If you remove the line
\textwidth=6.5in \textheight 9in \oddsidemargin=0in \topmargin=-0.5in
the table is centred between the given margins. You load a lot of packages, but does not load geometry, which will help you to set proper margins in a much easier way, see example 2.
However, I suggest that you load booktabs, array and dcolumn and set your tabular without vertical lines, but with better spacing. Using D
-columns from dcolumn, you are able to align your figure columns at the decimal separator. In my example you will see that I separate the second column and input the \pm
though by removing the space between the columns and replace it with the \pm
sign. See the array manual for further explanation. In the new example 1B, I have set the second row utilising a special feature in the dcolumn package, which centres the second column around the plus/minus sign. Example 1B is the new MWE, and my recommended solution.:
Example 1 – booktabs rules and D-columns
\documentclass[12pt]{article}
\usepackage[left=0.625in, right=0.625in]{geometry}
\usepackage{fancyhdr, booktabs, dcolumn}
\usepackage{setspace}
\onehalfspacing
\usepackage{float}
\usepackage[font=small, labelfont=bf]{caption}
%\textwidth=6.5in \textheight 9in \oddsidemargin=0in \topmargin=-0.5in
\cfoot{\thepage}
\begin{document}
\begin{table}[H]
\footnotesize
\setlength{\tabcolsep}{0.3em}
\centering
\caption{Ne-Kls Viscosity Data Ne75}
\medskip
\label{Ne$_{75}$Kls$_{25}$+XF Viscosity Data}
\begin{tabular}{@{}l
r@{$\,\pm\,$}l
D{.}{.}{3.2}cc
D{.}{.}{5.0}
D{.}{.}{4.0}
D{.}{.}{2.2}
D{.}{.}{-1.2}
D{.}{.}{4.2}
D{.}{.}{3.2}
D{.}{.}{4.0}
D{.}{.}{2.2}
@{}}
\toprule
Sample &
\multicolumn{2}{c}{Wt. \% F} &
\multicolumn{1}{c}{Mol \% F} &
Segment &
Method &
T (^{\circ}C) &
$T (K)$ &
\multicolumn{1}{c}{log$\eta$} &
A & B & C &
T^{12} & m \\
\midrule
Ne$_{100}$+0F & 0.00 & 0.00 & 0.00 & 9a & PP & 800 & 1073 & 12.07 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00 & 0.00 & 0.00 & 7b & PP & 828 & 1101 & 11.04 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00 & 0.00 & 0.00 & 8c & PP & 842 & 1115 & 10.79 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00 & 0.00 & 0.00 & 5 & PP & 856 & 1129 & 10.24 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22\\
& 0.00 & 0.00 & 0.00 & 6 & PP & 870 & 1143 & 9.95 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00 & 0.00 & 0.00 & 4 & PP & 885 & 1158 & 9.50 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00 & 0.00 & 0.00 & 3b & CC & 1538 & 1811 & 1.70 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00 & 0.00 & 0.00 & 5b & CC & 1548 & 1821 & 1.65 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00 & 0.00 & 0.00 & 2b & CC & 1557 & 1830 & 1.60 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00 & 0.00 & 0.00 & 1b & CC & 1572 & 1846 & 1.53 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00 & 0.00 & 0.00 & 4b & CC & 1588 & 1861 & 1.45 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
\midrule
Ne$_{100}$+2F & 0.93 & 0.10 & 3.35 & 9 & PP & 821 & 1094 & 10.21 & -4.31 & 7486.03 & 582.01 & 1024 & 39.24\\
& 0.93 & 0.10 & 3.35 & 4 & PP & 849 & 1122 & 9.67 & -4.31 & 7486.03 & 582.01 & 1024 & 39.24\\
