Atoms and the periodic table

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One of the first models of the atom was the “plum pudding” model, which was proposed by the physicist J. J. Thomson in the nineteenth century.

In the “plum pudding” model, atoms were thought to have a cloud of positive charge, and within that cloud there were tiny electrons with negative charge.

Sort of like the mid-nineteenth century british dessert — plum pudding --- where the plums were the electrons sitting within a delicious positive charge of pudding.

However, in the 20th century the physicist Ernest Rutherford realized that the plum pudding model wasn’t quite right.

He did a famous experiment where he took positively-charged alpha particles, and shot them at a piece of gold foil.

Most of these alpha particles passed straight through the gold foil and hit the detector, but some of them actually bounced off of the gold foil and hit the sides of the detector.

His explanation for this was that the atoms are mostly empty space, and that the majority of the alpha particles travel straight through the foil and strike the back wall of the detector, but that on very rare occasions, the alpha particles bounce off of something.

He visualized a very small but very dense object in the middle of the atom, that he called the nucleus, which the alpha particles were bouncing off of on those rare occasions.

Since the alpha particles are positively charged, Rutherford figured that the nucleus must also be positively charged since like charges repel.

So Rutherford gave us the idea that the modern atom consists of mostly empty space containing electrons, which are miniscule in size compared to the atom overall.

But in the middle of that empty space there is a very small but very dense nucleus.

Later, scientists showed that the nucleus actually consists of two types of particles: protons and neutrons.

Protons have a positive charge but neutrons have no charge.

Now, when looking at the periodic table you can find the simplest atom, which is hydrogen.

Hydrogen has the symbol H, and it has an atomic number equal to one.

Atomic numbers tell you the number of protons in the atom which means that Hydrogen has one proton in its nucleus.

The number of protons in the nucleus of an atom is unique to each element.

A typical hydrogen atom has a nucleus with one proton and one electron outside the nucleus.


The periodic table is a chart that organizes all the known elements according to their atomic number (number of protons in the nucleus). The elements are arranged in rows and columns, with each element occupying a specific place on the table. Each element has a unique symbol that is used to represent it. For example, hydrogen is represented by H, lithium by Li, and oxygen by O. The atomic number of an element is represented by a superscripted number to the left of the element's symbol. For example, the atomic number of hydrogen is 1, so it is represented as H1.

The elements are arranged in increasing order of atomic number. As you move from left to right across a row, the atomic numbers increase. As you move down a column, the atomic numbers increase.


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