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1 Atomic Structure – Learning Outcomes Describe the structure of the atom. State the location, relative charge, and atomic mass of the sub-atomic particles. HL: Define atomic number. HL: Draw the Bohr structure of the first 20 elements. HL: Define isotope. 2 Describe the Structure of the Atom An atom is the smallest particle of an element which still retains the properties of that element. Atoms have a solid core called the nucleus. The nucleus contains two types of sub-atomic particles – protons and neutrons. The nucleus is surrounded by mostly empty space. The empty space contains shells where another subatomic particle, electrons are found. 3 State Facts About Sub-Atomic Particles The location, charge, and atomic mass of sub-atomic particles are how they are distinguished. These masses and charges are so small (e.g. mass of proton = 0.000 000 000 000 000 000 000 000 001 67 kg), that we use new units more suitable. For mass, we use atomic mass units (u) and for charge, we use elementary charge. Particle Proton Neutron Electron Location Nucleus Nucleus Outside the nucleus Charge +1 0 -1 Atomic Mass 1 1 1 1850 by 2012rc – CC-BY-SA-3.0 4 Define Atomic Number 5 Define Atomic Number The atomic number of an atom is the number of protons in its nucleus. Most of the time, this is also the number of electrons the atom has orbiting it. The mass number of an atom is the sum of protons and neutrons in its nucleus. i.e. #neutrons + #protons = mass number 6 Draw Bohr Structure Protons and neutrons always exist in the nucleus. 7 Draw Bohr Structure Electrons are more complicated: They exist outside the nucleus. 8 Draw Bohr Structure They exist in pairs where possible. 9 Draw Bohr Structure Shells have a maximum amount of space. The first shell takes two electrons. A new shell is needed to add more. 10 Draw Bohr Structure Pairs on each shell spread away from each other. 11 Draw Bohr Structure The second and third shells hold 8 electrons each. Remember the first shell can only hold 2. 12 Draw Bohr Structure These rules combined give the Bohr structure of the atom. On our course, we need to be able to draw the Bohr structure of the first 20 elements. Draw the Bohr structure for: Hydrogen (𝐻11 ) 11 Sodium 𝑁𝑎23 Helium 𝐻𝑒42 14 Silicon 𝑆𝑖28 Lithium (𝐿𝑖73 ) 18 Argon 𝐴𝑟40 8 Oxygen 𝑂16 19 Potassium 𝐾39 10 Neon (𝑁𝑒20 ) 20 Calcium (𝐶𝑎40 ) 13 Define Isotope Each element is defined by its atomic number. Notice that the mass number on the periodic table comes with decimals. e.g. the mass number of hydrogen is given as 1.00794. Every hydrogen has 1 proton (that’s what makes it hydrogen), but some hydrogens have different numbers of neutrons in their nucleus. Hydrogen-1 has 1 proton, 0 neutrons (total mass 1) Hydrogen-2 has 1 proton, 1 neutron (total mass 2) Hydrogen-3 has 1 proton, 2 neutrons (total mass 3) 14 Define Isotope Isotopes are atoms with the same atomic number but different mass numbers. The 1.00794 given as the mass of hydrogen on the periodic table is an average value for the mass of hydrogen. Since hydrogen-1 is much, much more common than hydrogen-2 or hydrogen-3, the average mass (relative atomic mass) is very close to 1.