Theory — Atoms and the Periodic Table
The Structure of the Atom
Every atom has a dense central nucleus containing positively charged protons and neutral neutrons, surrounded by negatively charged electrons. The number of protons defines which element the atom is.
Mass number A = protons + neutrons
Neutrons = A − Z
Charge = protons − electrons (0 for a neutral atom)
A neutral atom has equal numbers of protons and electrons. If electrons are lost or gained, the atom becomes an ion — a cation (positive) if electrons are lost, an anion (negative) if electrons are gained. Atoms of the same element with different neutron counts are isotopes.
Organising the Elements
The periodic table arranges elements by increasing atomic number into periods (rows) and groups (columns). Elements in the same group have the same number of valence electrons — the outer-shell electrons that govern chemical behaviour — so they react similarly.
Group 13 → 3 Group 14 → 4 Group 15 → 5
Group 16 → 6 Group 17 → 7 Group 18 → 8 (2 for He)
Periodic Trends
Several properties change in regular ways across the table, driven by nuclear charge and how far the outer electrons sit from the nucleus.
Atomic Radius
Decreases left→right across a period (stronger pull); increases down a group (more shells).
Ionization Energy
Increases across a period; decreases down a group. Energy to remove an electron.
Electronegativity
Increases across a period; decreases down a group. Tendency to attract bonding electrons.
| Quantity | Definition | Trend |
|---|---|---|
| Atomic number Z | number of protons | increases across & down |
| Atomic radius | size of the atom | ↓ across period, ↑ down group |
| Ionization energy | energy to remove an electron | ↑ across period, ↓ down group |
| Electronegativity | pull on bonding electrons | ↑ across period, ↓ down group |
Apparatus
The equipment a real atomic-structure experiment uses. In the simulation these are modelled for you, but the readings correspond to what each instrument would measure.
Instructions — Running the Virtual Experiment
The Atom Builder lets you set the numbers of protons, neutrons, and electrons and reads out the element, mass number, and charge. The Periodic Table tab lets you click any of the first elements to read its data and switch on a colour map of each periodic trend. Record your readings in your lab report with screenshots.
Simulation — Atoms and the Periodic Table
Build the atom
Trend colour map
Cells coloured by element category. Pick a trend to see how it changes across the table.
Team Questions
Example Lab Report
Sample report demonstrating the expected format and level of detail. Use as a guide for your own submission, and include labelled screenshots of the atom builder and periodic table for each part.
Atoms and the Periodic Table
Chemistry | Section: [Your Section] | Date: [Date]
Lab Members: [Names of all members present]
Purpose
To build atoms from protons, neutrons, and electrons and determine the element, mass number, and charge, and to use the periodic table to read atomic numbers, symbols, and valence electrons and describe the periodic trends in atomic radius, ionization energy, and electronegativity.
Theory
The atomic number Z equals the number of protons and defines the element. The mass number A equals protons plus neutrons, so neutrons = A − Z. A neutral atom has equal protons and electrons; gaining or losing electrons forms an ion. Elements in the same group share the same valence-electron count and similar chemistry.
Results
| Atom | Z (protons) | Mass number A | Neutrons (A−Z) | Electrons | Charge |
|---|---|---|---|---|---|
| Carbon-12 | 6 | 12 | 6 | 6 | 0 |
| Sodium-23 | 11 | 23 | 12 | 11 | 0 |
| Chlorine-35 | 17 | 35 | 18 | 17 | 0 |
Periodic table observations: across Period 2 (Li → Ne) the atomic radius decreased and the ionization energy increased; down Group 1 (H → Li → Na → K) the radius increased and the ionization energy decreased.
Discussion
The element was determined entirely by the proton count, confirming that Z defines identity. Adding neutrons changed the mass number but not the element, illustrating isotopes. Removing electrons produced positive ions and adding electrons produced negative ions, matching charge = protons − electrons. The periodic-table trends agreed with theory: atomic radius decreased across a period because the growing nuclear charge pulls the same shell inward, while it increased down a group as new shells were added; ionization energy showed the opposite pattern. Any uncertainty here is qualitative — reading colour intensity by eye — rather than a measured value with an instrument.
Conclusion
Atomic number determines the element, mass number fixes the neutron count, and the electron balance sets the charge. The periodic table organises elements so that group members share valence electrons and chemistry, and the major trends (radius, ionization energy, electronegativity) vary predictably across periods and down groups.
Practice Questions
Show all work. Use Z = protons, A = protons + neutrons, neutrons = A − Z, charge = protons − electrons.