SkarpChapter 3 of 14
Inside the Atom: Elements, Ions, and the Periodic Table
Dive beneath the surface of matter to meet protons, neutrons, and electrons—and see how the periodic table secretly organizes the entire material universe.
Step 1: Zooming In – From Matter to Atoms
Atoms: The Next Level Down
All matter is made of tiny particles. An atom is the smallest unit of an element that still behaves like that element, such as one atom of oxygen or one atom of gold.
Structure of an Atom
Each atom has a central nucleus containing protons and neutrons, surrounded by an electron cloud where electrons move in the space around the nucleus.
Mostly Empty Space
Atoms are mostly empty space, but the forces between their particles make matter feel solid and give different substances their unique behaviors.
Link to Earlier Modules
Earlier you learned how particles move in solids, liquids, and gases. Now you will see how the type of atom and its electrons help explain those behaviors.
Step 2: Meet the Subatomic Particles
Protons
Protons (p⁺) are in the nucleus, have a +1 charge, and a relative mass of 1. The number of protons decides which element the atom is.
Neutrons
Neutrons (n⁰) are also in the nucleus, have no charge, and a relative mass of 1. They help stabilize the nucleus.
Electrons
Electrons (e⁻) move in the electron cloud around the nucleus, have a -1 charge, and a very small mass (about 1/1836 of a proton).
Big Picture
Protons and neutrons make the heavy nucleus. Electrons are light but control chemical reactions, bonding, and electricity.
Step 3: Atomic Number, Mass Number, and Isotopes
Atomic Number (Z)
The atomic number (Z) is the number of protons in the nucleus. It defines the element. For example, carbon has Z = 6 because all carbon atoms have 6 protons.
Mass Number (A)
The mass number (A) is the total number of protons plus neutrons in the nucleus: A = protons + neutrons. It is always a whole number.
Isotopes
Isotopes are atoms of the same element (same Z) with different numbers of neutrons, so different mass numbers. Example: carbon-12, carbon-13, carbon-14.
Average Atomic Mass
Modern periodic tables show average atomic mass, a weighted average of all natural isotopes, which is why values like chlorine's 35.5 are not whole numbers.
Step 4: Quick Practice – Identifying Atoms and Isotopes
Use this mini-activity to practice atomic number and mass number.
- For each atom, fill in the blanks (mentally or on paper):
- Atom A: 11 protons, 12 neutrons
- Atomic number Z = ?
- Mass number A = ?
- Element name = ?
- Atom B: 17 protons, 18 neutrons
- Atomic number Z = ?
- Mass number A = ?
- Element name = ?
- Atom C: 17 protons, 20 neutrons
- Atomic number Z = ?
- Mass number A = ?
- Element name = ?
- Now compare B and C:
- Do they have the same element? Why or why not?
- Are they isotopes of each other? Explain using Z and A.
Check yourself:
- Atom A: Z = 11, A = 23, element = sodium (Na)
- Atom B: Z = 17, A = 35, element = chlorine (Cl)
- Atom C: Z = 17, A = 37, element = chlorine (Cl)
So B and C are isotopes of chlorine: same atomic number, different mass numbers.
Step 5: Ions – When Atoms Gain or Lose Electrons
Neutral Atoms
In a neutral atom, protons = electrons, so the total charge is zero. Changing electrons can change the charge without changing the element.
Cations
A cation is a positive ion formed when an atom loses electrons. Example: Na → Na⁺ when sodium loses 1 electron (11 p⁺, 10 e⁻).
Anions
An anion is a negative ion formed when an atom gains electrons. Example: Cl → Cl⁻ when chlorine gains 1 electron (17 p⁺, 18 e⁻).
Real-World Ions
Table salt is made of Na⁺ and Cl⁻ ions. In your body, ions like Na⁺, K⁺, and Ca²⁺ are essential for nerve impulses and muscle contractions.
Step 6: Reading a Periodic Table Entry
Element Box Basics
A typical periodic table box shows: atomic number, symbol, name, and average atomic mass. Example: carbon has 6, C, carbon, and 12.01.
Atomic Number and Symbol
For carbon, atomic number 6 means 6 protons (and 6 electrons if neutral). The symbol C is its shorthand in formulas.
Average Atomic Mass
Carbon's 12.01 u is an average over its isotopes, mainly carbon-12 and carbon-13, so it is not exactly a whole number.
Try Oxygen
For oxygen, atomic number 8 means 8 protons. Its average atomic mass is about 16.00 u, matching mostly oxygen-16 atoms (8 p⁺, 8 n⁰).
Step 7: The Periodic Table – Groups, Periods, Metals, Nonmetals
Periods
Periods are horizontal rows. Elements in the same period have the same number of electron shells or energy levels around the nucleus.
Groups
Groups are vertical columns. Elements in a group usually have similar chemical properties and the same number of valence electrons.
