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2.2 Atomic Theory (Section 6.2 pg 153-156) • The idea of the atom is an ancient one, initially proposed by the Greeks. • Atomic Theory (the human understanding of atoms) has gone through many ‘revisions’ since: – John Dalton revised it in the early 1800s. He claimed all matter is made of atoms that are indestructible and unique to each element. • Atomic Theory (the human understanding of atoms) has gone through many ‘revisions’ since: – In 1898, J.J. Thomson discovered that the atom has tiny negatively charged particles called electrons. • Atomic Theory (the human understanding of atoms) has gone through many ‘revisions’ since: – Ernest Rutherford discovered that the atom also has positive particles called protons, that they were contained in a small, dense area in the middle (the nucleus), and that the nucleus accounted for nearly all the weight of an atom. • Atomic Theory (the human understanding of atoms) has gone through many ‘revisions’ since: – Niels Bohr proposed the idea of ‘electron shells’; i.e. the concept that electrons orbited around the nucleus. • Although Bohr’s ideas have been developed further, we can still use Bohr’s model to help understand the bonding behaviour and other properties of atoms. • Here are some key concepts: – Every atom is composed of three types of subatomic particles (proton, electron, neutron) – Proton = found in the nucleus, positively charged with an AMU (atomic mass unit) of 1. – Neutron = found in the nucleus, no charge (neutral) with an AMU of 1. – Electron = orbits the nucleus in an electron shell and has an AMU of 1/1800. • Each element has a unique number of protons in its nucleus; the # of protons is called an element’s atomic number. • E.g. H has the atomic #1; O has the atomic #8. • Although the atomic number (# of protons) for each element is the same, individual atoms in a sample may have different # of neutrons. • The mass number of an atom is the total number of protons and neutrons in the nucleus. • The mass number is written after the element name e.g. oxygen-18 represents O with a mass # of 18. • Different mass numbers don’t affect the physical or chemical properties of elements! • The atomic mass of an element is the average mass of the atoms of the element and reflects the abundance of the different mass numbers of the element (Fig.2 p.153). • When we draw the atom according to Bohr we call it a Bohr Diagram. There are certain rules to follow when drawing a Bohr Diagram (modified from p.155 in text): • Look at what symbol the element has and write this in the centre. Write the atomic # in front of it at the bottom. • Determine the # of shells needed. Find what row (period) it is in. This will be the # of electron shells. Draw them in. • The atomic # represents the # of electrons (e) needed. Start to fill the shells. The first shell 1 set of paired electrons (2 e). The first shell is ‘full’ when it has 2 e. • Fill the rest of the shells with your remaining e, obeying the following rules: Start by placing single e around the shell, then pair up any remaining e. The 2nd shell can take up to 8 e (4 pairs) The 3rd shell can take up to 8 e (4 pairs) The 4th shell can take up to 18 e (9 pairs) • Never add more electrons than you started with (never add more than the atomic #). • The 5 methods for writing a Bohr diagram shown below are all correct ways to represent O (Fig.6 p.155).