1. Section 4.3 How Atoms Differ

2. Objectives • Explain the role of atomic number in determining the identity of an atom. • Define an isotope and explain why atomic masses are not whole numbers. • Calculate the number of electrons, protons, and neutrons in an atom given its mass number and atomic number.

3. Atomic Number • Moseley (1887-1915) discovered that atoms ofeach element contain a unique number of positive charges in their nuclei. • Therefore, it was concluded that the number of protons identifies an atom as an atom of a particular element. • The number of protons in an atom is referred to as an element’s atomic number.

4. Atomic Number • Atomic number determines an element’s position in the periodic table - from left to right and top to bottom, elements are arranged by increasing atomic number. Carbon Element name ATOMIC NUMBER Chemical symbol Average atomic mass 6 C 12.011

5. Atomic Number • Recall that atoms are neutral. • The number of protons, therefore, MUST be equal to the number of electrons. • Atomic number = number of protons = number of electrons • Carbon, for example, has an atomic number of 6 and 6 protons and 6 electrons.

6. Complete the following table. Practice Problems

7. Isotopes • Recall that Dalton was wrong about atoms being indivisible. He was also wrong when he stated that all atoms of an element are identical. • All atoms of an element DO have the same number of protons and electrons. The number of neutrons, however, MAY DIFFER from atom to atom. • Atoms that have the same number of protons but different numbers of neutrons are called isotopes.

8. 3 Isotopes of Hydrogen • Each has 1 proton. • They differ in the number of neutrons they contain. • Because of this, each atom has a different mass. • Note that the isotopes of hydrogen have special names.

9. Isotopes • Elements are found in nature as a mixture of isotopes. • The percent abundance of the isotopes (percent of each isotope in a sample) is constant no matter where the sample is found. • Isotopes have the same chemical behavior because chemical behavior is determined by the number of ELECTRONS an element has.

10. Isotopes and Mass Number • To make it easy to identify the various isotopes of an element, scientists add a number after the element’s name. • This number is the mass number - the sum of the number of protons and neutrons in the nucleus of an atom of the isotope. • Mass # = # protons + # neutrons

11. Example • Oxygen-17 • oxygen has an atomic number of 8, so oxygen-17 has 8 protons and electrons. • Since mass # = # protons + # neutrons, then # neutrons = mass # - #protons. So, in oxygen-17, # neutrons = 17 - 8 or 9. • Oxygen-17 has 8 protons, 8 electrons, and 9 neutrons.

12. Mass number Isotopes Atomic number • Chemists use another shortened notation to identify isotopes. • The chemical symbol for the element is used. Written to the left of the symbol, a superscript indicates the mass number and a subscript indicates the atomic number.

13. Practice Problems

14. Atomic Mass Units • Recall that the masses of protons, neutrons, and electrons are VERY small numbers. These numbers are difficult to work with. • Scientists therefore developed a way to measure their mass relative to a standard atom (the carbon-12 atom). • One atomic mass unit(amu) is defined as 1/12 the mass of a carbon-12 atom.

15. Atomic Mass Units • The masses of the particles in terms of amu are: • electron = 0.000549 amu (We round to 0 amu) • proton = 1.007276 amu (We round to 1 amu) • neutron = 1.008665 amu (We round to 1 amu) • Note that the mass of a proton & neutron is slightly different; also, correct masses of atoms will not be whole numbers but decimals.

16. Atomic Mass • The atomic mass of an element is the weighted average mass of the isotopes of that element. • Atomic mass, therefore, takes into account the percent abundance of each isotope as well as the mass of an atom of that isotope. • The isotope in greater abundance will have a greater effect on determining the atomic mass.

17. Analyzing Atomic Mass • Analyzing an element’s atomic mass can SOMETIMES give you a clue as to what the most abundant isotope for that element is. • The atomic mass of fluorine is 18.998 amu. From this you can conclude that most fluorine occurs in the form of fluorine-19.

18. Calculating Atomic Mass • Multiply the % abundance of each isotope by its mass. 34.969(75.77%) = 26.496 amu 36.966(24.23%) = 8.9569 amu • Add the mass contributions of all the isotopes together. 35.453 amu is the atomic mass of chlorine. Chlorine-35 Chlorine-37 Atomic mass: 34.969 amu Atomic mass: 36.966 amu % abundance: 75.77% % abundance: 24.23%

19. Practice Problems • Use the following information to calculate the atomic mass of magnesium: magnesium-24: atomic mass 23.985, % abundance 78.99% magnesium-25: atomic mass 24.986, % abundance 10.00% magnesium-26: atomic mass 25.982, % abundance 11.01% • Use the following information to calculate the atomic mass of X; then identify X. 6X = 6.015 amu; 7.5% abundance 7X = 7.016 amu; 92.5% abundance