Chapter 2

1. Chapter 2 Atoms and Elements atom- smallest identifiable unit of an element element- a substance that cannot be broken down into simpler substances -there are about 91 naturally occurring elements -scientists have made over 20 elements Modern Atomic Theory and Laws That Led to It 1) Law of conservation of mass- matter is neither created nor destroyed

2. -when a reaction is carried out, the mass of the reactants must equal the mass of the products ex- 2Na + Cℓ2 2NaCℓ 7.7g 11.9g 19.6g 19.6g • Law of Definite Proportions- all samples of a given compound have the same proportions of their constituent elements ex- 18.0g H20 results in 16.0g O2 and 2.0g H2 mass ratio = 16.0g O2/2.0g H2 = 8 O2:1 H2 -this is true for all samples of water

3. Page 49 Ex 2.1 and For practice 2.1 • Law of Multiple Proportions- when two elements (A and B) form two different compounds, the mass of element B that combines with 1g of element A can be expressed as a ratio of small whole numbers Page 50 Ex- 2.2 For practice 2.2

4. John Dalton’s Atomic Theory • All elements are composed of atoms • All atoms of the same element have the same properties and are identical • Atoms combine in simple whole number ratios with other atoms to form compounds • Atoms of one element cannot be changed into atoms of another element

5. -Scientists began to think that atoms were composed of smaller particles Discovery of Electron -J.J. Thomson (1856-1940) -used a cathode ray tube -believed that the cathode ray was composed of tiny particles with an electrical charge -these are electrons -rays travel from – charged electrode (cathode) to + electrode (anode)

7. Discovery of Charge of Electron -Robert Millikan (1868-1953) -said electrons have a negative charge Plum Pudding Model -it was known that – charged particles attract + charged particles -it was also known that atoms were neutral -so there must be a + charged particle in the atom

8. -Thomson proposed that – charged electrons were small particles held within a + charged sphere -protons discovered by Eugen Goldstein (1850 – 1930)

9. Ernest Rutherford (1871-1937) -performed experiments that allowed him to conclude that the atom must have a positive mass in a much smaller space than proposed Nuclear Theory • most of atom’s mass and all of its + charge are in a small core called the nucleus • most of volume is empty space through which negative particles move • there are as many negatively charged particles as there are positive (protons) so the atom is neutral

10. -There were still some parts of atom missing James Chadwick (1891-1974) -discovered that missing mass was neutrons Subatomic Particles and the Atom protons(p+) – positively charged, found in nucleus, mass = 1.67 x 10-24g neutrons (n0) – neutrally charged, no charge, found in the nucleus, mass = 1.67 x 10-24g electrons (e-) – negatively charged, found surrounding nucleus in clouds or energy levels, mass = 9.11 x 10-28g **most of mass in the nucleus of an atom

11. atomic number- # of protons in an element, defines the element, the smaller # on the periodic table -since atoms are neutral: # of p+ = # of e- -so the atomic # also tells you the # of e- How many protons in the following? argon uranium iron lithium 18 92 26 3 *each element also has that # of e-

12. mass number- sum of the # of p+ and n0, larger # on the periodic table rounded to a whole # How do you find the # of n0? # n0 = mass # - atomic # How many neutrons in the following? argon uranium iron lithium 22 146 30 4 Can be written this way: Ar-40U-238Fe-56Li-7

13. atomic mass- larger # on the table not rounded, average mass of all the isotopes of that element isotopes- atoms with the same # of p+ and e- but different # of n0 ex- Ne-20 Ne-21 Ne-22 p+ = 10 p+ = 10 p+ = 10 e- = 10 e- = 10 e- = 10 n0 = 10 n0 = 11 n0 = 12 Page 59 Ex 2.3 and For Practice 2.3

14. natural abundance- the relative % of an isotope with respect to other isotopes of the same element -back to Ne isotope Which isotope of Ne would have the highest relative abundance? Ne-20, because it is closest to the atomic mass -Relative Abundance problems

15. Modern Periodic Table -grew from the work of Dmitri Mendeleev -arranged elements according to increasing mass -called periodic law- when elements are arranged in order of increasing mass/atomic #, similar properties will recur periods- horizontal rows groups/families- vertical columns, indentified by a # and a letter

16. Groups of the Periodic Table Alkali Metals- Group 1A elements Alkaline Earth Metals- Group 2A elements Transition Metals- Group B elements, bridge Inner Transition Metals- two rows at the bottom non-metals- upper and lower right side Metalloids- staircase- starting at boron Halogens- Group 7A Noble Gases- Group 8A or Group 0 -also called inert gases- they do not react to form compounds

17. Main group or Representative elements -Groups 1A to 8A minus the Transition metals Properties of Metals • conduct electricity • ductile (can be drawn into wires) • shiny/lustrous • malleable (can be pounded into sheets) -Most elements are solids Gases = H, O, N, F, Cℓ, He Ne, Ar, Kr, Xe, Rn Liquids = Hg, Br (Ga, Fr, Cs)