Week 5, Session 13: Atoms, Elements, and the Periodic Table

Week 5, Session 13: Atoms, Elements, and the Periodic Table

Why this session matters

Everything around you — air, water, your body, this screen — is built from atoms. Physical Science is about 40% of the test, and it starts here. If you understand the three particles inside an atom and how the periodic table is organized, a whole family of chemistry questions opens up.

The big idea in plain words

An atom is like a tiny solar system. In the center is a nucleus packed with protons (positive) and neutrons (neutral). Whizzing around it are electrons (negative). The number of protons is the atom's "ID number" — change it and you change the element. The periodic table is just an organized chart of all the elements, sorted by that ID number.

Picture it

Words you must own

• Atom: the smallest unit of an element.
• Proton: positive particle in the nucleus; its count sets the element.
• Neutron: neutral particle in the nucleus.
• Electron: negative particle orbiting the nucleus.
• Atomic number: the number of protons — the element's identity.
• Mass number: protons + neutrons.
• Element: a substance made of one kind of atom (all atoms have the same proton count).
• Isotope: atoms of the same element with different numbers of neutrons.
• Ion: an atom that has gained or lost electrons, giving it a charge.
• Group / period: a column / a row on the periodic table.

The science, step by step

1. Three particles: protons (+) and neutrons (0) sit in the nucleus; electrons (−) orbit around it.
2. Atomic number = protons = identity. Every carbon atom has 6 protons; that is what makes it carbon.
3. Neutral atoms have equal protons and electrons, so the charges cancel.
4. Mass number = protons + neutrons. Electrons are too light to count for mass.
5. Isotopes: same element, different neutron counts (so different mass numbers).
6. Ions: lose electrons → positive; gain electrons → negative.
7. The periodic table lists elements by atomic number. Elements in the same group (column) behave similarly because they have similar outer electrons.

Exam-style passage

Passage. A chemistry class recorded the particles in four atoms.

Watch me solve one

Question about the passage: Which two atoms are isotopes of the same element, and which atom is an ion?

• READ: Atomic number = protons. W has 6, X has 8, Y has 6, Z has 11.
• CHOOSE: W and Y are isotopes — both have 6 protons (both carbon) but different neutrons (6 vs 8). X is an ion — it has 8 protons but 10 electrons, so 2 extra negatives give it a 2− charge.
• CHECK: W, Y, and Z all have equal protons and electrons, so they are neutral; only X's electron count differs from its protons, making it the ion.

10 Guided Examples (cover the answer, then check)

Examples 1–6 use the four-atom passage.

1. Identity. What element is atom W?
   Answer: Carbon.
   Why: 6 protons means atomic number 6 = carbon.
2. Isotopes. Which atoms are isotopes of each other?
   Answer: W and Y.
   Why: Same protons (6), different neutrons.
3. Ion. Which atom is an ion, and what is its charge?
   Answer: X, charge 2−.
   Why: 10 electrons vs 8 protons = 2 extra negatives.
4. Mass number. What is the mass number of atom Z?
   Answer: 23.
   Why: protons + neutrons = 11 + 12.
5. Neutral. How do you know atom Z is neutral?
   Answer: Protons equal electrons (11 = 11).
   Why: Equal charges cancel.
6. Element Z. What element has 11 protons?
   Answer: Sodium (Na).
   Why: Atomic number 11 is sodium.
7. Charge of proton. What charge does a proton carry?
   Answer: Positive (+).
   Why: Protons are the positive nuclear particle.
8. Mass location. Where is almost all of an atom's mass?
   Answer: In the nucleus.
   Why: Protons and neutrons hold the mass; electrons are tiny.
9. Atomic number use. What does the atomic number tell you?
   Answer: The number of protons and the element's identity.
   Why: It defines the element.
10. Group meaning. Why do elements in the same group act alike?
    Answer: They have similar outer electrons.
    Why: Outer electrons drive chemical behavior.

The traps GED® loves

• Neutrons set the element? No — protons set the element; neutrons make isotopes.
• Mass number counts electrons? No — mass is protons + neutrons only.
• Ion means broken atom? No — an ion is just an atom with a charge from gaining/losing electrons.
• Electrons are positive? No — electrons are negative; protons are positive.

Quick Check

Your practice — 10 questions

Answer from memory. When counting charge, compare protons (+) with electrons (−).

Before you move on

• Say the three particles, their charges, and where each sits.
• Explain "isotope" and "ion" in one sentence each.
• Score at least 8 of 10 before Session 14.