Atomic Structure
Discover the building blocks of all matter -- protons, neutrons and electrons -- and learn how atomic number, mass number and electron configuration define each element.
Subatomic Particles
Every atom is made up of three types of subatomic particles: protons, neutrons and electrons. Protons and neutrons are found in the dense central nucleus, while electrons orbit the nucleus in electron shells (energy levels).
Proton
Charge: +1
Mass: 1 amu
Location: Nucleus
Neutron
Charge: 0
Mass: 1 amu
Location: Nucleus
Electron
Charge: -1
Mass: ~1/1836 amu
Location: Electron shells
Key point: In a neutral atom, the number of protons equals the number of electrons, so the overall charge is zero. If an atom gains or loses electrons, it becomes an ion.
Atomic Number and Mass Number
Every element is uniquely identified by its atomic number (Z) -- the number of protons in the nucleus. The mass number (A) is the total number of protons and neutrons.
Nuclide Notation
126C
Carbon-12: 6p, 6n, 6e
2311Na
Sodium-23: 11p, 12n, 11e
5626Fe
Iron-56: 26p, 30n, 26e
Calculating neutrons: Number of neutrons = Mass number (A) - Atomic number (Z). For example, 2311Na has 23 - 11 = 12 neutrons.
Isotopes and Electron Configuration
Isotopes are atoms of the same element (same number of protons) but with different numbers of neutrons. They have the same chemical properties but different physical properties (such as mass and radioactive behaviour).
Isotopes of Carbon
126C
6 protons, 6 neutrons
Most abundant (98.9%)
136C
6 protons, 7 neutrons
Stable, 1.1% abundance
146C
6 protons, 8 neutrons
Radioactive (carbon dating)
Electron Configuration
Electrons fill energy levels (shells) from lowest to highest energy. The maximum number of electrons in each shell: Shell 1 = 2, Shell 2 = 8, Shell 3 = 8 (for the first 20 elements).
Hydrogen (1)
1
Carbon (6)
2, 4
Sodium (11)
2, 8, 1
Chlorine (17)
2, 8, 7
Key Vocabulary
Atomic Number (Z)
The number of protons in the nucleus of an atom. It defines the element and its position on the periodic table.
Mass Number (A)
The total number of protons plus neutrons in the nucleus. A = Z + number of neutrons.
Isotopes
Atoms of the same element with the same number of protons but different numbers of neutrons, giving different mass numbers.
Ion
An atom that has gained or lost electrons, giving it a net positive (cation) or negative (anion) charge.
Worked Examples
An atom of phosphorus has atomic number 15 and mass number 31. How many protons, neutrons and electrons does it have?
Protons = atomic number = 15
Neutrons = mass number - atomic number = 31 - 15 = 16
Electrons = protons (neutral atom) = 15
Write the electron configuration for potassium (Z = 19).
Shell 1: 2 electrons (maximum for first shell)
Shell 2: 8 electrons (maximum for second shell)
Shell 3: 8 electrons (maximum for third shell, first 20 elements)
Shell 4: 1 electron (remaining electrons: 19 - 2 - 8 - 8 = 1)
Electron configuration: 2, 8, 8, 1
Explain why 3517Cl and 3717Cl are isotopes.
Both have atomic number 17, meaning they both have 17 protons -- so they are both chlorine.
Cl-35 has 35 - 17 = 18 neutrons.
Cl-37 has 37 - 17 = 20 neutrons.
They are isotopes because they are the same element (same Z) with different mass numbers (different number of neutrons).
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
The atomic number of an element tells you the number of:
Question 2
How many neutrons does 4020Ca have?
Question 3
Isotopes of an element differ in their number of:
Question 4
What is the electron configuration of aluminium (Z = 13)?
Question 5
Where is the majority of an atom's mass located?
Key Concepts Summary
- ●Atoms consist of protons (+1), neutrons (0) in the nucleus, and electrons (-1) in shells.
- ●The atomic number (Z) = number of protons; the mass number (A) = protons + neutrons.
- ●Isotopes are atoms of the same element with different numbers of neutrons.
- ●Electrons fill shells from lowest energy: 2, 8, 8 for the first three shells.
- ●The nucleus contains nearly all the atom's mass but occupies a tiny fraction of its volume.