Atoms, Elements and Compounds

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1. 1

   Name the three subatomic particles

   proton +1 · neutron 0 · electron −1

   Protons and neutrons sit in the nucleus and carry nearly all the mass. Electrons occupy shells around it and have almost no mass. An atom is neutral because protons and electrons are equal in number.

2. 2

   Use the atomic number to identify the element

   atomic number = number of protons

   The proton count defines which element an atom is — change it and you have a different element entirely. This is why the periodic table is ordered by atomic number rather than by mass.

3. 3

   Find neutrons from the mass number

   neutrons = mass number − atomic number

   The mass number counts protons plus neutrons. Subtracting the atomic number leaves the neutrons, and atoms of the same element with different neutron counts are isotopes.

4. 4

   Distinguish elements from compounds

   O₂ is an element (one type of atom); H₂O is a compound

   An element contains only one kind of atom even when those atoms are joined in pairs. A compound contains different elements chemically bonded in fixed proportions.

5. 5

   Distinguish compounds from mixtures

   compound: bonded, fixed ratio · mixture: unbonded, any ratio

   A mixture can be separated by physical means such as filtering or distillation because nothing is bonded. Separating a compound requires a chemical reaction, since bonds must be broken.

Three categories, one distinguishing question

Ask two questions and the category follows. Are the atoms all the same kind? If yes, it is an element. If no, are they chemically bonded? If yes, a compound; if no, a mixture. Air is a mixture, water is a compound, and oxygen gas is an element.

The confusion usually comes from O₂ being called an element despite containing two atoms. The count of atoms is irrelevant — what matters is whether they are the same kind, and both are oxygen.

• Proton +1, neutron 0, electron −1
• Atomic number = protons = which element
• Mass number = protons + neutrons
• Element: one type of atom
• Compound: different atoms bonded, fixed ratio
• Mixture: not bonded, separable physically

Isotopes

Atoms of the same element always have the same number of protons but can differ in neutrons. These variants are isotopes, and they behave identically in chemical reactions because chemistry depends on electrons rather than on the nucleus.

They differ in mass, which is why relative atomic masses on the periodic table are rarely whole numbers — they are averages weighted by how common each isotope is.

Why mixtures separate and compounds do not

Nothing holds the components of a mixture together, so physical differences — boiling point, particle size, magnetism, solubility — can be exploited to pull them apart. Distillation, filtration and chromatography all work this way.

A compound's atoms are held by chemical bonds, so separating them means breaking bonds, which requires a chemical reaction. Electrolysis of water is a standard example, and it needs electrical energy rather than a filter.

How we teach atomic structure

We teach the two-question test for element, compound and mixture, because students who classify by memory of examples fail on unfamiliar substances. A rule they can apply beats a list they have seen.

We also connect atomic number to the periodic table immediately. Seeing that the table is ordered by proton count makes both topics easier and prevents the table being learned as an arbitrary grid to memorise.