We present Chandra observations of 2106 radio-quiet quasars in the redshift range 1.7 ≤ z ≤ 2.7 from the Sloan Digital Sky Survey (SDSS), through data release fourteen (DR14), that do not contain broad absorption lines (BAL) in their rest-frame UV spectra. This sample adds over a decade worth of SDSS and Chandra observations to our previously published sample of 139 quasars from SDSS DR5 which is still used to correlate X-ray and optical/UV emission in typical quasars. We fit the SDSS spectra for 753 of the quasars in our sample that have high-quality (exposure time ≥ 10 ks and off-axis observation angle ≤ 10 ) X-ray observations, and analyze their X-ray-to-optical SED properties ( α ox and ∆α ox ) with respect to the measured CIV and MgII emission-line rest-frame equivalent width (EW) and the CIV emission-line blueshift. We find significant correlations (at the ≥ 99.99% level) between α ox and these emission-line parameters, as well as between ∆α ox and C iv EW. Slight correlations are found between ∆α ox and C iv blueshift, Mg ii EW, and the C iv EW to Mg ii EW ratio. The best-fit trend in each parameter space is used to compare the X-ray weakness ( ∆α ox ) and optical/UV emission properties of typical quasars and weak-line quasars (WLQs). The WLQs typically exhibit weaker X-ray emission than predicted by the typical quasar relationships. The best-fit relationships for our typical quasars are consistent with predictions from the disk-wind quasar model. The behavior of the WLQs compared to our typical quasars can be explained by an X-ray "shielding" model.

We investigate systematically the X-ray emission from type 1 quasars using a sample of 1825 Sloan Digital Sky Survey non-broad absorption line (non-BAL) quasars with Chandra archival observations. A significant correlation is found between the X-ray-to-optical power-law slope parameter (α OX) and the 2500 Å monochromatic luminosity (L 2500Å), and the X-ray weakness of a quasar is assessed via the deviation of its α OX value from that expected from this relation. We demonstrate the existence of a population of non-BAL X-ray-weak quasars, and the fractions of quasars that are X-ray weak by factors of ≥6 and ≥10 are 5.8% ± 0.7% and 2.7% ± 0.5%, respectively. We classify X-ray-weak quasars (X-ray weak by factors of ≥6) into three categories based on their optical spectral features: weak emission-line quasars (WLQs; C iv rest-frame equivalent width < 16 Å), red quasars (Δ(g − i) > 0.2), and unclassified X-ray-weak quasars. The X-ray-weak fraction of within the WLQ population is significantly higher than that within non-WLQs, confirming previous findings that WLQs represent one population of X-ray-weak quasars. The X-ray-weak fraction of within the red quasar population is also considerably higher than that within the normal quasar population. The unclassified X-ray-weak quasars do not have unusual optical spectral features, and their X-ray weakness may be mainly related to quasar X-ray variability.

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