Lecture 3 Atomic theory, atomic number, isotopes, atomic mass, Periodic table Atomic Theory Dalton (1803 – 1810) All maAer is composed of extremely small parDcles called atoms. All atoms of a given element are idenDcal, having the same size, mass, and chemical properDes. The atoms of one element are different from the atoms of all other elements. Compounds are composed of atoms of more than one element. Chemical reacDons are rearrangements Atoms are indivisible × A chemical reacDon involves only the separaDon, combinaDon, or rearrangement of atoms; it does not result in their creaDon or destrucDon. (Lavoisier ConservaDon of maAer) An atom consists of a nucleus + electrons Nucleus consists of protons and neutrons and is surrounded by electrons The number of protons is called the atomic number Z (German Zahl for number) and defines the element All atoms of a given element have the same atomic number In a neutral atom the number of electrons equals the number of protons electron If nucleus were size of a basketball electrons would be 14 miles away proton neutron Most of atom is empty space! size of nucleus (1.75 − 15.) × 10
size of atom 2 × 10
−10
m
−15
m
1869 Dimitri Mendeleev Arranged elements on basis of atomic mass 1913 Henry Moseley (1888-­‐1915) Determined charge on nucleus of all known elements Arranged elements on basis of atomic number Par$cle Mass (kg) Charge (C) Rela$ve charge proton 1.673x10-­‐27 +1.602x10-­‐19 +1 neutron 1.675x10-­‐27 0 electron 9.109x10-­‐31 -­‐1.602x10-­‐19 -­‐1 0 The number of neutrons in the nucleus of an atom is variable and we will represent this number by the leAer N The number of neutrons plus the number of protons in a nucleus is called the mass number and is represented by the symbol A. Atoms with the same Z but different A are called Isotopes. Isotopes Same Z different N so different A A
Z
X≡ X
A
A=Z+N !
Most atoms have several isotopes, some have one (monoisotopic) 19F only one isotope 39K 93.220 % 12C 98.89% 41K 6.770 % 13C 1.11% 40K 0.012 % There are three isotopes of Hydrogen Z N A 1H 1 0 1 2H 1 1 2 Deuterium 3H 1 2 3 TriDum Hydrogen(proDum) Atomic Mass Unit 1/12th the mass of 12C Represented by the symbol u u = 1.66054018 x 10-­‐24 g The mass of an atom is oeen given as a mulDple of u Par$cle Mass (kg) Mass (amu) proton 1.673x10-­‐27 1.008 neutron 1.675x10-­‐27 1.009 electron 9.109x10-­‐31 5.485x10-­‐4 Atomic Mass of an element Weighted average of the masses of all the naturally occurring isotopes of an element A (Ne) Mass (u) % abundance 20 19.9924 90.46 21 20.9938 0.2700 22 21.9914 9.250 Calculate the atomic mass of the element Ne ⎛ 90.46 ⎞
⎛ 0.2700 ⎞
⎛ 9.250 ⎞
⎜⎝
⎟⎠ 19.9924 + ⎜⎝
⎟⎠ 20.9938 + ⎜⎝
⎟⎠ 21.9914 = 20.1760
100
100
100
Average atomic mass of Ne 20.18 amu An isotope of Ne consists of 10 protons and 10 neutrons (A=20) a proton has a mass of 1.008 amu a neutron has a mass 1.009 amu Why is the mass of this isotope 19.9924 amu and not the sum of the masses of its consDtuent parDcles, which is 20.17 amu? Atoms may gain or loose electrons and form IONS −
Cl + e → Cl
−
chlorine atom becomes the chloride ion NegaDve ions are called anions +
Na → Na + e
−
sodium atom becomes the sodium ion PosiDve ions are called caDons Non metals(negaDve ions) anions atom name Nega$ve ion name O oxygen O2-­‐ oxide N nitrogen N3-­‐ nitride F fluorine F-­‐ fluoride Br bromine Br-­‐ bromide Metals (posiDve ions) caDons atom name Posi$ve ion name Na sodium Na+ sodium ion Ca calcium Ca2+ calcium ion Al aluminum Al3+ aluminum ion P block D block S block F block What is an ion and how does it differ from an atom? Classify the following as a metal, semimetal, or non-­‐metal Na metal Ge metalloid Cl nonmetal Mg metal N nonmetal Cu metal A monatomic ion contains 15 protons, 16 neutrons, and 18 electrons. Which ion of what isotope is this? 15 protons means its Phosphorous 16 neutrons + 15 protons means the atomic mass A = 31 18 electrons means there are 3 extra electrons resulDng in a charge of 3-­‐ 31
P
3−
Which group contains the halogens? Which group contains the alkali metals ? Which group contains the noble gases? What do we call the elements in group 2A?