Science · Chemistry foundations
Atoms, Elements, Ions, and the Periodic Table
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Start here: key ideas
- Atoms contain protons, neutrons, and electrons.
- Atomic number equals the number of protons.
- Mass number equals protons plus neutrons.
- Isotopes have different numbers of neutrons.
- Ions form when atoms gain or lose electrons.
- Periodic-table groups often share valence-electron patterns.
What you’ll be able to do
- Identify protons, neutrons, and electrons.
- Find protons, neutrons, and electrons from atomic information.
- Tell isotopes from ions.
- Use group patterns to predict valence electrons.
Learn the parts of an atom, read atomic notation, and use simple periodic-table patterns.
No earlier lesson is required. Basic addition and subtraction are enough.
Inside the atom
An atom has a tiny nucleus surrounded by electrons. The nucleus contains positive protons and neutral neutrons. Negative electrons occupy the space outside the nucleus.
Protons and neutrons have most of the atom’s mass. Electrons have much less mass. A neutral atom has equal numbers of protons and electrons.
Charge comes from the balance of protons and electrons. Each proton is +1 and each electron is −1. For example, 11 protons and 10 electrons give a 1+ charge. Neutrons do not affect charge.
The proton count is also called the atomic number. Find it before working out neutrons or charge. Changing an electron count changes charge, not the element.
OpenStax College, Anatomy & Physiology (2014 CC BY archive) · CC BY 3.0 · https://github.com/philschatz/anatomy-book/blob/eadaa6b2992f4e4010f45d58ad4db065e398093e/resources/202_Two_Models_of_Atomic_Structure.jpg · https://creativecommons.org/licenses/by/3.0/ · Original source image reproduced unchanged; explanatory caption added.
Isotopes and ions change different counts
The number of protons fixes the element. Isotopes are atoms of the same element with different numbers of neutrons. They have the same atomic number but different masses.
An ion forms when electrons change. Losing electrons makes a positive ion. Gaining electrons makes a negative ion. Ordinary ion formation does not change the nucleus.
Carbon-12 and carbon-14 are isotopes because both have 6 protons but different neutron counts. Sodium and sodium ion are not isotopes when only electrons differ. Changing the proton number would make a different element.
Reading atomic notation
The atomic number is the number of protons. The mass number is protons plus neutrons. Therefore, neutrons = mass number − atomic number.
Example: an atom with atomic number 8 and mass number 16 has 8 protons and 8 neutrons. If it has 10 electrons, its charge is 2− because it has two more electrons than protons.
Worked example. For an ion with atomic number 12, mass number 24, and charge 2+: protons = 12; neutrons = 24 − 12 = 12; electrons = 12 − 2 = 10. A positive charge means electrons were lost.
A periodic-table atomic mass is usually a weighted average over isotopes, not the integer mass number of one atom. For a named isotope, use its stated mass number. Worked comparison: chlorine-35 and chlorine-37 each have 17 protons, with 18 and 20 neutrons respectively. A chloride ion has 18 electrons and charge −1; it is not a positive ion.
OpenStax College, Chemistry (CC BY archive) · CC BY 3.0 · https://github.com/philschatz/chemistry-book/blob/a6ac3918e61cbedf0589c5b43e748dc8a7edf5e2/resources/CNX_Chem_02_03_AtomSym.jpg · https://creativecommons.org/licenses/by/3.0/ · Original source image reproduced unchanged; explanatory caption added.
Using the periodic table
The periodic table is ordered by atomic number. Rows are periods. Columns are groups. Elements in one group often have similar chemical behavior because they have similar numbers of valence electrons, the outer electrons involved in bonding.
For main-group elements, group 1 usually has one valence electron and group 17 has seven. Noble gases in group 18 have full outer levels and are usually less reactive.
Metals are mainly on the left and center of the table and often lose electrons. Nonmetals are mainly on the right and often gain or share electrons. These are useful patterns, not rules for every detail.
Supporting explanation: atomic size
Down a group, added electron shells generally make atoms larger. Across a period, increasing nuclear attraction generally pulls the electrons in the same outer shell closer, making atoms smaller. Use these supplied relationships to interpret a comparison; no electron-configuration notation or shielding calculations are needed.
Key terms
- atom: The smallest unit of an element that keeps its chemical properties.
- proton: A positively charged particle in the nucleus.
- neutron: An uncharged particle in the nucleus.
- electron: A negatively charged particle outside the nucleus.
- atomic number: The number of protons in an atom.
- mass number: The total number of protons and neutrons.
- isotope: Atoms of one element with different neutron counts.
- ion: A charged atom or group of atoms.
Summary
- Protons identify an element.
- Atomic number equals protons.
- Mass number equals protons plus neutrons.
- Isotopes differ in neutrons.
- Ions differ in electrons.
- Valence electrons help predict chemical behavior.
Protons pick the element; neutrons change the isotope; electrons change the charge.
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