Particles in the atom and atomic radius

At the heart of every atom is a tiny, dense nucleus, which contains almost all the atom's mass. This nucleus is made of two types of particles: positively charged protons and neutral neutrons. Whizzing around the nucleus in specific energy levels, or shells, are negatively charged electrons. Because the nucleus is so small compared to the whole atom, most of an atom is actually empty space. In any neutral atom, the number of positive protons is exactly balanced by the number of negative electrons, resulting in an overall charge of zero. The properties of these three fundamental particles are crucial to understanding chemistry.

Properties of Subatomic Particles:

Every element is defined by the number of protons in its atoms; this is called the proton number (Z), or atomic number. For example, any atom with 6 protons is a carbon atom. The total number of protons and neutrons in the nucleus is the nucleon number (A), also known as the mass number. Since protons and neutrons make up nearly all the atom's mass, this number gives you the relative mass of the atom.

We use a standard notation to represent this information: $^A_Z X$, where X is the element's symbol, A is the nucleon number (top left), and Z is the proton number (bottom left).

From this, you can figure out everything about a neutral atom:

• Number of protons = Z
• Number of electrons = Z (in a neutral atom)
• Number of neutrons = A - Z (nucleon number minus proton number)

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Because they have the same number of protons and electrons, isotopes have identical chemical properties. However, their different neutron numbers mean they have different nucleon numbers and different masses. For example, chlorine exists as two main isotopes: chlorine-35 ($^{35}_{17}\text{Cl}$) with 18 neutrons, and chlorine-37 ($^{37}_{17}\text{Cl}$) with 20 neutrons. Both have 17 protons.

Ions are formed when a neutral atom gains or loses one or more electrons. They are electrically charged particles.

• Cations are positive ions formed when an atom *loses* electrons. There are now more protons than electrons. Example: A magnesium atom ($^{24}_{12}\text{Mg}$) has 12p and 12e. It loses 2 electrons to form a magnesium ion, $\text{Mg}^{2+}$, which has 12p and 10e.
• Anions are negative ions formed when an atom *gains* electrons. There are now more electrons than protons. Example: An oxygen atom ($^{16}_8\text{O}$) has 8p and 8e. It gains 2 electrons to form an oxide ion, $\text{O}^{2-}$, which has 8p and 10e.

Because subatomic particles have different charges, they behave differently when they pass through an electric field. An electric field is a region between two oppositely charged plates, one positive and one negative.

Imagine firing parallel beams of protons, neutrons, and electrons through this field:

• Protons, being positively charged (+1), are attracted to the negative plate and repelled by the positive plate. Their path will curve towards the negative plate.
• Neutrons, having no charge (0), are unaffected by the electric field. They will pass straight through without any deflection.
• Electrons, being negatively charged (-1), are attracted to the positive plate and repelled by the negative plate. Their path will curve towards the positive plate.

An important distinction is the *amount* of deflection. Both protons and electrons have the same magnitude of charge (1), but an electron has a much, much smaller mass (1/1840th of a proton). This means the electron is deflected much more easily and follows a more curved path than the proton.

The atomic radius is a measure of the size of an atom, typically the distance from the nucleus to the outermost electron shell. For comparison, we often use the covalent radius, which is half the distance between the nuclei of two identical atoms bonded together.

A key trend you must know is how atomic radius changes across a period in the Periodic Table. As you move from left to right across a period (e.g., from Sodium to Chlorine in Period 3), the atomic radius *decreases*.

This happens for three main reasons:

1. Increasing Nuclear Charge: As you move across a period, one proton is added to the nucleus for each new element. This increases the positive charge of the nucleus.
2. Same Principal Energy Level: The extra electron for each element is added to the same outer shell (or principal energy level).
3. Shielding is Similar: Because the electrons are added to the same shell, the number of inner shells of electrons (which 'shield' the outer electrons from the full pull of the nucleus) remains constant. The shielding effect doesn't increase much.

The result is that the increasingly positive nucleus exerts a stronger pull on the electrons in the same outer shell, pulling them closer and causing the atomic radius to shrink.