4.3 How Atoms Differ

4.3 Atomic Number – number of protons in an atom On periodic table -top left number (whole #). Identifies element. Ex. He = 2 Has 2 protons Elements are arranged by increasing atomic number.

Since atoms are neutral: # protons (+) = # electrons (-) What is the atomic number for Li? How many protons? Electrons? Ans. 3 protons, 3 electrons Use the periodic table to find the atomic number for oxygen. Ans. 8 protons, 8 electrons

Isotopes – atoms of the same element having different number of neutrons Have same # protons & electrons (same element) Have different masses (since different # neutrons) What element is shown in the diagram? How many protons and electrons in each isotope? How many neutrons? What is the mass?

Mass # = # protons + # neutrons Ex. B-11 (boron with mass# of 11) Mass of electrons is not counted (negligible). # neutrons = mass # - atomic # # neutrons = 11 - 5 = 6 Mass on periodic table average mass of all the isotopes of a given element. Boron has 2 isotopes, B-10 and B-11 Which is more common?

Given the Isotope C-14: (red = protons, black = neutrons) 1) Find the atomic #. 2) Find the # protons. 3) Find the # electrons. 4) Find the neutrons. 5) What is the mass #? 6) What is carbon’s atomic mass? APE: atomic# = #protons = #electrons (neutral) 1) 6 2) 6 3) 6 4) 14-6=8 5) 14 6) 12.011

C-12 Isotope Find the atomic #, protons, & electrons. What is its mass #? How many neutrons? 12-6 = 6 What is the atomic mass? Compare to the C-14 isotope.

Compare the 3 isotopes of hydrogen: H-1 H-2 H-3 How many p+ for each? What is the atomic #? How many e-? What is the mass# How many no? What is the atomic mass for H? 1.00794 amu

Isotopic Notation 14 C or 14 C 6 Used to show mass # (top) and atomic # (bottom) 1) Find atomic #, number protons, number of electrons. 2) How many neutrons? Ans. 1) 6,6,6 2)14-6=8

Quiz 1) What is lithium’s atomic number? 2) How many protons & electrons does Li have? 3) What does N-13 mean? Find the # protons, at.#, neutrons, electrons. 4) Write the name of the isotope having 8 protons and 9 neutrons. 5) Write the isotopic notation for this isotope. 17O 8

Fill in the Chart: Symbol Isotope Name Protons Neutrons Electrons 31P 15 8 7 Al-27

IONS – Charged Particles (Atoms with a charge) Negative ions are formed when electrons are gained. Ex. F – At. # = 9 F-1 has 10 electrons Positive ions are formed when electrons are lost. Ex. Mg – At. # = 12 Mg2+ has 10 electrons

* Find p+, e-, n0 for isotopes of ions: Ca2+ - 41 20 protons 20 electrons if neutral 18 electrons has a +2 charge (lost 2) 21 neutrons P3- - 31 15 p+, 15 e-, (if neutral) + 3 = 18 electrons

This is nearly equal to mass of a proton or neutron: Mass of Individual Atoms – since atomic masses are so tiny, scientists assign atomic mass units (amu). 1 amu = 1/12 the mass of 1 atom of C –12. This is nearly equal to mass of a proton or neutron: 1 p+ = 1.007 276 amu 1 n0 = 1.008 665 amu 1 e- = 0.000 549 amu

Atomic Mass is a weighted average. To find a weighted average: (% as decimal)(Mass 1)+ (% as dec.)(Mass 2) Ex. Find the average mass of football team if: 92.0% weigh 200. lbs. 8.00% weigh 180. lbs. Ave. mass = (0.920)(200) + (0.0800)(180) = 184.0 + 14.4 = 198 lbs.

Atomic mass of element – Weighted average mass of the isotopes of that element. Isotopes % Abundance Atomic Mass Cl-35 75.770% 34.969 amu Cl-37 24.230% 36.966 amu At.Mass= (%Cl-35 as dec.)(Mass) +(%Cl-37)(Mass) = (0.75770)(34.969) + (0.24230)(36.966) = 26.496 amu + 8.957 amu = 35.453 amu Check answer on Periodic Table √ Ans. Closest to Cl-35 (76% abund.)

Quiz Review 1) Look over all vocabulary. 2) Look over scientists & contributions: Democritus, Dalton, Rutherford 3) Know parts of the atom. (Atom chart) 4) Know how to read isotopic symbols. 5) Be able to use symbol & isotope name to find protons, neutrons & electrons. 6) Find atomic mass & element, given isotopes & % abundance.

The End