Atom Builder
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flagWhat you'll discover
- arrow_forwardName the three subatomic particles and where each lives in the atom
- arrow_forwardShow that the proton count alone decides which element an atom is
- arrow_forwardBuild isotopes by changing neutrons and ions by changing electrons
- arrow_forwardFill electron shells in the 2, 8, 8 pattern for the first twenty elements
Inside the atom
Every atom is built from just three particles. Protons (positive) and neutrons (neutral) are packed into a tiny, dense nucleus at the centre. Electrons (negative), nearly two thousand times lighter, occupy the space around it arranged in shells.
The scale is hard to imagine: if the nucleus were a football placed at the centre of a stadium, the nearest electrons would orbit out by the far stands, and everything between would be empty space. Almost all of an atom's mass sits in the nucleus, yet almost all of its size comes from the electron cloud.
Protons define the element
The single number that decides an atom's identity is its proton count, called the atomic number Z. One proton makes hydrogen, six make carbon, eight make oxygen — no exceptions, ever. Change the proton count and you have literally changed the element.
The mass number A is protons plus neutrons, since both weigh about one atomic mass unit. In the simulation, watch the element name flip the instant you add or remove a proton, while adding neutrons changes only the mass number. The periodic table is simply all the elements lined up in order of Z.
Isotopes and ions
Atoms of the same element can still differ. Isotopes have the same protons but different neutrons: carbon-12 and carbon-14 are both carbon, but the heavier one is radioactive and lets archaeologists date ancient remains. Too many or too few neutrons makes a nucleus unstable.
Ions differ in electrons instead. A neutral atom has equal protons and electrons; strip an electron away and the atom becomes a positive ion (cation), add an extra and it becomes a negative ion (anion). Sodium losing one electron to become Na⁺ is the first step in forming common salt.
Electron shells: 2, 8, 8
Electrons cannot crowd anywhere they like — they fill shells of fixed capacity, starting nearest the nucleus. The first shell holds 2 electrons, the second 8, and the third 8 (for the first twenty elements). So sodium's eleven electrons arrange as 2, 8, 1.
The outermost electrons, called valence electrons, decide an element's chemistry. Atoms with full outer shells (helium, neon, argon) are famously unreactive, while atoms one electron away from a full shell — like sodium or chlorine — react eagerly. Chemistry is, at heart, the story of atoms chasing complete shells.