Atomic Structure

What is an atom?
An atom is the smallest unit of an element, having the properties of that element.

Some fast facts on atoms:

• All atoms are built from just three particles - Protons, Neutrons and Electrons.
• The centre of an atom is called the nucleus and this contains the protons and neutrons. The nucleus takes up less than 1% of the volume of an atom.
• Protons and neutrons have virtually the same mass. The proton and neutron each have a relative mas of one.
• Protons have a positive charge, but neutrons are neutral.
• The other 99% of the atom is empty space occupied by moving electrons.
• These electrons have a mass of about 2000 times less that of a proton or neutron.
• Electrons have a negative charge, which cancels out the positive charge on the proton.
• Electrons move around very rapidly, and occupy different shells at different distances from the nucleus.

Now, each shell can hold a certain maximum number of electrons. The 1st shell can hold 2 electrons, the 2nd shell can hold 8 electrons while the 3rd shell can also hold 8 electrons. For elements after calcium, their third shell can hold up to 18 electrons.

The atomic number is the number of protons in an atom while the mass number is the number of protons AND the number of neutrons in an atom.

Isotopes

Some atoms come from the same element, but have different masses. These different atoms are called isotopes. Isotopes are atoms with the same atomic number, but with different mass numbers.

An example of an isotope is a chlorine isotope. Two chlorine isotopes, chlorine-35 and chlorine-37, both have 17 protons and electrons each, but chlorine-35 has 18 neutrons and chlorine-37 has 20 neutrons. Hence, they have different mass numbers, different masses and hence different physical properties from its other isotopes, as their properties depend on the mass of its atoms and molecules.

So, isotopes in a nutSHELL (pun intended):

Isotopes of the same element have the same number of protons, electrons, atomic number and chemical properties, but have different numbers of neutrons, mass numbers and physical properties.

Electronic Structure & Electronic Configuration

How to write electronic configuration:

Earlier we just saw that each shell can hold a maximum number of electrons. The 1st shell can hold 2 electrons, the 2nd shell can hold 8 electrons while the 3rd shell can also hold 8 electrons. For elements after calcium, their third shell can hold up to 18 electrons.

So now, for example, let's take a Nitrogen atom. A nitrogen atom has 7 electrons so it's electronic configuration can be written as 2.5

A Magnesium atom has 12 electrons, so its electronic configuration can be written as 2.8.2

An Argon atom has 18 electrons, so its electronic configuration can be written as 2.8.8

Valency

The shell which is the furthest away from the nucleus that is still occupied by electrons is called the valence shell, or the outer shell. The electrons in the valence shell are known as valence electrons, and in a chemical reaction, only these valence electrons are involved in the chemical bonding between these atoms.

The number of electrons an atom uses to form bonds is called its valency. So basically, an atom with 1, 2, 3 or 4 valence electrons would have a valency of 1, 2, 3 and 4 respectively, and an atom with 5, 6, 7 or 8 valence electrons would have a valency of 3, 2, 1 and 0 respectively.