What is the Structure of the Atom?
Every atom is built from three subatomic particles: protons and neutrons packed into a tiny, dense nucleus, and electrons moving around it in shells. The number and arrangement of these particles determine an element's identity and chemical behavior.
An atom consists of a positively charged nucleus (protons + neutrons) surrounded by negatively charged electrons in shells; the number of protons — the atomic number — defines which element it is.
- 1↓NucleusA dense core made of protons (positive) and neutrons (neutral).
- 2↓ProtonsPositively charged particles; their count = atomic number (Z).
- 3↓NeutronsNeutral particles that add mass and stabilize the nucleus.
- 4↓Electron shellsNegatively charged electrons orbit the nucleus in energy levels (shells).
- 5Valence electronsOutermost-shell electrons that determine how the atom bonds with others.
Step-by-step worked examples
How many protons, neutrons, and electrons are in a neutral carbon-12 atom (atomic number 6)?
Atomic number Z = 6 → protons = 6 Mass number A = 12 → neutrons = A − Z = 12 − 6 = 6 Neutral atom → electrons = protons = 6
An ion has 11 protons and 10 electrons. What is its charge?
Charge = protons − electrons = 11 − 10 = +1 This is a Na⁺ ion (sodium cation).
An atom has 17 protons and a mass number of 35. How many neutrons does it have?
Neutrons = mass number − protons = 35 − 17 = 18 This is chlorine-35 (Cl-35).
Flashcards
Quick quiz
Q1.Which particle has no electric charge?
Q2.What determines an element's atomic number?
Q3.An atom with 6 protons and 7 neutrons has a mass number of…
Q4.Where is most of an atom's mass located?
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Common mistakes
Thinking electrons have significant mass. — Correct: Electrons are about 1836 times lighter than protons — nearly all mass is in the nucleus.
Confusing atomic number with mass number. — Correct: Atomic number = protons only; mass number = protons + neutrons.
Assuming all atoms of an element have the same mass. — Correct: Isotopes of the same element can have different neutron counts and masses.
Believing electrons orbit like planets in fixed paths. — Correct: Electrons exist in probability clouds (orbitals) within shells, not fixed circular paths.
FAQ
What is the structure of the atom?
An atom has a central nucleus of protons and neutrons, surrounded by electrons arranged in shells.
What is atom structure made of?
Three particles: protons (+), neutrons (neutral), and electrons (−).
What are examples of atomic structure?
Hydrogen (1 proton, 1 electron), carbon (6 protons, 6 neutrons, 6 electrons), and sodium (11 protons, 11 electrons) are common examples.
How do you find the number of neutrons in an atom?
Subtract the atomic number (protons) from the mass number: neutrons = A − Z.