Element Crossword It's a Gas! In this International Year of Chemistry, find the gas. Credit: Thomas Jefferson National Accelerator Facility - Office of Science Education, http://education.jlab.org/elementcrossword/gas_01.html Across 2. This gas can be used to fill party balloons. 4. This gas is used to make yellow-green 'neon' signs. 6. This gas is widely used in luminous signs. 7. Nearly 1% of the earth's atmosphere is... 9. About 21% of the earth's atmosphere is... 10. Most of the visible universe is made of... 11. This gas is used in strobe lights. Down 1. When combined with tin, this gas helps keep your teeth strong. 3. This gas helps keep swimming pools clean. 5. About 78% of the earth's atmosphere is... 8. A radioactive gas.