1 2 3 1 2 4 7

  9. Isotopes • Isotopes – have the same number of p+ but diff. numbers of no. • Example: • All carbon atoms have 6 p+. • Some carbon atoms have 6 no, and some have 7 no. • Some carbon atoms have A = 12, and some have A = 13.

  10. Oxygen - 16 Oxygen - 17 Oxygen - 18 O - 16 O - 17 O - 18 A Z Isotopes • Three isotopes of oxygen: 8 p+ 8 p+ 8 p+ 8 no 9 no 10 no

  11. Average Atomic Mass • Average Atomic Mass – weighted average of the masses of all atoms in a sample. • The atomic mass given on the P.T. is a weighted average.

  12. Average Atomic Mass of Neon Atomic Number Avg. Atomic Mass

  13. Calculating Average Atomic Mass • The element silicon has three naturally-occurring isotopes: • Isotope Abundance Mass (amu) • Si-28 92.23% 27.977 • Si-29 4.68% 28.976 • Si-30 3.09% 29.974 • What is the average atomic mass of silicon?

  14. Calculating Average Atomic Mass • What is the atomic mass of silicon? • First convert all percentages to decimals: • 92.23%  0.9223 • 4.68%  0.0468 • 3.09%  0.0309 • Next multiply the decimals by the masses: • 0.9223 x 27.977 amu = 25.80 amu (4 SF) • 0.0468 x 28.976 amu = 1.36 amu (3 SF) • 0.0309 x 29.974 amu = 0.926 amu (3 SF) • Now add the results: • 25.80 + 1.36 + 0.926 = 28.09 amu (2 dec. places) • This agrees with the mass given on the periodic table.

  15. Calculating Average Atomic Mass • The element chlorine has two naturally-occurring isotopes: • Isotope Abundance Mass (amu) • Cl-35 75.78% 34.969 • Cl-37 24.22% 36.966 • What is the average atomic mass of chlorine?

  16. Calculating Average Atomic Mass x 34.969 amu x 36.966 amu = 26.50 amu = 8.953 amu 35.45 amu 0.7578 0.2422

  17. Calculating Average Atomic Mass • Iron has four naturally-occurring isotopes: • Isotope Abundance Mass(amu) • Fe-54 5.845% 53.940 • Fe-56 91.754% 55.935 • Fe-57 2.199% 56.935 • Fe-58 0.282% 57.933 • What is the average atomic mass of iron?

  18. Calculating Average Atomic Mass x 53.940 amu x 55.935 amu x 56.935 amu x 57.933 amu = 3.153 amu = 51.323 amu = 1.207 amu = 0.163 amu • 0.05845 • 0.91754 • 0.02119 • 0.00282 55.846 amu

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