1. 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 Chancourtois was 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 1/2000 th 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. 1Na + 11 protons 11 – 1 = 10 electrons Ex. 2Cl- 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. 1an ion of cobalt with 25 electrons will have what charge? 27 -25 = +2 Ex. 2an ion of sulfur has 18 electrons. What is its charge? 16 – 18 = -2 You can now complete B.5 Extra Practice packet