Structure of the Atom:

An atom is the basic unit of matter, consisting of a nucleus (containing protons and
neutrons) surrounded by a cloud of electrons. Atoms are the smallest units that retain
the properties of an element and are the building blocks of all substances.

Every atom consists of three main subatomic particles:

o Protons (p⁺): Positively charged, found in the nucleus.
o Neutrons (n⁰): Neutral (no charge), also found in the nucleus.
o Electrons (e⁻): Negatively charged, moves/resolve around the nucleus in shells.
The Nucleus:
 Very small compared to the size of the whole atom.
 Contains nearly all the mass of the atom.
 Extremely dense and positively charged due to the presence of protons.
Electron Shells:
 Electrons occupy orbitals or shells around the nucleus.
 These shells are regions of space where there is a high probability of finding
an electron.
 Electrons are spread out in this empty space, which makes the atom mostly
empty.
Why Atoms Are Mostly Empty Space:
 The nucleus is about 1/100,000 the size of the atom.
 Most of the volume of an atom is taken up by the space where electrons move.
 Even though electrons move rapidly, they are tiny compared to the nucleus
and are spread far apart.
Rutherford’s Gold Foil Experiment (Evidence):
 In 1909, Ernest Rutherford conducted an experiment that led to this understanding.
 Most alpha particles passed straight through a thin gold foil — showing the
atom is mostly empty.
 Some particles were deflected — indicating a dense, positively charged nucleus.
Atoms are made up of three types of subatomic particles:
 Protons
 Neutrons
 Electrons
Each of these particles has specific properties like charge and mass.

Subatomic Particles:
Particle Location in Atom Relative Charge Relative Mass
Proton nucleus +1 (positive) 1
Neutron nucleus 0 (neutral) 1
Electron shells –1 (negative) 1/1836
Detailed Explanation:
🔴 Protons
 Found in the nucleus of the atom.
 Have a positive charge (+1).
 Relative mass is 1 unit.
 The number of protons = atomic number.
 Protons determine the identity of the element.
⚪ Neutrons
 Also found in the nucleus.
 Have no charge (neutral).
 Relative mass is 1 unit.
 Neutrons help stabilize the nucleus.
 The number of neutrons = mass number – atomic number.
🔵 Electrons
 Move in shells or energy levels around the nucleus.
 Have a negative charge (–1).
 Very tiny mass: about 1/1836 of a proton’s mass.
 In a neutral atom, the number of electrons = number of protons.
Atoms are described using numbers that tell us how many particles they contain.
These include:
1. Atomic Number (also called Proton Number):
 Definition: The number of protons in the nucleus of an atom.
 Symbol: Z
 Every element has a unique atomic number.
 It also tells you the number of electrons in a neutral atom.
Example:
 Hydrogen has 1 proton → Atomic number = 1
 Oxygen has 8 protons → Atomic number = 8
2. Mass Number (also called Nucleon Number):
 Definition: The total number of protons and neutrons in the nucleus of an atom.
 Symbol: A
Formula:
Mass Number (A) = Number of Protons (Z) + Number of Neutrons
Example:
 Carbon has 6 protons and 6 neutrons → Mass number = 12
 Sodium has 11 protons and 12 neutrons → Mass number = 23
3. Number of Neutrons:
Once you know the atomic number and mass number, you can calculate the number of
neutrons:
Number of Neutrons = Mass Number−Atomic Number
Example:
 Chlorine has mass number 35, atomic number 17 → Neutrons = 35 – 17 = 18

Summary Table

Term Symbol What It Tells You

Atomic Number Z Number of protons (and electrons if neutral)
Proton Number Z Same as atomic number
Mass Number A Number of protons + neutrons
Nucleon Number A Same as mass number

A = Z + N
(Mass number = Protons + Neutrons)
Distribution of Mass and Charge in an Atom
1. Distribution of Mass:
 Almost all the mass of an atom is concentrated in the nucleus.
 The nucleus contains:
o Protons (mass = 1 unit each)
o Neutrons (mass = 1 unit each)
 Electrons have very tiny mass (about 1/1836 of a proton), so they
contribute almost nothing to the atom’s total mass.
 Even though electrons move throughout a large space around the nucleus,
their mass is negligible.
2. Distribution of Charge:
 Protons are positively charged (+1).
 Electrons are negatively charged (–1).
 Neutrons have no charge (neutral).
In a neutral atom:
 The number of protons = number of electrons.
 This means the positive and negative charges cancel out, making the
whole atom electrically neutral.

Behaviour of Protons, Neutrons, and Electrons in an Electric Field.

