Elements, compounds and the periodic table

Element identity is a proton-count rule

The word element can identify a type of atom or a substance consisting of that type of atom. In the first sense, oxygen means atoms with eight protons. In the second, an oxygen-gas sample can consist mainly of O₂ molecules, each containing two oxygen atoms. Being molecular does not make it a compound: a compound contains atoms of more than one element.

Keep the sample distinct from the ideal chemical identity. An iron object can contain alloying elements, surface oxides or other impurities. Calling it “iron” may be useful everyday shorthand, but a precise composition claim needs evidence. The symbol Fe identifies the element, not every possible sample sold under an iron label.

Same element, different neutron count

Carbon-12 and carbon-13 both have six protons. They differ in neutron count: six and seven respectively. Such atoms are isotopes of the same element. Their electron arrangements and many chemical properties are closely related, but isotopic substitution can affect masses, motion and reaction behaviour. “Same element” does not mean identical in every physical respect.

A change from a neutral atom to an ion changes the electron count, not which isotope it is. Conversely, changing the neutron count changes the isotope even if the atom stays neutral. Treat element, isotope and charge as separate descriptors. This avoids using one label to answer three different questions.

Symbols connect to a shared table

An element symbol has one or two letters in ordinary current use: the first is capitalized and any second letter is lower case. Co denotes cobalt; CO in a chemical formula denotes carbon and oxygen together. Correct letter case is part of the meaning, not decoration. Some symbols come from historical names, so they do not always match the first letters of an English name.

The modern periodic table orders elements by increasing atomic number. A horizontal row is a period; a vertical column is a group. IUPAC group numbers run from 1 to 18. The layout brings related chemical patterns together. Later lessons connect those patterns to electronic structure and explain why simple trends have exceptions.

A table cell contains different kinds of information

A typical cell gives a symbol, atomic number and an atomic-weight value. The atomic number is a proton count. An atomic-weight value often summarizes isotope composition and is generally not an integer. It should not be used as the neutron count of a particular atom. To calculate neutron count, you need the mass number of the specified isotope.

Elements in one group have related properties, but the group is not a promise of identical behaviour or one charge in every compound. Temperature, phase, structure and the particular reaction still matter. Use the table as an organized entry point into chemistry, then check the claim and its scope.

Worked examples

Try it yourself

1. Do Co and CO convey the same composition?

   Show the answer and reasoning

   No.

   Co is the cobalt symbol. CO is a formula containing one carbon and one oxygen atom per molecule.

2. What number orders the modern periodic table?

   Show the answer and reasoning

   Atomic number, Z.

   It counts protons; ordering by mass alone would not give the modern identity rule.

3. Can two isotopes of an element have different proton counts?

   Show the answer and reasoning

   No.

   Different proton counts define different elements. Isotopes differ in neutron count.

4. Is a pure O₂ sample a compound because it contains molecules?

   Show the answer and reasoning

   No. It is an elemental substance.

   Each molecule contains only oxygen atoms. A compound contains more than one element.

What to carry forward

• Separate element identity, isotope identity and ion charge.
• Element-symbol letter case carries meaning.
• Periods are rows and groups are columns.
• A tabulated atomic weight is not the mass number of every atom in a sample.