& 0.93 & 0.10 & 3.35 & 17-18 & CC & 1502 & 1775 & 1.98 & -4.31 & 7486.03 & 582.01 & 1024 & 39.24\\
& 0.93 & 0.10 & 3.35 & 13-14-15 & CC & 1550 & 1823 & 1.71 & -4.31 & 7486.03 & 582.01 & 1024 & 39.24\\
& 0.93 & 0.10 & 3.35 & 36-38-39 & CC & 1585 & 1859 & 1.54 & -4.31 & 7486.03 & 582.01 & 1024 & 39.24\\
& 0.93 & 0.10 & 3.35 & 9-10-11 & CC & 1589 & 1861 & 1.53 & -4.31 & 7486.03 & 582.01 & 1024 & 39.24\\
\midrule
Ne$_{100}$+4F & 2.42 & 0.07 & 8.39 & n.d. & PP & 679 & 952 & 10.72 & -4.31 & 8395.10 & 393.24 & 908 & 28.81\\
& 2.42 & 0.07 & 8.39 & n.d. & PP & 712 & 985 & 9.87 & -4.31 & 8395.10 & 393.24 & 908 & 28.81\\
& 2.42 & 0.07 & 8.39 & n.d. & CC & 1513 & 1786 & 1.74 & -4.31 & 8395.10 & 393.24 & 908 & 28.81\\
& 2.42 & 0.07 & 8.39 & n.d. & CC & 1561 & 1834 & 1.52 & -4.31 & 8395.10 & 393.24 & 908 & 28.81\\
& 2.42 & 0.07 & 8.39 & n.d. & CC & 1585 & 1858 & 1.41 & -4.31 & 8395.10 & 393.24 & 908 & 28.81\\
& 2.42 & 0.07 & 8.39 & n.d. & CC & 1589 & 1862 & 1.39 & -4.31 & 8395.10 & 393.24 & 908 & 28.81\\
\midrule
Ne$_{100}$+6F & 3.62 & 0.07 & 12.03 & n.d. & PP & 596 & 869 & 11.11 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62 & 0.07 & 12.03 & n.d. & PP & 622 & 895 & 10.67 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62 & 0.07 & 12.03 & 3 & PP & 651 & 924 & 10.03 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62 & 0.07 & 12.03 & n.d. & PP & 660 & 933 & 9.85 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62 & 0.07 & 12.03 & n.d. & CC & 1514 & 1787 & 1.51 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62 & 0.07 & 12.03 & n.d. & CC & 1558 & 1831 & 1.32 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62 & 0.07 & 12.03 & n.d. & CC & 1581 & 1854 & 1.23 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62 & 0.07 & 12.03 & n.d. & CC & 1582 & 1855 & 1.22 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
\bottomrule
\end{tabular}
\end{table}
\end{document}
Example 1B – special D-column
\documentclass[12pt]{article}
\usepackage[left=0.625in, right=0.625in]{geometry}
\usepackage{fancyhdr, booktabs, dcolumn, array}
\usepackage[font=small, labelfont=bf]{caption}
%\textwidth=6.5in \textheight 9in \oddsidemargin=0in \topmargin=-0.5in
\cfoot{\thepage}
\begin{document}
\begin{table}
\renewcommand{\arraystretch}{1.2}
\setlength{\tabcolsep}{0.3em}
\footnotesize
\centering
\medskip
\caption{$Ne_{75}Kls_{25}+XF$ Viscosity Data%
\label{Ne-Kls Viscosity Data Ne75-2}%
}
\begin{tabular}{@{}
>{$}l<{$}
D{+}{\,\pm\,}{3,3}
D{.}{.}{3.2}
cc
D{.}{.}{5.0}
D{.}{.}{4.0}
D{.}{.}{2.2}
D{.}{.}{-1.2}
D{.}{.}{4.2}
D{.}{.}{3.2}
D{.}{.}{4.0}
D{.}{.}{2.2}
@{}}
\toprule
\multicolumn{1}{@{}l}{Sample} &
\multicolumn{1}{c}{$Wt. \% F$} &
\multicolumn{1}{c}{$Mol \% F$} &
Segment &
Method &
T (^{\circ}C) &
T (K) &
\multicolumn{1}{c}{log$\eta$} &
A & B & C &
T^{12} &
m \\
\midrule
Ne_{100}+0F
& 0.00+0.00 & 0.00 & 9a & PP & 800 & 1073 & 12.07 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00+0.00 & 0.00 & 7b & PP & 828 & 1101 & 11.04 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00+0.00 & 0.00 & 8c & PP & 842 & 1115 & 10.79 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00+0.00 & 0.00 & 5 & PP & 856 & 1129 & 10.24 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22\\
