I take it you already know what an atom is.
Everything around you that takes up space is made up of atoms.
It is the smallest possible unit that retains the properties of elements. 

Democritus, a Greek philosopher, first proposed the idea of atoms.
The word, “atom” comes from the Greek word, “Atomos” which means “uncuttable” or “indivisible”.

If you could divide any object infinitely, at one point the object will lose all its properties and turn into something completely different. For example, any glass will turn into oxygen and silicon.
And these particles that you got at the last step, which can’t be broken anymore are called atoms.
Atoms are so small that if you were to put 10 million hydrogen atoms in a line, it will only be 1 meter long.

So, right at this moment, a group of atoms is reading about atoms written by another group of atoms using a device that is just another group of atoms.
Today, the target is to learn about the atomic structure.
Let’s proceed to the topic.

 

Structure of an Atom

You should know by now that even an atom can be divided and that it is made up of many smaller particles, mainly protons, neutrons, and electrons.
Though atom means indivisible, it is called atom anyway.
Pretty weird, right?
Anyways, let’s take a look at the structural diagram of an atom, Shall we?

An atom can be divided into two parts- the nucleus and the electron orbitals.
The nucleus is made up of positively charged protons and neutral neutrons.
So, protons and neutrons are also called nucleons as they make up the nucleus.
As protons and neutrons are almost 2000 times heavier than an electron and so the nucleus contains almost all the mass of the atom. Even so, the nucleus takes up almost no space.
Try to imagine how dense it is. It is about 1017kgm-3.
If the earth was an atom, the nucleus would only be the size of a football court.
As a result, atoms are mostly empty.

As it stands, we too are mostly empty, everything else is.
What do you think?

Now, take a look at the electrons.
They move around the nucleus following certain paths called electron orbitals which can also be called electron shells.
There are multiple definite electron shells of varying distances from the nucleus.
Electrons arrange themselves among these shells following few rules.
The arrangement of electrons will need a discussion of its own.
So, let’s skip it for now and stay on track.
Electron shells can be designated with alphabets.
Suppose, n denotes the order of the shells.
So, for 1st shell n=1, then we can call it K-shell. In the same way for the 2nd shell, n=1 and it is called L shell.
Correspondingly, this goes on as M, N, O, etc.

Normally atoms are neutral as a whole. Which means there are the same numbers of protons and electrons in an atom. But in the case of ions, they vary.

That’s the gist of it. You will learn more as you go.
Now let’s take a look into other things related to an atom.

Atomic number

The number of protons in the nucleus of an atom is called the atomic number.
The atomic number indicates that of which element the atom is.
You can think of it as some sort of identification number assigned to different groups.
For example, all the atoms of hydrogen in the universe have 1 proton each, atoms of calcium have 20 protons each.
The number of protons i.e., the number of electrons in the atoms decides the chemical properties of the element.
It is denoted by “Z”.

 If you are told that an atom has twenty-six protons, what element would it be? Of course, it is iron.
Now you know. Also, if you look at the periodic table of elements, you will see that they are arranged according to their atomic numbers. There are more mysteries to the atomic number than meets the eye.
It is not just a number but is what differentiates an element from others.
How fascinating, just a number can be!!

Mass number

Remember nucleon? Protons and neutrons are collectively called nucleons. You know the reason. The total number of nucleons in an atom i.e., the total number of protons and neutrons is called the mass number. It doesn’t have a unit but it is equal to the value of mass expressed in the a.m.u. unit. We can discuss it while discussing quantitative chemistry and stoichiometry but not now. You might be thinking now, “Where did the electrons go?”. They are still there. But compared to protons and neutrons, their weight is completely negligible and not counted. Don’t feel pity, unlike neutrons they have a complete effect on the charge of the atom and expressing the chemical properties.

The mass number is denoted by “A”. From the mass number and atomic number, we can easily know the number of neutrons. 

