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Almost all of an atom's mass sits in a tiny nucleus at its centre; the atom's size comes from the far larger cloud of electrons around it. When you press on a table, the electron clouds of its atoms and yours push back against each other, so your hand does not pass through.

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Science

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We know how small the nucleus is thanks to experiments at the University of Manchester. In 1909 Hans Geiger and Ernest Marsden, working with Ernest Rutherford, fired alpha particles, tiny fragments shot out by radioactive atoms, at a very thin sheet of gold. Most went straight through, but a few bounced sharply back. In 1911 Rutherford concluded that almost all of an atom's mass is packed into a tiny central nucleus. We now know it holds more than 99.9% of the mass and is 27,000 to 60,000 times narrower than the atom. Two effects keep atoms from overlapping. Electrons all carry negative charge, so they repel each other. And the Pauli exclusion principle, a rule of quantum physics stated by Wolfgang Pauli in 1925, forbids two electrons from being in exactly the same state, so electron clouds resist being squeezed together.

Türkçe oku
Atomun kütlesinin neredeyse tamamı ortadaki minicik çekirdektedir; atomun büyüklüğünü ise çevresini saran, çok daha geniş elektron bulutu belirler. Masaya bastırdığınızda elinizdeki atomların elektron bulutlarıyla masanınkiler birbirini iter; eliniz de bu yüzden masanın içinden geçmez.

Çekirdeğin ne kadar küçük olduğunu Manchester Üniversitesi'ndeki deneylerden öğrendik. 1909'da Ernest Rutherford'la çalışan Hans Geiger ve Ernest Marsden, çok ince bir altın varağa alfa parçacıkları, yani radyoaktif atomların fırlattığı minik parçacıklar gönderdi. Parçacıkların çoğu varaktan dosdoğru geçti, ama birkaçı sertçe geri sekti. Rutherford 1911'de, atomun kütlesinin neredeyse tamamının ortadaki minicik bir çekirdekte toplandığı sonucuna vardı. Bugünkü ölçümlere göre çekirdek, kütlenin yüzde 99,9'undan fazlasını taşır ve çapı atomunkinden 27.000 ila 60.000 kat küçüktür. Atomların iç içe geçmesini iki etki önler. Elektronların hepsi eksi yüklüdür, bu yüzden birbirini iter. Ayrıca Wolfgang Pauli'nin 1925'te ortaya koyduğu bir kuantum fiziği kuralı olan Pauli dışarlama ilkesi, iki elektronun tamamen aynı durumda bulunmasını yasaklar; bu yüzden elektron bulutları sıkıştırılmaya direnir.

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Background reading: Atom
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