Compounds and Chemical Formulae Flashcards

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Note: We may include questions that have multiple correct answers. It's useful to remember specific examples to understand these concepts better.

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What is a compound?

A compound is a substance made when two or more different elements are chemically joined together in a fixed ratio.

What is a chemical formula?

A chemical formula uses element symbols and numbers to show which elements are in a substance and how many atoms of each element it has.

What does a small number written after a symbol in a chemical formula show?

It shows how many atoms of that element are in one unit of the compound.

What does it mean if there is no small number after an element symbol in a formula?

It means there is just one atom of that element in one unit of the compound.

What does the formula H2O tell you about water?

H2O shows water is made from hydrogen and oxygen and that each unit has 2 hydrogen atoms and 1 oxygen atom.

In the formula CO2, how many carbon and oxygen atoms are there?

CO2 has 1 carbon atom and 2 oxygen atoms in each unit of the compound.

In the formula MgCl2, how many magnesium and chlorine atoms are there?

MgCl2 has 1 magnesium atom and 2 chlorine atoms in each unit of the compound.

What is meant by a fixed ratio of atoms in a compound?

The elements in a compound always join in the same number ratio, for example water is always 2 hydrogen atoms to 1 oxygen atom.

How can you tell from a formula if a substance is an element or a compound?

An element has just one type of symbol, like O2, but a compound has two or more different symbols, like CO2 or H2O.

Why do compounds usually have different properties from the elements they are made from?

When elements join to form a compound, new bonds and structures form, so the compound has different properties to the original elements.