TEAS Ready⚗️ Science · Chemistry · Lesson 1 of 6 · 5 min
Atomic structure and the periodic table
⚛️ Every drug, fluid and tissue you will ever study is built from about 90 kinds of atoms. Learn their parts once and the periodic table starts doing the work for you.
💡 Key idea
Protons set the element, neutrons set the isotope, and electrons set the charge and the chemistry.
🧠 The idea
An atom has a dense nucleus holding PROTONS (charge +1) and NEUTRONS (no charge), surrounded by ELECTRONS (charge −1) that are far lighter. The ATOMIC NUMBER is the number of protons; it defines the element, so every carbon atom has 6. The MASS NUMBER is protons plus neutrons. Atoms of one element with different neutron counts are ISOTOPES, such as carbon-12 and carbon-14. A neutral atom has equal protons and electrons; an atom that gains electrons becomes a negative ion (anion), and one that loses electrons becomes a positive ion (cation). The periodic table is ordered by atomic number. Rows are periods; columns are groups, and elements in a group share the same number of valence (outer-shell) electrons, so they react alike. Metals sit on the left, nonmetals on the upper right. Moving left to right across a period, atoms get smaller and electronegativity rises; moving down a group, atoms get larger.
✏️ Worked example
An ion has 17 protons, 18 neutrons and 18 electrons. Step 1: 17 protons means atomic number 17, which is chlorine. Step 2: mass number = 17 + 18 = 35, so this is chlorine-35. Step 3: charge = 17 − 18 = −1. The particle is a chloride ion, Cl⁻, an anion because it gained one electron.
📌 Takeaways
- Atomic number = protons; mass number = protons + neutrons.
- Gaining electrons makes an anion (−); losing electrons makes a cation (+).
- Same group = same valence electrons = similar reactions.
📖 Terms in this lesson
Isotope: Atoms of the same element with the same number of protons but different numbers of neutrons; for example, carbon-12 and carbon-14.
✅ Check yourself
Question 1 of 3
An atom has 11 protons and 12 neutrons. What is its mass number?
- 12
- 11
- 1
- 23
Show the answer and rationale
Answer: D
Mass number is protons plus neutrons: 11 + 12 = 23. Eleven is the atomic number, which identifies the element (sodium), and 12 is only the neutron count, not the total mass number.
Question 2 of 3
Which particle determines which element an atom is?
- Electron
- Valence shell
- Proton
- Neutron
Show the answer and rationale
Answer: C
The number of protons (the atomic number) defines the element. Changing neutrons gives a different isotope of the same element, and changing electrons gives an ion of the same element.
Question 3 of 3
Elements in the same vertical column of the periodic table have the same
- number of protons
- number of valence electrons
- number of neutrons
- atomic mass
Show the answer and rationale
Answer: B
Columns are groups, and members of a group share the same number of valence electrons, which is why they react similarly. Their proton counts, neutron counts and atomic masses all increase going down the column.
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