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 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 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).