Metals vs Nonmetals
Metals (left/center) are shiny, conductive, and form cations. Nonmetals (right side and hydrogen) are often dull, poor conductors, and form anions or share electrons.
Special Families
Key groups: Group 1 alkali metals (very reactive), Group 17 halogens (very reactive), Group 18 noble gases (very unreactive and stable).
Step 8: Valence Electrons and Reactivity Patterns
Valence Electrons
Valence electrons are outermost electrons. They control bonding and reactions because they are the easiest to gain, lose, or share.
Group Patterns
Group 1: 1 valence e⁻, form +1 ions. Group 2: 2 valence e⁻, form +2 ions. Group 16: 6 valence e⁻, form 2− ions. Group 17: 7 valence e⁻, form −1 ions.
Noble Gases
Group 18 noble gases have full outer shells (8 valence e⁻, except He with 2). They are very stable and mostly unreactive.
Reactivity Trends
Metals become more reactive going down a group (easier to lose e⁻). Nonmetals like halogens become less reactive going down (harder to gain e⁻).
Step 9: Quick Check – Atoms, Ions, and the Table
Test your understanding with this question.
An atom of magnesium has atomic number 12 and mass number 24. It loses two electrons to form an ion. Which statement is correct?
- The ion has 12 protons, 12 neutrons, and 10 electrons, and is written Mg²⁺.
- The ion has 10 protons, 12 neutrons, and 12 electrons, and is written Mg²⁻.
- The ion has 12 protons, 10 neutrons, and 12 electrons, and is written Mg²⁺.
- The ion has 12 protons, 12 neutrons, and 14 electrons, and is written Mg²⁻.
Show Answer
Answer: A) The ion has 12 protons, 12 neutrons, and 10 electrons, and is written Mg²⁺.
Atomic number 12 means 12 protons. Mass number 24 means 24 − 12 = 12 neutrons. A neutral Mg atom has 12 electrons; losing 2 gives 10 electrons and a 2+ charge, so Mg²⁺ with 12 p⁺, 12 n⁰, 10 e⁻.
Step 10: Flashcard Review – Key Terms
Use these flashcards to quickly review the core ideas from this module.
- Atom
- The smallest unit of an element that still has that element's properties; made of protons, neutrons, and electrons.
- Proton
- Positively charged subatomic particle in the nucleus; atomic number equals the number of protons.
- Neutron
- Neutral subatomic particle in the nucleus; adds mass and helps stabilize the nucleus.
- Electron
- Negatively charged, very light particle moving in the electron cloud; controls chemical behavior.
- Atomic number (Z)
- Number of protons in an atom's nucleus; defines the element.
- Mass number (A)
- Total number of protons plus neutrons in the nucleus of a specific atom.
- Isotope
- Atoms of the same element (same Z) with different numbers of neutrons and different mass numbers.
- Ion
- A charged particle formed when an atom gains or loses electrons; can be a cation (+) or anion (−).
- Valence electrons
- Electrons in the outermost energy level of an atom; involved in bonding and reactions.
- Group vs period
- Group: vertical column with similar properties and valence electrons. Period: horizontal row with the same number of electron shells.
Key Terms
- ion
- An atom or group of atoms that has a net electric charge due to losing or gaining electrons.
- atom
- The smallest unit of an element that still shows that element's chemical properties.
- anion
- A negatively charged ion formed when an atom gains one or more electrons.
- group
- A vertical column in the periodic table whose elements often have similar properties and the same number of valence electrons.
- metal
- An element that is typically shiny, conducts heat and electricity well, and tends to form positive ions.
- cation
- A positively charged ion formed when an atom loses one or more electrons.
- period
- A horizontal row in the periodic table whose elements have the same number of electron shells.
- proton
- A positively charged particle in the nucleus of an atom; its number defines the element.
- isotope
- One of two or more forms of an element with the same number of protons but different numbers of neutrons.
- neutron
- A neutral particle in the nucleus of an atom that contributes to its mass and stability.
- electron
- A negatively charged, very light particle that moves in the space around the nucleus.
- nonmetal
- An element that usually does not conduct heat or electricity well and tends to gain or share electrons.
- metalloid
- An element with properties intermediate between metals and nonmetals.
- reactivity
- A measure of how easily an element undergoes a chemical reaction, often by gaining or losing electrons.
- mass number
- The total number of protons and neutrons in the nucleus of an atom, symbol A.
- noble gases
- Group 18 elements with full outer electron shells, making them very stable and mostly unreactive.
- atomic number
- The number of protons in the nucleus of an atom, symbol Z.
- periodic table
- A chart of all known elements arranged by increasing atomic number and repeating patterns of properties.
- valence electrons
- Electrons in the outermost energy level of an atom that are involved in chemical bonding.
- subatomic particle
- A particle smaller than an atom, such as a proton, neutron, or electron.