Atoms are so small that even today we cannot see them directly, so scientists built pictures of the atom, atomic models, that explain experimental results, and changed them whenever a new experiment did not fit.
Dalton's model of an atom as an unbreakable ball gave way to Thomson's model once the electron was discovered, and after alpha particles fired at gold foil were seen bouncing back, to Rutherford's model with a nucleus at the center.
Today an atom is described as a tiny nucleus of protons and neutrons with electrons around it, electrically neutral because it has as many electrons as protons.
Choose the four models in turn to see how the picture of the atom changed with experiments, and in the lower picture count the protons, neutrons and electrons of a lithium atom.
What Is an Atom?
⚛️ What If You Keep Splitting Matter?
①The idea of the atom began with the thought that splitting matter again and again must end at tiny pieces that cannot be split further.
②An atom is about 10⁻¹⁰ m across; you would need 100 million atoms in a row to make about 1 cm.
③The nucleus is about 100,000 times smaller than the atom, yet almost all of the atom's mass is in it.
How the Atomic Model Changed
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Atomic models from Dalton → Thomson → Rutherford → Bohr
💡 Why the Model Changed
①Dalton (1803): saw the atom as a ball that cannot be split.
②Thomson (1904): discovered that negatively charged electrons come out of atoms and proposed electrons embedded in a ball of positive charge.
③Rutherford (1911): alpha particles fired at thin gold foil mostly passed straight through, but very rarely bounced back sharply, so he proposed a small, heavy, positive nucleus at the center.
④Bohr (1913): explained that electrons move only in fixed orbits around the nucleus, each with a set energy.
The Structure of an Atom
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A lithium atom: 3 protons, 4 neutrons, 3 electrons
What an atom is made of
atom = nucleus (protons + neutrons) + electrons
With as many electrons as protons, the whole atom is electrically neutral.
Atomic number and mass number
atomic number = number of protons, mass number = protons + neutrons
Atoms with the same number of protons are the same element; the lithium in the picture has atomic number 3 and mass number 7.
How Ions Form
Positive ion
atom − electron → positive ion (+)
Sodium (Na) loses its 1 outermost electron and becomes Na⁺.
Negative ion
atom + electron → negative ion (−)
Chlorine (Cl) gains 1 electron and becomes Cl⁻.
💡 Why Ions Form
①Atoms tend toward a stable state with the outermost electron shell full.
②Metal atoms such as sodium and magnesium have few outer electrons and easily lose them to become positive ions; the sodium in the Bohr picture has just 1 outer electron.
③Non-metal atoms such as chlorine and oxygen have nearly full outer shells and easily gain electrons to become negative ions.
④Losing or gaining electrons leaves the number of protons unchanged, so the element stays the same.
Summary
The Essentials
atom = nucleus (protons + neutrons) + electrons · atomic number = number of protons
The atomic model was revised to fit experiments, from Dalton → Thomson → Rutherford → Bohr.
🎯 Exam Points
①Dalton (hard ball) → Thomson (ball with embedded electrons) → Rutherford (nucleus and electrons) → Bohr (fixed orbits).
②The nucleus is made of protons (+) and neutrons (no charge); electrons (−) are outside it.
③A neutral atom has as many electrons as protons.
④A positive ion has lost electrons (+); a negative ion has gained them (−).
⑤Atomic number = number of protons; mass number = protons + neutrons.
Examples and Unit Test
Example 1
An atom has 11 protons, 11 electrons and 12 neutrons. What are its atomic number and mass number?
1
The atomic number equals the number of protons, and the mass number is the number of protons plus neutrons.
2
There are 11 protons, so the atomic number is 11 and the mass number is 11 + 12 = 23.
▸ Atomic number 11, mass number 23
Atomic number 11 is sodium; electrons are so light that they are not counted in the mass number.
Example 2
What particle does a sodium atom (Na) become when it loses 1 electron, and what is its charge?
1
When an atom loses electrons it has more protons than electrons and becomes a positively charged ion.
2
Sodium loses its 1 outer electron and becomes the sodium ion Na⁺, with 11 protons and 10 electrons.
▸ Positive ion Na⁺ (+ charge)
Take the 1 outer electron away from the sodium (2, 8, 1) in the Bohr picture and the remaining shell is full.
Unit-test style
Which statement about atomic structure and ions is correct?
①The atomic number equals the number of neutrons
②An atom has equal numbers of protons and electrons, so it is electrically neutral
③When an atom gains an electron it becomes a cation
④The mass number is the sum of the numbers of electrons and protons
⑤The nucleus is made only of electrons
▸ ② An atom has equal numbers of protons and electrons, so it is electrically neutral
1
An atom has as many positive protons as negative electrons, so overall it is neutral; ② is correct.
2
The atomic number equals the number of protons (①), gaining electrons makes a negative ion (③), the mass number is protons plus neutrons (④), and the nucleus is made of protons and neutrons (⑤).