Number of neutrons= Mass number – Atomic number= A-Z

If you’re wondering, why “kg” is not used, it is because calculation involving chemical reactions using the kg unit is a bit more troublesome.  Firstly, substances react in a definite proportion based on moles. Moles describes the amount of substance. Each mole of an element contains an amount equal to its mass number when expressed in gram. For example, 40 grams of calcium means 1 mol (unit of the mole) calcium.  Leave mole for another day.

Moreover, although atoms are so little that you can’t measure their mass using balance, it is possible to determine the mass using other means. But problems remain. For example, the mass of an oxygen atom is 2.661×10-23 kg.  Why memorize and use this small value when we can use atomic numbers with beautiful thin figures. It’s a different case if you have a very good memory and like chubby figures. 

 

Coming up next is an interesting topic We have always wondered about our origin and history. For this reason, archaeologists are always searching for things that once existed in the past. A very important part of their work is to determine the age of the discovered things. And the way they do it is by using isotopes. But do you know what an isotope is? Let’s find out.

 

Isotope

You have seen that varying the number of electrons changes the charges of an atom, and varying the number of protons converts the atom to an atom from another element, like completely changing its identity. What if the number of neutrons is changed while keeping others the same? We will get an atom of the same element but with a different mass. Did you guess it right?

And these atoms of the same element of varying masses are called “isotopes”.

Isotopes are different versions of the same product. They may have a few special functions but they are the same thing.

One question comes to mind, is mass the only difference? The answer is no. As the mass changes, other physical properties like the density of the substance, the stability of the nucleus, radioactivity, etc. also change. But the chemical properties remain the same. As stated earlier, it is the number of protons and electrons which determines the chemical properties.

Isotopes are generally written by writing the name first then adding the mass number. For example, hydrogen has three isotopes. They are written as, hydrogen-1, hydrogen-2, and hydrogen 3.

 

 

Here protium, deuterium, tritium are the common names for the isotopes. You can see that the number of protons and the electrons are the same for all three of them. Most of the hydrogen atoms around us or in us are protium. Deuterium is rare and tritium even rarer. Have you heard the term, “heavy water”? It is water but instead of hydrogen-1 (protium), it has hydrogen-2 (deuterium) in it. Thus, the name- heavy water.

Isotopes are important because of their different physical properties. They can be detected and differentiated from others easily. Also, as the nuclear structure changes, isotopes of some elements show radioactivity. These isotopes have various uses e.g., carbon-14 is used for determining the age of fossils. Even the age of the earth and moon was determined in this way but the isotope used was potassium-40!! The earth is 4.534 billion years old while the moon is 4.53 billion years old!! So the theory that the moon was formed by a big meteor knocking the earth might be true then.

Also, sometimes they are used as a substitute in chemical reactions to determine the reaction mechanism.

Now, onto the last topic for today.

 

Isotopic symbol

The isotopic symbol is literally the symbol of isotopes. It is also called the nuclide symbol. You might already know that each element has a symbol of its own e.g., Na for sodium, Fe for iron, O for Oxygen, Li for lithium, etc. You can use isotopic symbols in the same way to express them. Look at the following figure, 

From this symbol, you will know a lot of information at a glance. You can calculate the number of neutrons from A and Z. And also know the number of electrons. As the number of protons can’t change, a positive charge means fewer electrons and a negative charge means more extra electrons.

**Note that both the protons and electrons have the same but opposite charge and the charge of an electron is considered one unit.

So 136C means it is a carbon atom which means it has six protons. And it has a mass number of 13. So, the number of neutrons is, (13-6) or 7. Moreover, it has no charge so the number of electrons is the same as protons and is 6 too.

Consider 3316S2- , this sulfide ion has 16 protons, 17 neutrons (33 – 16 = 17) and 18 electrons (16 + 2 = 18).

 

And that’s about the things that cover the basic knowledge of atomic structure.

Congratulations, you have obtained the basic knowledge of the structure of an atom.