The element actinium is unstable and radioactive. Use the periodic table to predict the formula for actinium chloride, given the formula of lanthanum chloride, LaCl₃.

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Periodic table of the elements Alkali metals Alkaline-earth metals Transition metals Other metals Other nonmetals period - 2 3 4 5 6 7 group 1* 3 H 2 4 Li 11 Na 19 K 37 Rb 55 Cs 87 Fr Be 12 Mg 20 21 22 23 Ca Sc Ti V 38 39 40 41 Sr Y Zr 56 57 72 73 74 Ba La Hf Ta W 88 89 104 105 Ra Ac Rf 3 lanthanoid series 6 actinoid series 7 4 58 Ce 90 Th 5 es Db Sg 7 Halogens Noble gases Pa U Rare-earth elements (21, 39, 57-71) and lanthanoid elements (57-71 only) Actinoid elements 8 9 27 6 24 25 26 28 Cr Mn Fe Co Ni 45 47 48 Ag Cd Sn Sb 79 80 81 82 83 42 43 44 46 Nb Mo Tc Ru Rh Pd 75 76 77 78 Re Os Ir Pt Au Hg TI Pb Bi 107 108 109 110 111 112 113 114 Bh Hs Mt Ds Rg Cn Nh FI 59 60 61 62 63 Pr Nd Pm Sm Eu 91 92 93 94 95 Np Pu Am 10 64 Gd 96 Cm 11 29 30 Cu Zn 12 97 Bk 13 5 6 B 14 65 66 67 Tb C 49 14 31 32 Ga Ge 50 7 13 15 Al Si P 33 34 35 As Se Br 68 15 Dy Ho Er 98 99 100 Cf Es Fm N 16 17 8 OF 16 S 84 9 10 18 2 Po At 115 116 117 Mc Lv Ts He 17 18 CI 69 70 71 Tm Yb Lu 101 102 103 Md No Lr Ne Ar 51 52 53 54 Te I 85 86 36 Kr Xe Rn 118 Og