Atoms and the Periodic Table The Atom Ch. 4

Democritus (400 BC) Greek philosopher. All things are composed of tiny, indivisible particles: called: Atomos means “something that cannot be cut”.

Lavoisier (1789) French scientist. Came up with the law of conservation of Mass. “Matter is neither created nor destroyed.”

Dalton (1803) Came up with the first “official” atomic Theory: Elements are made of tiny particles called atoms. All atoms of a given element are the same.

Dalton Atoms of different elements are different. Atoms of different elements combine to form compounds. Atoms cannot be subdivided, created, or destroyed.

Thomson (1906) Experimented with cathode rays Discovered that cathode rays are negatively charged Developed the “plum pudding” model of the atom

Cathode Ray Experiment Thomson passed negatively charged Cathode rays through a tube of Hydrogen: the rays bent toward the positively charged plate. He tried it again with different gases and always got the same result. He realized that all matter must contain these negatively charged particles (electrons).

“Plum Pudding” Thomson said atoms were probably composed of negatively charged particles mixed with a positively charged substance.

History Summary: Democritus – Atomos Lavoisier – Law of Conservation of Mass Dalton – First official Atomic Theory Thomson – Plum Pudding Model, theory of electrons

Subatomic Particles Subatomic Particles Charge Symbol Mass (kg) Electron 1- e- 9.02x10-31 Proton 1+ p+ 1.673x10-27 Neutron no 1.675x10-27 Electrons are about 1837 times smaller than protons, so they do not significantly contribute to an atom’s mass. 1 neutron has about the same mass as 1 proton, so we calculate the mass of an atom to be protons + neutrons. Ex. Hydrogen – 1.00794 AMU Hydrogen’s mass is essentially that of 1 proton H has 1 proton, 0 neutrons and 1 electron

Atomic Number An element is defined by its number of protons Ex. All examples of Nitrogen (number 7 on the periodic table) have 7 protons. If you change this number, you change the element! Ex. If we change to 8 protons, what do we now have? Oxygen

Calculating Number of Neutrons All the atoms of one element have the same number of protons in their nucleus, but electrons & neutrons differ From the Periodic Table: Atomic Number (Z) = # of protons Mass Number (A) = # of protons + # of neutrons Atomic Symbol = X

Calculating Number of Neutron Example: For Ca (Calcium) Z = 20 = (20 protons) A = 40 = (20 protons + neutrons) Number of Neutrons = A - Z A – Z = 40 – 20 = 20 neutrons  Ca has 20 protons and 20 neutrons

The Nucleus & Standard Atomic Notation Z = # of protons A = # of protons + # of neutrons X = Ca A Z 40 20

Inferring the Number of Electrons You can infer the number of electrons from Z Protons have a positive charge = p+  But, the atom is neutral on the periodic table  Therefore, there must be enough electrons to BALANCE the number of protons If the atom has p+ = 5  and NO CHARGE  e- = 5  BALANCE

Atomic Mass Number always a whole number mass # = protons + neutrons always a whole number ROUND from atomic mass on the Periodic Table! © Addison-Wesley Publishing Company, Inc.

Isotopes Mass # (P + N) Atomic # (P) Nuclear symbol: Isotopes are atoms of the same element with different mass numbers/different number of neutrons. Nuclear symbol: Mass # (P + N) Atomic # (P) Hyphen notation: carbon-12 (C-12)

Isotopes © Addison-Wesley Publishing Company, Inc.

Isotopes Chlorine-37 atomic #: 17 mass #: 37 # of protons: # of electrons: # of neutrons: 17 37 20

Relative Atomic Mass 12C atom = 1.992 × 10-23 g – that’s small! atomic mass unit (amu) – used by scientists to compare the size of really small particles 1 amu = 1/12 the mass of a 12C atom © Addison-Wesley Publishing Company, Inc. 1 p = 1.007276 amu 1 n = 1.008665 amu 1 e- = 0.0005486 amu I’m so small nobody notices me

Average Atomic Mass Weighted average of all isotopes. This is the crazy number on the Periodic Table. You should round to the nearest tenth. Avg. Atomic Mass OR Avg. Atomic Mass = (mass)(% in nature) + (mass)(% in nature)

Average Atomic Mass EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16O, 0.04% 17O, and 0.20% 18O. Avg. Atomic Mass 16.0 g/mol This is the same number you get when you round the atomic mass to the nearest tenth on the Periodic Table! OR Avg. Atomic Mass = (16)(.9976) + (17)(.0004) + (18)(.020) = 16.0 g/mol

Average Atomic Mass EX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37. Avg. Atomic Mass 35.4 g/mol OR Avg. Atomic Mass = (35)(.8) + (37)(.2) = 35.4 g/mol

Electrostatic forces These are the forces of attraction between electrons and protons

Nuclear force Different from electrostatic forces, this is the force that keeps nucleons (particles found in the nucleus of an atom) together. Fun fact: the total mass of the nucleus is actually slightly greater than the mass of the protons and the neutrons together- when you break a nucleus down, some of that mass is released as energy, which we use in nuclear power plants and nuclear weapons