TOPIC 3 Atoms and the Periodic Table

All known Atoms are arranged in the Periodic Table(PT). Each type of Atom is known as an Element. There are approx 118 known elements. Columns down the way in the PT are known as Groups and Rows across the way are called Periods.

Names of Groups Group 1 is called the Alkali Metals. This is because they form alkalis when placed in water. Group8 or 0 is called the Noble gases. The elements in this group are extremely unreactive Certain groups have specific names Group 7 is called the Halogens. This group is the most reactive of the non-metals

Atomic Numbers and Mass Numbers The Atomic Number: This tells us the number of protons in an atom The Atomic Mass No: This tells us the number of protons and neutrons added together. x Atomic No. Atomic Mass No. For an atom to be neutral it must have the same no. of Protons and Electrons No.of Neutrons.= Atomic mass – Atomic number eg 23 Na 11 To calculate the no.of Neutrons in an atom

Atoms All things are made up of Atoms. Atoms are so small and light that a special scale called Atomic Mass Units had to be made up (amu). Atoms are made up of three sub-atomic particles called, Protons, Neutrons and Electrons p n e Protons and neutrons are found in the nucleus The Nucleus Electrons orbit the nucleus ParticleMass(amu)Charge Proton1 + Electron0 - Neutron10

Chemical Definitions Atoms are so light that we find we have to use a special instrument to measure their mass. It is called a Mass Spectrometer. Using this instrument it has been found that atoms of the same element have slightly different masses. These are called Isotopes. In order to determine the mass of an atom we have to take an average of the masses, taking into account the isotopes. This is called the Relative Atomic Mass (r.a.m). Isotopes are atoms of the same element with different Mass Numbers because they have different numbers of Neutrons.

Ions When Atoms join they must collide with each other. Since the electrons are on the outside of the atom, it is these that are affected and not the protons or the neutrons. Ions are Charged atoms –These are formed when atoms either lose or gain electrons. When an Atom loses electrons a Positive Ion is formed When an Atom gains electrons a Negative Ion is formed

To calculate the number of protons,neutrons and electrons in Atoms and Ions Mg No.of p = 12 No.of e = 12 No.of n = =12 Mg No. of p = 12 No. of e = 12-2 = 10 No. of n = 24-12=12 O 16 8 No.of p = 8 No.of e = 8 No.of n = 16-8 = 8 O No. of p = 8 No. of e = 8+2 = 10 No. of n = 16-8= 8

Electron Arrangement Atoms join together so as to achieve their most stable electron arrangement(EA). xx xx x x Electrons are arranged in Energy Levels(or shells) around the nucleus. The first Energy Level can take up to 2 electrons The second energy level can take up to 8 electrons The third energy level can take up to 18 electrons(but at this stage we shall consider it can take up to 8 electrons)

Examples of electron arrangements A Ca Atom has 20 electrons therefore it has an EA of 2,8,8,2 A Ca 2+ Ion has 18 electrons (since it has lost two) therefore it has an EA of 2,8,8 A S Atom has 16 electrons therefore it has an EA of 2,8,6 A S 2 - Ion has 18 electrons (since it has gained two) therefore it has an EA of 2,8,8 Atoms join so as to be closest to their nearest most stable Electron Arrangement ie 2 or 2,8 or 2,8,8 etc.