& 0.00+0.00 & 0.00 & 6 & PP & 870 & 1143 & 9.95 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00+0.00 & 0.00 & 4 & PP & 885 & 1158 & 9.50 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00+0.00 & 0.00 & 3b & CC & 1538 & 1811 & 1.70 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00+0.00 & 0.00 & 5b & CC & 1548 & 1821 & 1.65 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00+0.00 & 0.00 & 2b & CC & 1557 & 1830 & 1.60 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00+0.00 & 0.00 & 1b & CC & 1572 & 1846 & 1.53 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00+0.00 & 0.00 & 4b & CC & 1588 & 1861 & 1.45 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
\midrule
Ne_{100}+2F
& 0.93+0.10 & 3.35 & 9 & PP & 821 & 1094 & 10.21 & -4.31 & 7486.03 & 582.01 & 1024 & 39.24\\
& 0.93+0.10 & 3.35 & 4 & PP & 849 & 1122 & 9.67 & -4.31 & 7486.03 & 582.01 & 1024 & 39.24\\
& 0.93+0.10 & 3.35 & 17--18 & CC & 1502 & 1775 & 1.98 & -4.31 & 7486.03 & 582.01 & 1024 & 39.24\\
& 0.93+0.10 & 3.35 & 13--14--15 & CC & 1550 & 1823 & 1.71 & -4.31 & 7486.03 & 582.01 & 1024 & 39.24\\
& 0.93+0.10 & 3.35 & 36--38--39 & CC & 1585 & 1859 & 1.54 & -4.31 & 7486.03 & 582.01 & 1024 & 39.24\\
& 0.93+0.10 & 3.35 & 9--10--11 & CC & 1589 & 1861 & 1.53 & -4.31 & 7486.03 & 582.01 & 1024 & 39.24\\
\midrule
Ne_{100}+4F
& 2.42+0.07 & 8.39 & n.d. & PP & 679 & 952 & 10.72 & -4.31 & 8395.10 & 393.24 & 908 & 28.81\\
& 2.42+0.07 & 8.39 & n.d. & PP & 712 & 985 & 9.87 & -4.31 & 8395.10 & 393.24 & 908 & 28.81\\
& 2.42+0.07 & 8.39 & n.d. & CC & 1513 & 1786 & 1.74 & -4.31 & 8395.10 & 393.24 & 908 & 28.81\\
& 2.42+0.07 & 8.39 & n.d. & CC & 1561 & 1834 & 1.52 & -4.31 & 8395.10 & 393.24 & 908 & 28.81\\
& 2.42+0.07 & 8.39 & n.d. & CC & 1585 & 1858 & 1.41 & -4.31 & 8395.10 & 393.24 & 908 & 28.81\\
& 2.42+0.07 & 8.39 & n.d. & CC & 1589 & 1862 & 1.39 & -4.31 & 8395.10 & 393.24 & 908 & 28.81\\
\midrule
Ne_{100}+6F
& 3.62+0.07 & 12.03 & n.d. & PP & 596 & 869 & 11.11 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62+0.07 & 12.03 & n.d. & PP & 622 & 895 & 10.67 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62+0.07 & 12.03 & 3 & PP & 651 & 924 & 10.03 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62+0.07 & 12.03 & n.d. & PP & 660 & 933 & 9.85 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62+0.07 & 12.03 & n.d. & CC & 1514 & 1787 & 1.51 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62+0.07 & 12.03 & n.d. & CC & 1558 & 1831 & 1.32 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62+0.07 & 12.03 & n.d. & CC & 1581 & 1854 & 1.23 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62+0.07 & 12.03 & n.d. & CC & 1582 & 1855 & 1.22 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
\bottomrule
\end{tabular}
\end{table}
\end{document}
Example 2 – original code as MWE
Here is you code as an MWE, unnecessary packages removed, but geometry added. As you will see, you tabular now centres between the left and right margins.
