Atomic Structure & the Periodic Table: The Map of Matter

All matter is built from atoms, and every atom is built from just three kinds of particle. The protons in an atom's core determine which element it is; the electrons around the outside determine how it reacts. The periodic table arranges all the known elements so that their patterns — in size, reactivity, and behavior — line up in neat rows and columns. Understanding that structure is the foundation of all chemistry.

This guide covers subatomic particles, atomic number and mass, isotopes and ions, and how to read the periodic table, with worked and practice questions matched to the level seen in ACT Science and chemistry at Northside Tutoring.

Why Atomic Structure Matters

Atomic structure underlies all of chemistry and many ACT Science passages. Knowing how to read an element's particles and its place on the periodic table lets you reason through chemistry questions quickly. (The SAT has no science section.)

The Three Particles

Inside the Atom

Nearly all of an atom's mass is in the tiny nucleus, while the electrons — far lighter — take up almost all of its volume.

Atomic Number & Mass

Two numbers define an atom:

The atomic number identifies the element — every carbon atom has 6 protons. In a neutral atom, the number of electrons equals the number of protons. To find neutrons, subtract: neutrons = mass number − atomic number.

Isotopes & Ions

Change the neutrons and you get an isotope — same element, different mass (carbon-12 vs. carbon-14). Change the electrons and you get an ion — an atom with a charge. Losing electrons makes a positive ion (cation); gaining electrons makes a negative ion (anion).

Reading the Periodic Table

The table is organized so that structure predicts behavior. Periods are the horizontal rows; groups (or families) are the vertical columns, and elements in the same group behave similarly because they have the same number of outer (valence) electrons. Metals sit on the left, nonmetals on the right, with metalloids along the staircase between them.

Where You'll See This — Test by Test

Atomic structure supports ACT Science chemistry passages; the SAT has no science section and the SSAT doesn't test it. It's core high-school and AP Chemistry.

Watch the Lesson

Sometimes a diagram needs a voice. In the short video below, one of our Northside tutors walks through the core idea and works through test-style problems in real time.

Worked Example Problems

These problems are calibrated to the difficulty you'll actually see on test day. Try each one before opening the solution.

Common Mistakes to Avoid

Practice Problems — You Try

Three problems below. Work each before checking the solution.

The Northside Method — How We Teach This 1-on-1

Reading a blog is a great starting point. But there's a meaningful gap between understanding a concept and reflexively applying it under timed conditions. That gap is exactly what our tutors close.

Every Northside student works through a four-step framework:

1. Assessment. We diagnose which specific skills are slowing your student down — not just whether they "get it" in the abstract.
2. Perfect-match coach. We pair them with an elite tutor (we accept only the top 1% of applicants) whose teaching style fits how your student actually learns.
3. Bespoke plan. A roadmap built around your student's target score, target timeline, and current pacing data.
4. Data-driven adjustment. Every session ends with a check on whether the student's accuracy and speed are moving in the right direction.

And if a student meets all eligibility requirements but doesn't hit the defined score improvement? We provide 5 additional hours of cohort learning at no cost. That's the Northside guarantee — built on 25 years of measured outcomes.