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NOTES 4 – Atomic Structure The Idea of the Atom Democritus (~400 BC) • Greek philosopher • “All matter is made of tiny, indivisible particles” A tomos means unable to be divided Democritus thought atoms were the smallest pieces of matter The Development of the Modern Atomic Model John Dalton (1803) • Each element has different size and shape atoms • Atoms of different elements combine to form compounds JJ Thompson (1897) • Atoms are solid masses of smaller charged particles, half are positive and half are negative The Development of the Modern Atomic Model Ernst Rutherford (1905) • Gold Foil Experiment • Most of the atom is empty space • There is a tiny bunch of positively charged particles in the center of an atom The Gold Foil Experiment The Development of the Modern Atomic Model Niels Bohr (1911) • Negatively charged particles orbit the bunch of positively charged particles • The orbiting, negative particles are organized into energy levels The Modern Atomic Model The Modern Atomic Model Subatomic particles – tiny particles that make up atoms There are three types of subatomic particles: 1. Protons (p+) – positively charged particles 2. Neutrons (n0) – particles with no charge 3. Electrons (e-) – negatively charged particles Nucleus Nucleus - the dense center part of the atom • Contains the protons and neutrons • Nucleus contains 99.99% of the mass of an atom • Electrons orbit nucleus Atoms are extremely small Comparing Subatomic Particles Particle Location Size & Mass Charge Proton (p+) Nucleus Equal to Neutron Positive (+) Neutron (n0) Nucleus Equal to Proton Neutral (0) Electron (e-) 10,000X smaller Outside Nucleus than protons Negative (-) and neutron Atoms are Neutral Every atom has equal numbers of protons (p+) and electrons (e-) Ex. Oxygen atoms have 8 p+ and 8 eThe negatives cancel out the positives • 8 protons • 8 electrons • Total charge = = = + 8 charge - 8 charge 0 Count the protons and electrons in this fluorine atom Electrons in Energy Levels Electrons surround the nucleus in energy levels The number of energy levels depends on how many electrons the atom has How Many Electrons in Each Level? 1st energy level holds 2 e2nd energy level holds up to 8 e3rd energy level holds up to 18 eAtoms with 2 e- or less have 1 energy level Atoms with 3 to 10 e- have 2 energy levels Atoms with more than 10 e- have at least 3 energy levels Valence Electrons Valence electrons - The electrons in the outer most energy level Valence electrons determine the reactivity of an atom • Will it bond with other atoms? Ex. Oxygen atoms have 8 electrons • 2 in the first level • 6 in the second level • How many valence e- does an oxygen atom have? 6 The periodic table gives information about 1 atom of any element The periodic table gives information about 1 atom of any element The periodic table gives information about 1 atom of any element The periodic table gives information about 1 atom of any element The periodic table gives information about 1 atom of any element The periodic table gives information about 1 atom of any element Atomic Number = Number of protons & electrons • Ex. 1 oxygen atom has 8 protons and 8 electrons Mass Number = Number of protons + neutrons • Ex. 1 oxygen atom has 16 protons + neutrons • To figure out the # of neutrons, subtract the atomic number from the mass number • 16 – 8 = 8 so an oxygen atom has 8 neutrons How many protons, neutrons, and electrons are in one atom of sodium? Atomic number is 11, so there are: • 11 protons • 11 electrons Mass number is 23, so there are: • 23 – 11 = 12 • 12 neutrons