We report the detection of radio recombination line (RRL) H_40{alpha}_ towards 75 sources, with data obtained from ACA (Atacama Compact 7 m Array) observations in the ATOMS (ALMA Three-millimeter Observations of Massive Star-forming regions) survey of 146 active Galactic star-forming regions. We calculated ionized gas mass and star formation rate (SFR) with H_40{alpha}_ line emission. The mass of ionized gas is significantly smaller than molecular gas mass, indicating that ionized gas is negligible in the star-forming clumps of the ATOMS sample. The SFR estimated with RRL H_40{alpha}_ agrees well with that calculated with the total bolometric luminosity (L_bol_) when SFR => 5 M_{sun}_.Myr^-1^, suggesting that millimetre RRLs could well sample the upper part of the initial mass function and thus be good tracers of SFR. We also study the relationships between L_bol_ and the molecular line luminosities (L'_mol_) of CS J = 2-1 and HC3N J = 11-10 for all the 146 ATOMS sources. The L_bol_ - L'_mol_ correlations of both the CS J = 2-1 and HC3N J = 11-10 lines appear approximately linear and these transitions have success in predicting L_bol_ similar to that of more commonly used transitions. The L_bol_ -to- L'_mol_ ratios or SFR-to-mass ratios (star formation efficiency) do not change with galactocentric distances (R_GC_). Sources with H_40{alpha}_ emission (or H II regions) show higher L_bol_ -to- L'_mol_ ratios than those without H_40{alpha}_ emission, which may be an evolutionary effect.