1. Basic Concept:
An electric field exerts a force on charged particles:
 Positive charges are attracted to the negative plate.
 Negative charges are attracted to the positive plate.
 Neutral particles are not affected by the electric field.
2. Particle Behaviour in an Electric Field:
Particle Charge Direction of Deflection Reason
Proton +1 (positive) Deflects toward the negative plate Opposites attract
Electron –1 (negative) Deflects toward the positive plate Opposites attract
Neutron 0 (neutral) No deflection No charge, so no force

3. Description:
 Electrons are much lighter than protons, so they are deflected more
sharply.
 Protons deflect in the opposite direction to electrons but less sharply due
to their larger mass.
 Neutrons continue in a straight line, unaffected by the field.
4. Diagram (Text-based):

5. Important Note:
 Only charged particles are affected by an electric field.
 The direction and amount of deflection depends on:
o The type of charge
o The mass of the particle

How to Determine Protons, Neutrons, and Electrons in Atoms and

Ions?
Term Symbol Meaning
Number of protons in the nucleus (also electrons in
Atomic Number Z
a neutral atom)
Mass Number A Total number of protons + neutrons
Shows if it's an atom (neutral) or ion (positive or
Charge –/+
negative)
1. Protons
 The number of protons = atomic number (Z)
 This never changes for a given element.
2. Neutrons
Neutrons = Mass number (A) − Atomic number (Z)
3. Electrons
For a Neutral Atom:
Electrons=Protons
For an Ion:
 Positive ion (cation) → Electrons = Protons – Charge
 Negative ion (anion) → Electrons = Protons + Charge
4. Example Table
Symbol Atomic No. (Z) Mass No. (A) Charge Protons Neutrons Electrons
O 8 16 0 8 8 8
O²⁻ 8 16 –2 8 8 10
Na 11 23 0 11 12 11
Na⁺ 11 23 +1 11 12 10
Cl⁻ 17 35 –1 17 18 18

Atomic Radius and Ionic Radius Trends

1. Atomic Radius
Definition: The atomic radius is the distance from the center of the nucleus to the
outermost electron shell.
Variation across a Period (Left to Right)
 Atomic radius decreases from left to right across a period.
Why?
 Number of protons increases → nuclear charge increases.
 Electrons are added to the same shell, so no increase in shielding.
 Stronger attraction between the nucleus and electrons pulls them closer.
 So, atoms become smaller.
Example:
Atomic radius from Na → Mg → Al → Si → P → S → Cl → Ar decreases.
Variation down a Group (Top to Bottom)
 Atomic radius increases down a group.
Why?
 More electron shells are added.
 Outer electrons are farther from the nucleus.
 Increased shielding effect reduces the attraction between nucleus and outer
electrons.
 So, atoms become larger.
Example:
Atomic radius from Li → Na → K → Rb → Cs increases.
2. Ionic Radius
Definition: The ionic radius is the radius of an ion (atom that has gained or lost electrons).
Across a Period (for Ions)
  Cations (positive ions): Smaller than their atoms
o Loss of electrons → less repulsion, and fewer electrons pulled closer
by nucleus.
 Anions (negative ions): Larger than their atoms
o Gain of electrons → more repulsion between electrons → size
increases.
Across a period (e.g., Na⁺ to Cl⁻):
 Ionic radius first decreases (for cations),
 then jumps and decreases again (for anions).
Down a Group (for Ions)
 Ionic radius increases down a group.
 Same reason as atomic radius: more electron shells = larger size.
Example:
Ionic radius from F⁻ → Cl⁻ → Br⁻ → I⁻ increases.
Summary Table
Trend Across a Period Down a Group
Atomic Radius Decreases Increases
Cation Size Decreases Increases
Anion Size Decreases (after jump) Increases

Definition of Isotope
Isotopes are atoms of the same element that have the same number of protons
but different numbers of neutrons.
Important Points:
 Same atomic number (same number of protons)
 Different mass number (because of different number of neutrons)
 Chemically similar (since they have the same number of electrons)
Example:
 Carbon-12: 6 protons, 6 neutrons
 Carbon-14: 6 protons, 8 neutrons
Both are isotopes of carbon because they have 6 protons, but different neutrons.

Why Isotopes Have the Same Chemical Properties?

Isotopes of the same element have the same chemical properties.
Explanation:
 Chemical properties of an element depend on the number and arrangement
of electrons, especially the outer (valence) electrons.
 All isotopes of an element have the same number of protons → so they
have the same number of electrons (in neutral atoms).
 Since their electron configurations are the same, they behave the same
way in chemical reactions.
Example:
 Hydrogen-1 (¹H) and Hydrogen-2 (²H or deuterium) both have:
o 1 proton
o 1 electron
 Even though they have different numbers of neutrons, they both react the
same with oxygen to form water.
Chemical properties depend on electrons, not neutrons.

Why Isotopes Have Different Physical Properties?

Statement:
Isotopes of the same element have different physical properties, such as mass
and density.
Explanation:
 Isotopes have the same number of protons but different numbers of
neutrons.
 This means their masses are different because neutrons add to the mass of
the atom.
 Since mass affects physical properties like density and mass-related
behavior, isotopes differ physically.
 For example:
o Heavier isotopes have greater mass.
o This affects properties like density, melting point, and boiling point
(though we focus here on mass and density).
Example:
 Carbon-12 (6 protons + 6 neutrons) is lighter than Carbon-14 (6 protons +
8 neutrons).
 Carbon-14 atoms are heavier, so samples containing more Carbon-14 will
have a slightly higher density.

Important point:
Different neutron numbers → different mass → different physical properties like
mass and density.