"et al." is an abbreviation of "et alli". "et" is a complete word hence no full-stop is required BUT "al." is an abbreviation of "alli" hence a full stop is required.
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Li [as the author's last name] and others is the English equivalent of Li et al. In the word by word translation, the conjunction 'et' means 'and'. The syllable 'al.' is an abbreviation for 'alii'....
Et al. is an abbreviation for the Latin phrase et alii, which means "and others". Since al. is an abbreviation, it is properly spelled with a period/full stop.