Atom is the smallest fraction of an element thatCan exist , and still show the characteristics ofThe element .Atoms are composed essentially of electrons (1Negative charge ) , protons (1positive charge ),andNeutrons (no charge ) .Atoms of different elements are distinguished byThe number of protons in the nucleus .
The measuring unit of tiny masses in atom is called (atomicmass unit ) (amu) .1 amu = 1.661 x 10-24 grams .The mass of an electron = 9.11 x 10-28 g or 5.4 x10-4 amuThe mass of a proton = 1.67 x10-24 g or 1.007 amu .The mass of a neutron = 1.68 x 10-24 g or 1.009 amu .an atomic mass unit or amu is one twelfth of the mass of anunbound atom of carbon-12. It is a unit of mass used toexpress atomic masses and molecular masses.Also Known As: unified atomic mass unit (u), Dalton (Da)or universal mass unit
Atomic number and atomic mass :What makes an atom of one element different fromAnother ? For example , how does carbon atomDiffers from oxygen . Each element haveA characteristic number of protons , so that , theNumber of protons in the nucleus of the atom isIs called the atomic number .Because the atom has no net electrical charge ,The number of electrons it contains must equalThe number of protons .
Elements are made of one or more than one same atom . Each element has a name and symbol , symbols are made of 1 or 2 letters taken from the name of the element . The first letter of symbol is derived from the the 1st letter of the element . For example C,N,O ,F . Some elements have symbols derived from their Latin names .for example (Na) ,natrium ,
Element Symbol Latin NameAntimony Sb StibiumCopper Cu CuprumGold Au AurumIron Fe FerrumLead Pb PlumbumMercury Hg HydragyrumPotassium K KaliumSilver Ag ArgentumSodium Na NatriumTin Sn Stannum
the symbol C-12 represents the carbon atom whichcontain six protons and six neutrons . The atomicnumber is shown by the subscript , and the super-script , called the mass number , which it is thetotal number of protons plus neutrons in the atom . 12 ← mass number (protons + neutrons) C 6 ← atomic number (protons or electrons)
Periods and groups :The period is a horizontal row of elements . theperiodic table consists of seven periods .groups or families are columns of elements in theperiodic table .the elements of a particular groupor family share many similarities , as in humanfamily , the similarities due to physical andchemical properties that are related to similaritiesin electronic structure (the way in which electronsare arranged in the atom) .
Isotopes : isotopes have kinds of elements having the same number of protons but different number of neutrons . For example hydrogen contains one proton in the neucleous and zero neutron ,deutrium contains 1 proton &1 neutron and tritium contains 1proton & 2 neutrons .Mass num. = number of protons + no. of neutrons .Atomic number = number of protons .
Some important properties of elements:Electronegativity :Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons . The Pauling scale is the most commonly used. Fluorine (the most electronegative element) is assigned a value of 4.0, and values range down to caesium and francium which are the least electronegative at 0.7 .
Fluorine, the most reactive non-metal, is assigned the highest value since it has the greatest attraction for the electron being shared by the other element. Oxygen is also highly electronegative and has a strong attraction for electrons.Metals have low electronegativities since they have weak attraction for any shared electrons.When two unlike atoms are convalently bonded, the shared electrons will be more strongly attracted to the atom of greater electronegativity. Such a bond is said to be polar. A polar bond results in the unequal sharing of the electrons in the bond.
The difference in electronegativities of two elements can be used to predict the nature of the chemical bond. Bond type can be described as belonging to one of three classes:nonpolar covalentpolar covalentionicWhen differences are 1.7 or greater, the bond is usually ionic.Less than 1.7, the bond is usually covalent, and unless the difference is less than 0.5 the bond has some degree of polarity.Differences of less than 0.5 are considered to be nonpolar.
Ionization energy :The energy needed to remove one or more electrons ( the outermost, or highest energy, electron) from a neutral atom in the gas phase to form a positively charged ion (cation ) .In general, the first ionization energy increases as we go from left to right across a row of the periodic table.The first ionization energy decreases as we go down a column of the periodic table.
Electron affinity :Electron affinity reflects the ability of an atom to accept an electron to form a negative charge element (anion ) .In general, electron affinity increases as we go from left to right across a row of the periodic table.Electron affinity decreases as we go down a column of the periodic table.
Atomic size (atomic radius) :In general, atomic size decreases as we go from left to right across a row of the periodic table.Atomic size increases as we go down a column of the periodic table.Atomic size for neutral atom is smaller than the negative charged ion for the same element .Atomic size for neutral atom is larger than the positive charged ion for the same element .