Question

1. explain the difference between:

a. an atom and an element.
b. a molecule and a compound.
extension questions

1. it is often useful to separate matter. physical methods of separation (filtering, distillation) do not require a chemical change. in other words, no chemical bonds are broken or formed during the separation. chemical methods of separation (decomposition, electrolysis) require a chemical change. in other words, chemical bonds are broken and/or formed during the separation.

a. is straining cooked pasta from water a physical or chemical separation?
b. is using a fuel cell to separate water into hydrogen and oxygen a physical or chemical separation?
c. which type(s) of matter (mixtures/compounds/elements) could be separated by physical methods?
d. which type(s) of matter (mixtures/compounds/elements) would need to be separated by chemical methods?

1. students in a chemistry course were asked the following question on a unit exam: “draw a diagram representing an element using circles as atoms.”

a. the following diagrams represent two typical answers given by students. which drawing is the best representation of an element? explain.
b. imagine that the atom in drawing b had been removed by physical separation from one of the samples in model 1. which samples could have been the source of the atom in drawing b?

Explanation:

14a. An atom is the smallest unit of an element that retains its chemical properties. An element is a pure substance made up of only one type of atom.
14b. A molecule is a group of two or more atoms held together by chemical bonds. A compound is a substance made up of two or more different types of elements chemically combined in a fixed ratio.
15a. Straining cooked pasta from water is a physical separation as no chemical bonds are broken or formed; it is just separating two substances based on physical properties.
15b. Using a fuel - cell to separate water into hydrogen and oxygen is a chemical separation as water molecules are decomposed into their elemental components through a chemical reaction, breaking chemical bonds.
15c. Mixtures can be separated by physical methods because the components of a mixture are not chemically bonded and can be separated based on differences in physical properties like size, density, boiling point etc.
15d. Compounds need to be separated by chemical methods because the elements in a compound are chemically bonded and require a chemical change to break those bonds.
16a. Drawing A is a better representation of an element. An element can exist as a collection of atoms of the same type. Drawing A shows multiple atoms of the same type, which is more representative of how elements often exist in nature (e.g., oxygen gas is $O_2$, a collection of oxygen atoms). Drawing B shows just one atom, and while an element can be thought of in terms of a single atom, in most cases, elements are found as collections of atoms.
16b. Since physical separation can only be used to separate mixtures, the source of the atom in Drawing B could have been a mixture that contained the element as a component.

Answer:

14a. An atom is the smallest unit of an element; an element is a pure - substance of one atom type.
14b. A molecule is a group of atoms; a compound is made of different elements in a fixed ratio.
15a. Physical separation.
15b. Chemical separation.
15c. Mixtures.
15d. Compounds.
16a. Drawing A; elements often exist as collections of atoms.
16b. A mixture.