Which atom can accommodate an octet of electrons but doesn't necessarily have to?

I don't get balancing chemical equations, can someone explain?

Calculate the density of a substance that has a mass of 250g and a volume of 50. Ml A-50 g/mL B-5.0 g/mL C-0.50 g/mL D-0.005 g/mL

Which best defines partial pressure in a mixture of gases

In two or more words complete sentences, differentiate between inner (terrestrial) planets and outer (Jovian) planets.

What is the percent yield of a reaction in which 200. g of phosphorus trichloride reacts with excess water to form 91.0 g of hcl and aqueous phosphorous acid (h3po3) ?

Sarah heated some orange coloured crystals in a test tube. The crystals turned green and fizzed. After the fizzing stopped, the test tube was allowed to cool. A green powder was left in the tube.

In which orbital does an electron in a bromine atom experience the greatest effective nuclear charge?

Kool-Aid is made up of sugar (C6H12O6), food coloring, and water. It is an example of a (n) element heterogeneous mixture homogeneous mixture compound

A laboratory experiment requires 250 millimeters of water for boiling. It also requires 100 millimeters of water for a cooling process.

Which set of procedures and observations indicates a chemical change? A) Ethanol is added to an empty beaker and the ethanol eventually disappears. B) A solid is gently heated in a crucible and the solid slowly turns to liquid.