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WonderScience fact

More than 99.9% of an atom's mass is packed into its nucleus, a centre tens of thousands of times smaller across than the atom itself. The rest of the atom is filled by a thin cloud of electrons. If an atom were a large stadium, its nucleus would be about the size of a pea.

Physics

Science

The story behind the card

The nucleus, made of protons and neutrons, was discovered in 1911 by Ernest Rutherford in Manchester. His colleagues Hans Geiger and Ernest Marsden had fired tiny charged particles at thin gold foil; most passed straight through, but a few bounced back, which only made sense if the atom's positive charge and mass were packed into a tiny core. Electrons are not little balls in orbits; they are better described as a cloud showing where each is likely to be found. Solid things feel solid for two reasons. The negatively charged electrons of neighbouring atoms repel one another, and a rule of quantum physics called the Pauli exclusion principle stops electrons from crowding into the same state. Together these keep atoms from being squeezed into each other.

Türkçe oku
Bir atomun kütlesinin %99,9'undan fazlası çekirdeğinde toplanır. Çekirdeğin çapı atomun kendisinden on binlerce kat küçüktür; atomun geri kalanını seyrek bir elektron bulutu doldurur. Atom büyük bir stadyum olsaydı, çekirdeği bir bezelye tanesi kadar olurdu.

Proton ve nötronlardan oluşan çekirdeği 1911'de Manchester'da Ernest Rutherford keşfetti. Çalışma arkadaşları Hans Geiger ve Ernest Marsden ince bir altın folyoya elektrik yüklü minik parçacıklar göndermişti. Parçacıkların çoğu folyodan dosdoğru geçti, ama birkaçı geri sekti. Bu ancak atomun pozitif yükünün ve kütlesinin minicik bir merkezde toplanmasıyla açıklanabilirdi. Elektronlar yörüngede dönen küçük toplar değildir; her birinin nerede bulunabileceğini gösteren bir bulut olarak tarif edilmeleri daha doğrudur. Katı cisimlerin katı hissettirmesinin iki nedeni vardır. Komşu atomların negatif yüklü elektronları birbirini iter; ayrıca Pauli dışarlama ilkesi denen bir kuantum kuralı, elektronların aynı duruma sıkışmasını engeller. Bu ikisi birlikte atomların iç içe geçmesini önler.

Sources and further reading

Background reading: Atom
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