HKNIC - The classical concept - Inside an Atom
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The classical concept - Inside an Atom
After we have found out that matter is made up of atoms, the next question people began to ask was “What is inside an atom?” Scientists began to stumble upon the answer when they noticed that electricity has certain strange effects – when putting a voltage difference across a vacuum, they found minute particles carrying a negative charge – now known as electrons – flowing across the vacuum from the electrode at one end and passing the opposite electrode to produce a patch of greenish glow at the other end of the tube. Scientists also noticed that some particles would come out from certain types of elements. From this point onward, the idea that an atom is an indivisible particle is not altogether too accurate.
 
What we might now call a “classical” view of the atom emerged in the early 20th century. This says that an individual atom has a heavy nucleus made up of protons and neutrons. Protons and neutrons are visualised as particles, with protons having a positive electrical charge and neutrons having no electrical charge. Spinning around the nucleus are electrons, which have a negative electrical charge. An atom is electrically neutral when its number of protons equals its number of electrons.
An atom is of course very small. It will take about 10 million atoms to line up to give 1 millimetre. An atom is also largely empty, and matter really concentrates in the atomic nucleus. This was established in the early 20th century, when scientists shot atomic particles at matter, only to discover that most passed straight through as if there was nothing in their path.

The number of protons in an atom tells us what kind of element it is. In other words, if one atom has a different number of protons in its nucleus to another atom, it belongs to a different element. For example, an atom with one proton in its nucleus belongs to the element hydrogen, while an atom with two protons belongs to the element helium. The element uranium has 92 protons in its atomic nucleus, the largest number of protons found in an element occurring in nature.

Elements whose atoms contain fewer protons are known as “lighter” elements, while those with more protons are known as “heavier” elements. Lighter elements have fewer electrons than heavier elements so as to keep their atoms electrically neutral. A hydrogen atom, for example, has one electron to balance its proton, while helium has two electrons to balance its two protons.

In the real world, electrons may, however, be shared among atoms and jump from atom to atom. This phenomenon allows all kinds of chemical reactions to take place, when two dissimilar atoms share their electrons so as to become a compound, and allows electronic appliances to operate when electrons jump about inside a microchip. By contrast, scientists believed that protons and neutrons would always remain inside the nucleus – this is indeed the case as long as we do not use the tricks in nuclear physics, that is, touch on the nucleus of the atom.

The number of protons gives chemical identity to an element, that is, atoms of one element always behave chemically in the same way when reacting with the atoms of another element. For example, all hydrogen atoms will react chemically with oxygen, or burn, in the same way to give water vapour. While the number of protons in an element is constant, however, the number of neutrons may change and these variations are called nuclides – sometimes referred to as isotopes. For example, although hydrogen atoms usually have no neutrons in their nuclei, a small percentage have one and some have two.

Nuclides of the same element have slightly different physical behaviours. Water is a compound of hydrogen and oxygen. But when the hydrogen contains neutrons it is slightly heavier than normal water and so is called “heavy water”. However, in the subject of nuclear physics, nuclides of the same element may behave quite differently which can be seen in the section on What is Radiation?.