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Atomic structure and periodic table General objectives Learners should acquire an understanding of the structure and characteristics of atoms Learners should be aware of the periodic table as a method of classifying elements The Atom An atom is the smallest particle of an element that can take part in a chemical reaction. P N e An atom consists of a nucleus containing nucleons namely protons and neutrons. Electrons move around the nucleus in pathways called orbits or shells Proton Neutron Electron Relative mass Relative charge Location In an atom of an element, the number of protons always equals the number of electrons Atomic number (proton number) The number of protons in the nucleus of an atom Atomic number is characteristic of each element No two elements can have the same atomic number Mass number (nucleon number) The sum of protons and neutrons in an atom Atomic number and mass number are often written together with the symbol for an atom Mass number Symbol Atomic number Symbol for an atom of carbon atomic number = Mass number = Atomic structure and the periodic table A periodic table is a way of organising the known elements according to their similarities In the table, elements are arranged in order of their atomic number Build up of electrons in shells The horizontal rows are called periods (numbered from 1-7) Each period represents the filling of one electron shell Shell number 1 can contain only 2 electrons, therefore period one has only 2 elements, hydrogen and helium. Shell number 2 can contain 8 electrons. Period 2 has 8 elements going from Li to Ne Shell number 3 can contain 18 electrons. However, after only 8 electrons it appears to be full and then shell number 4 begins the fill The number of the period is also the number of the shells in the atoms of that period. Groups The vertical columns in the periodic table are called groups The groups are numbered from l to Vll The last group is numbered Vlll or 0 From group l to Vll, all the members of the group have the same number of outermost electrons. This is the same as the group number The noble gas configuration All members of group 0, except helium have 8 outermost electrons. This is a very stable arrangement called a stable octet, and these elements do not take part in chemical reactions Atoms tend to attain noble state/structure by loosing, gaining or sharing of electrons Valency Electrons When electrons take part in chemical reaction, it is only the outermost electrons that actually take part The outermost electrons are called valency electrons (because they are involved in bonding) Valency is the combining power of an atom Example Lithium has a valency of 1. By loosing 1 electron it can achieve the noble gas electronic structure of helium Li He Fluorine also has a valency of 1. By gaining 1 electron it can have the electronic structure of Neon F Ne The electronic structure of elements 1 to 20 The significance of valency electrons is to take part in a chemical reaction. The table below summaries how atoms in each group achieve a noble gas configuration Group Outermost electrons l ll lll lV V Vl Vll To achieve noble gas configuration Valency Isotopes Isotopes are atoms of the same elements having the same number of protons but different number of neutrons Isotopes of the same element usually have identical chemical properties but they have different physical properties Many elements consist of two or more isotopes Hydrogen There are three different types of hydrogen atom H H H Hydrogen Deuterium Tritium In any sample of hydrogen, both deuterium and tritium are present in extremely small quantities Chlorine There are two isotopes of chlorine Cl Cl In any sample of chlorine gas about 25% of the atoms in the sample of the 37 Cl type while 75% are of the 35 Cl type. This gives the average mass of 35.5 Carbon There are two common isotopes of carbon C C Because carbon 14 is radioactive, its presence can be easily detected. The amount of this isotope in bone can be used to tell how old the bone is. The process is called carbon dating.