# Chapter 3 Atomic Structure and Periodic Table.docx -...

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Page1Chapter 3: Atomic Structure And PeriodicTable:Earlier, we’ve studied that elements are the purest substances of all. Andeach element has its own type of atoms. When scientists first discoveredthe atom, they believed it was a spherical structure like marbles. Lateron other scientists discovered that there the atom is made up of evensmaller sub-atomic particles.The Atomic Structure:It was discovered that an atom is made up of three types of sub-atomicparticles, these areprotons,neutronsandelectrons. It was alsodiscovered that in the center of an atom, there is aNucleuswhich ismade up of protons and neutrons. Around the nucleus there are energyshells in whichelectronsare. Electrons are always orbiting the nucleus inthe energy shells.Kareem Mokhtar©
Page2Protons and Neutrons are said to be Nucleons because together theymake the nucleus. Each type of these sub-atomic particles has its ownphysical properties which are explained in this table:ParticleSymbolMassChargeProtonp1+1Neutronn1Electronѐ1/200 (Negligible)-1These physical properties tell us several factors:The mass of the atom is concentrated in the nucleus, this isbecause the mass of an electron is negligible.A neutron has no charge, it is neutralAn atom is also neutral, this is because it always contains asmuch positive protons as negative electrons.The mass of the sub-atomic particles is in atomic mass unit (AMU). Thisis because they are so light that they can’t be measured in grams.There are two numbers given to each type of atoms:Atomic/Proton Number:it is the number of protons in an atom. Andsince an atom has an equal number of protons and electrons, it is thenumber of electrons in the atom too.Mass/Nucleon Number:it is the number of protons + number ofnucleons in the nucleus of an atom. It is called mass number because itsvalue is also mass of the atom.Kareem Mokhtar©
Page3If the atomic number is the number of protons, and the mass number isthe number of protons and neutrons, then subtracting the protonnumber from the mass number will give you the number of neutrons inthe atom.When we represent the atom of an element, we give it a symbol of oneor two letters where the first letter is always in capitals and the secondone is in lowercase. The mass number goes above the symbol and theatomic number goes below the symbol.Example:4He2This represent the Helium AtomIts symbol is HeIts mass number is 4Its atomic number is 2It has 2 protons, 2 neutrons (4-2=2), and 2 electrons.24Mg12This represents the Magnesium AtomIts symbol is MgIts mass number is 24Its atomic number is 12It has 12 protons, 12 neutrons (24-12=12), and 12 electrons.

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Atom, Chemical element, Kareem Mokhtar