\documentclass[12pt]{article}
\usepackage[left=0.625in, right=0.625in]{geometry}
\usepackage{fancyhdr}
\usepackage{setspace}
\onehalfspacing
\usepackage{float}
\usepackage[font=small, labelfont=bf]{caption}
%\textwidth=6.5in \textheight 9in \oddsidemargin=0in \topmargin=-0.5in
\cfoot{\thepage}
\begin{document}
\begin{table}[H]
\scriptsize
\centering
\caption{Ne-Kls Viscosity Data Ne75}
\medskip
\label{Ne$_{75}$Kls$_{25}$+XF Viscosity Data}
\begin{tabular}{|c|cccccccccccc|}
\hline
\hline
Sample & Wt. \% F & Mol \% F & Segment & Method & $T$ ($^{\circ}C$) & $T$ (K) & log$\eta$ & A & B & C & T$^{12}$ & m \\
\hline
\hline
Ne$_{100}$+0F & 0.00$\pm$0.00 & 0.00 & 9a & PP & 800 & 1073 & 12.07 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00$\pm$0.00 & 0.00 & 7b & PP & 828 & 1101 & 11.04 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00$\pm$0.00 & 0.00 & 8c & PP & 842 & 1115 & 10.79 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00$\pm$0.00 & 0.00 & 5 & PP & 856 & 1129 & 10.24 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22\\
& 0.00$\pm$0.00 & 0.00 & 6 & PP & 870 & 1143 & 9.95 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00$\pm$0.00 & 0.00 & 4 & PP & 885 & 1158 & 9.50 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00$\pm$0.00 & 0.00 & 3b & CC & 1538 & 1811 & 1.70 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00$\pm$0.00 & 0.00 & 5b & CC & 1548 & 1821 & 1.65 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00$\pm$0.00 & 0.00 & 2b & CC & 1557 & 1830 & 1.60 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00$\pm$0.00 & 0.00 & 1b & CC & 1572 & 1846 & 1.53 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
& 0.00$\pm$0.00 & 0.00 & 4b & CC & 1588 & 1861 & 1.45 & -4.31 & 7025.89 & 646.54 & 1074 & 41.22 \\
\hline
Ne$_{100}$+2F & 0.93$\pm$0.10 & 3.35 & 9 & PP & 821 & 1094 & 10.21 & -4.31 & 7486.03 & 582.01 & 1024 & 39.24\\
& 0.93$\pm$0.10 & 3.35 & 4 & PP & 849 & 1122 & 9.67 & -4.31 & 7486.03 & 582.01 & 1024 & 39.24\\
& 0.93$\pm$0.10 & 3.35 & 17-18 & CC & 1502 & 1775 & 1.98 & -4.31 & 7486.03 & 582.01 & 1024 & 39.24\\
& 0.93$\pm$0.10 & 3.35 & 13-14-15 & CC & 1550 & 1823 & 1.71 & -4.31 & 7486.03 & 582.01 & 1024 & 39.24\\
& 0.93$\pm$0.10 & 3.35 & 36-38-39 & CC & 1585 & 1859 & 1.54 & -4.31 & 7486.03 & 582.01 & 1024 & 39.24\\
& 0.93$\pm$0.10 & 3.35 & 9-10-11 & CC & 1589 & 1861 & 1.53 & -4.31 & 7486.03 & 582.01 & 1024 & 39.24\\
\hline
Ne$_{100}$+4F & 2.42$\pm$0.07 & 8.39 & n.d. & PP & 679 & 952 & 10.72 & -4.31 & 8395.10 & 393.24 & 908 & 28.81\\
& 2.42$\pm$0.07 & 8.39 & n.d. & PP & 712 & 985 & 9.87 & -4.31 & 8395.10 & 393.24 & 908 & 28.81\\
& 2.42$\pm$0.07 & 8.39 & n.d. & CC & 1513 & 1786 & 1.74 & -4.31 & 8395.10 & 393.24 & 908 & 28.81\\
& 2.42$\pm$0.07 & 8.39 & n.d. & CC & 1561 & 1834 & 1.52 & -4.31 & 8395.10 & 393.24 & 908 & 28.81\\
& 2.42$\pm$0.07 & 8.39 & n.d. & CC & 1585 & 1858 & 1.41 & -4.31 & 8395.10 & 393.24 & 908 & 28.81\\
& 2.42$\pm$0.07 & 8.39 & n.d. & CC & 1589 & 1862 & 1.39 & -4.31 & 8395.10 & 393.24 & 908 & 28.81\\
\hline
Ne$_{100}$+6F & 3.62$\pm$0.07 & 12.03 & n.d. & PP & 596 & 869 & 11.11 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62$\pm$0.07 & 12.03 & n.d. & PP & 622 & 895 & 10.67 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62$\pm$0.07 & 12.03 & 3 & PP & 651 & 924 & 10.03 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62$\pm$0.07 & 12.03 & n.d. & PP & 660 & 933 & 9.85 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62$\pm$0.07 & 12.03 & n.d. & CC & 1514 & 1787 & 1.51 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62$\pm$0.07 & 12.03 & n.d. & CC & 1558 & 1831 & 1.32 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62$\pm$0.07 & 12.03 & n.d. & CC & 1581 & 1854 & 1.23 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
& 3.62$\pm$0.07 & 12.03 & n.d. & CC & 1582 & 1855 & 1.22 & -4.31 & 8526.37 & 324.43 & 832 & 27.57\\
\hline
\end{tabular}
\end{table}
\end{document}
\textwidth=6.5in \textheight 9in \oddsidemargin=0in \topmargin=-0.5in