Atoms & Elements

What Is an Atom?

An atom is the smallest particle of an element that retains the chemical properties of that element. All matter — solids, liquids, and gases — is made of atoms. Atoms are incredibly tiny: a single human hair is about one million atoms wide.

Atoms were once thought to be indivisible (the word "atom" comes from the Greek for "uncuttable"), but we now know they are made of even smaller subatomic particles.

Subatomic Particles

Every atom contains three types of subatomic particles:

Particle	Location	Relative charge	Relative mass
Proton	Nucleus	+1	1
Neutron	Nucleus	0	1
Electron	Shells around nucleus	-1	1/1836 (negligible)

Key points:

• The nucleus is at the centre of the atom and is extremely small and dense
• The nucleus contains protons and neutrons, so it carries a positive charge overall
• Electrons orbit the nucleus in shells (energy levels)
• A neutral atom has equal numbers of protons and electrons, so overall charge is zero
• Almost all of an atom's mass is in the nucleus; electrons contribute virtually nothing

Atomic Number and Mass Number

Every element is defined by two key numbers:

Atomic number (Z) = number of protons in the nucleus

• This is unique to each element — no two elements share the same atomic number
• In a neutral atom, the number of electrons equals the atomic number

Mass number (A) = total number of protons + neutrons

Number of neutrons = Mass number - Atomic number

Example: Sodium (Na) has atomic number 11 and mass number 23.

• Protons = 11
• Neutrons = 23 - 11 = 12
• Electrons = 11 (neutral atom)

Elements

An element is a pure substance made of only one type of atom. There are 118 known elements, each listed on the periodic table.

Examples:

• Hydrogen (H) — atomic number 1; lightest element
• Carbon (C) — atomic number 6; basis of all life
• Oxygen (O) — atomic number 8; essential for respiration
• Iron (Fe) — atomic number 26; magnetic metal
• Gold (Au) — atomic number 79; unreactive precious metal

Elements cannot be broken down into simpler substances by chemical means.

Electronic Configuration

Electrons occupy shells (energy levels) around the nucleus. Each shell can hold a maximum number of electrons:

• Shell 1 (closest to nucleus): max 2 electrons
• Shell 2: max 8 electrons
• Shell 3: max 8 electrons (for the first 20 elements)

Electrons fill the innermost shell first before moving to the next.

Element	Atomic number	Configuration
Hydrogen	1	1
Carbon	6	2, 4
Oxygen	8	2, 6
Sodium	11	2, 8, 1
Chlorine	17	2, 8, 7
Calcium	20	2, 8, 8, 2

The outer shell electrons (valence electrons) determine how an atom bonds with other atoms. Atoms are most stable when their outer shell is full (8 electrons, or 2 for the first shell).

Isotopes

Isotopes are atoms of the same element with the same number of protons but a different number of neutrons. They have the same atomic number but different mass numbers.

Examples:

• Carbon-12 (6 protons, 6 neutrons) and Carbon-14 (6 protons, 8 neutrons) — both are carbon
• Hydrogen-1 (no neutrons), Deuterium (1 neutron), Tritium (2 neutrons)

Isotopes have the same chemical properties but different physical properties (e.g., mass).

Some isotopes are radioactive (unstable nuclei that decay and emit radiation), such as Carbon-14, which is used in radiocarbon dating.

The Periodic Table Overview

The periodic table arranges elements in order of increasing atomic number. Elements are organised into:

• Periods (horizontal rows) — elements in the same period have the same number of electron shells
• Groups (vertical columns) — elements in the same group have the same number of outer shell electrons, giving them similar chemical properties

Group 1 = alkali metals (very reactive), Group 7 = halogens (very reactive non-metals), Group 0 = noble gases (very unreactive, full outer shells).

Models of the Atom (History)

Our understanding of atomic structure has evolved:

1. Dalton (1803) — atoms are solid, indivisible spheres
2. Thomson (1897) — discovered electrons; proposed "plum pudding" model (negative electrons embedded in positive sphere)
3. Rutherford (1911) — discovered the nucleus via gold foil experiment; proposed nuclear model
4. Bohr (1913) — electrons orbit in fixed shells at set energy levels
5. Modern quantum model — electrons exist in probability clouds (orbitals), not fixed paths

Key Terms

Term	Definition
Atom	Smallest particle of an element with that element's chemical properties
Proton	Positively charged particle in the nucleus
Neutron	Neutral particle in the nucleus
Electron	Negatively charged particle orbiting the nucleus in shells
Atomic number	Number of protons in an atom; defines the element
Mass number	Total number of protons and neutrons in an atom
Isotope	Atoms of the same element with different numbers of neutrons
Electronic configuration	Arrangement of electrons in shells around the nucleus

Worked Example

Q: An atom has 15 protons and a mass number of 31. Give its electronic configuration and number of neutrons.

Step 1: Atomic number = number of protons = 15 (this is phosphorus).

Step 2: Number of neutrons = mass number - atomic number = 31 - 15 = 16 neutrons.

Step 3: Electronic configuration — 15 electrons fill shells in order:

• Shell 1: 2
• Shell 2: 8
• Shell 3: 5

Configuration = 2, 8, 5

Common Mistakes

• Confusing atomic number and mass number — atomic number is the number of protons only; mass number includes protons AND neutrons.
• Thinking electrons have significant mass — electron mass is negligible compared to protons and neutrons.
• Saying isotopes have different numbers of protons — isotopes always have the same number of protons (otherwise they would be different elements).
• Forgetting that in a neutral atom, electrons = protons — when an atom gains or loses electrons it becomes an ion, not a different element.

Tips and Tricks

• Mnemonic for subatomic particles: Protons are Positive, Neutrons are Neutral, Electrons are Electric (negative).
• To find neutrons: mass number minus atomic number.
• Atoms in the same group of the periodic table react similarly — same number of outer electrons = same reactivity pattern.
• Remember: atomic number never changes for an element — if it changes, it becomes a different element (nuclear reaction, not a chemical one).