1. Periodic Trends ChemCom Unit 1B

2. Phases of the moon The phases of the moon repeat at regular intervals or periods. - This is termed “periodicity”

3. The history of the periodic table • Johann Dobereiner – Law of Triads (1817)

4. The history of the periodic table • 1862- Alexandre-Emile Béguyer de Chancourtoiswas the first person to organize the elements by atomic mass

5. John Newlands – Law of Octaves 1865

6. Dmitri Mendeleev – 1872

7. Early Periodic tables • The tables of the 1800’s were organized according to two characteristics • Atomic mass • Physical property • Hydrogen atoms have the lowest mass, oxygen atoms are about 16 times more massive, sulfur was about 32 times more massive • An average atomic mass was assigned to each element based on these comparisons

8. Combining capacity with other elements, such as chlorine and oxygen • Chemical property • One atom of potassium (K) or cesium (Cs) combines with only one atom of chlorine to produce KCl or CsCl • One atom of magnesium (Mg) or strontium (Sr) combines with two atoms of chlorine to produce MgCl2 or SrCl2 • Vertical columns • Elements with similar chemical properties • Horizontal arrangements • Based on increasing atomic mass • Creators of early periodic tables were unable to explain the similarities in properties among neighboring elements • 50 years after Mendeleev, the reason for these patterns was proposed

9. The Electrical Nature of Matter • Like charges repel • Unlike charges attract - + + - + -

10. Every electrically neutral (uncharged) atom contains equal numbers of positively and negatively charged particles: • Protons – positively charged particles found in the nucleus (center) of the atom • The number of protons identifies what element the atom is • Electrons – negatively charged particles found outside the nucleus of the atom • An electrically neutral atom of chlorine contains • 17 protons • 17 electrons

11. Ions (atoms or groups of atoms that are NOT electrically neutral) contain different numbers of protons and electrons • A potassium atom with 19 protons and 18 electrons will have a charge of: • +1 • Because there is one more proton than there are electrons.

12. Neutrons • Electrically neutral particles within the nucleus of most atoms • Only hydrogen contains no neutrons • Does not affect the charge of the atom

13. The Modern Periodic table • Elements are placed in sequence according to their increasing atomic number (number of protons) rather than their atomic mass • Early periodic tables used atomic mass rather than atomic number • Works well for relatively small atoms • Does not work well for larger/ more massive atoms • Note that Cobalt (Co) and Nickel (Ni) are not in mass order

14. The nucleus of the atom • A concentrated region of positive charge in the center of the atom • Contains both protons (positive charge) and neutrons (neutral charge) • The total number of protons and neutrons in the nucleus is called the mass number • Electrons are not included in the mass number because each electron is only about 1/2000th the mass of a proton or neutron

15. Atoms of the same element • All have the same number of protons • May have differing numbers of neutrons • Atoms with the same number of protons but different numbers of neutrons are known as isotopes • Isotopes are atoms of the same element with different mass numbers • Atomic masses • Are decimals because it represents the average mass of all of the different isotopes of that particular element.

16. Counting Subatomic Particles • Atomic number: • Equals the number of protons • Equals the number of electrons (in a neutral atom) • Atomic mass: • Is an average of the masses of all the possible isotopes of a particular element • Rounded to the nearest whole number, it equals the mass number for the most common isotope • Mass number: • Equals the number of protons + number of neutrons

17. In a neutral atom: • # Protons = # electrons = atomic number • # Neutrons = mass number – # protons • In an ion (charged atom): • # Protons = atomic number • # Electrons = # protons – charge • Ex. 1 Na+ • 11 protons • 11 – 1 = 10 electrons • Ex. 2 Cl- • 17 protons • 17 – (-1) = 18 electrons • # Neutrons = mass number - # protons

18. To find the charge of an ion when the number of protons and electrons is known: • Charge = # Protons - # electrons • Ex. 1 an ion of cobalt with 25 electrons will have what charge? • 27 -25 = +2 • Ex. 2 an ion of sulfur has 18 electrons. What is its charge? • 16 – 18 = -2 • You can now complete B.5 Extra Practice packet