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).