First 12 Elements Study Guide

Summary: A complete study guide for the first 12 elements of the periodic table (Hydrogen through Magnesium), covering atomic structure, electron configuration, classification, and properties. Tags: elements chemistry periodic-table quiz Created: 2026-08-03 Last Updated: 2026-08-03

First 12 Elements Study Guide

This guide covers everything you need to know about the first 12 elements (H to Mg) — the same material tested in the ⚛️ First 12 Elements Quiz.

Quick Reference Table

Element-by-Element Details

1. Hydrogen (H)

• Atomic number: 1
• Mass number: 1
• Protons: 1 | Electrons: 1 | Neutrons: 0
• Electron configuration: 1
• Group 1, Period 1
• Classification: Non-metal
• State at room temperature: Gas
• Key fact: Hydrogen is the simplest and most abundant element in the universe. It is the only element in Period 1 with no neutrons in its most common isotope.

2. Helium (He)

• Atomic number: 2
• Mass number: 4
• Protons: 2 | Electrons: 2 | Neutrons: 2
• Electron configuration: 2
• Group 0, Period 1
• Classification: Noble gas
• State at room temperature: Gas
• Key fact: Helium is a noble gas with a full outer shell (2 electrons). It is inert and does not react with other elements.

3. Lithium (Li)

• Atomic number: 3
• Mass number: 7
• Protons: 3 | Electrons: 3 | Neutrons: 4
• Electron configuration: 2,1
• Group 1, Period 2
• Classification: Metal (Alkali metal)
• State at room temperature: Solid
• Key fact: Lithium is the lightest metal and the first element in Period 2. It has 1 electron in its outer shell, making it highly reactive.

4. Beryllium (Be)

• Atomic number: 4
• Mass number: 9
• Protons: 4 | Electrons: 4 | Neutrons: 5
• Electron configuration: 2,2
• Group 2, Period 2
• Classification: Metal (Alkaline earth metal)
• State at room temperature: Solid
• Key fact: Beryllium has 2 electrons in its outer shell. It is a lightweight, strong metal used in aerospace applications.

5. Boron (B)

• Atomic number: 5
• Mass number: 11
• Protons: 5 | Electrons: 5 | Neutrons: 6
• Electron configuration: 2,3
• Group 3, Period 2
• Classification: Metalloid
• State at room temperature: Solid
• Key fact: Boron is the only metalloid in the first 12 elements. It sits on the staircase boundary between metals and non-metals.

6. Carbon (C)

• Atomic number: 6
• Mass number: 12
• Protons: 6 | Electrons: 6 | Neutrons: 6
• Electron configuration: 2,4
• Group 4, Period 2
• Classification: Non-metal
• State at room temperature: Solid
• Key fact: Carbon is the basis of organic chemistry and all known life. It has 4 outer electrons, allowing it to form 4 covalent bonds.

7. Nitrogen (N)

• Atomic number: 7
• Mass number: 14
• Protons: 7 | Electrons: 7 | Neutrons: 7
• Electron configuration: 2,5
• Group 5, Period 2
• Classification: Non-metal
• State at room temperature: Gas
• Key fact: Nitrogen makes up about 78% of Earth’s atmosphere. It exists as N₂ molecules with a triple covalent bond.

8. Oxygen (O)

• Atomic number: 8
• Mass number: 16
• Protons: 8 | Electrons: 8 | Neutrons: 8
• Electron configuration: 2,6
• Group 6, Period 2
• Classification: Non-metal
• State at room temperature: Gas
• Key fact: Oxygen makes up about 21% of Earth’s atmosphere. It is essential for respiration and combustion.

9. Fluorine (F)

• Atomic number: 9
• Mass number: 19
• Protons: 9 | Electrons: 9 | Neutrons: 10
• Electron configuration: 2,7
• Group 7, Period 2
• Classification: Non-metal (Halogen)
• State at room temperature: Gas
• Key fact: Fluorine is the most reactive non-metal. It is a halogen with 7 outer electrons — just 1 short of a full shell.

10. Neon (Ne)

• Atomic number: 10
• Mass number: 20
• Protons: 10 | Electrons: 10 | Neutrons: 10
• Electron configuration: 2,8
• Group 0, Period 2
• Classification: Noble gas
• State at room temperature: Gas
• Key fact: Neon has a full outer shell (2,8) and is chemically inert. It is used in neon signs due to its reddish-orange glow when electrically excited.

11. Sodium (Na)

• Atomic number: 11
• Mass number: 23
• Protons: 11 | Electrons: 11 | Neutrons: 12
• Electron configuration: 2,8,1
• Group 1, Period 3
• Classification: Metal (Alkali metal)
• State at room temperature: Solid
• Key fact: Sodium is the first element in Period 3. Like Lithium, it has 1 outer electron and is highly reactive — it reacts vigorously with water.

12. Magnesium (Mg)

• Atomic number: 12
• Mass number: 24
• Protons: 12 | Electrons: 12 | Neutrons: 12
• Electron configuration: 2,8,2
• Group 2, Period 3
• Classification: Metal (Alkaline earth metal)
• State at room temperature: Solid
• Key fact: Magnesium burns with a bright white flame and is used in flares and fireworks. It has 2 outer electrons.

Study Tips

1. Learn the order: H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg — atomic numbers 1 through 12.
2. Periods: Period 1 has 2 elements (H, He). Period 2 has 8 elements (Li through Ne). Period 3 starts with Na and Mg.
3. Groups: Elements in the same group have the same number of outer electrons.
   • Group 0 (noble gases): He, Ne — full outer shells, inert
   • Group 1 (alkali metals): H, Li, Na — 1 outer electron, very reactive
   • Group 2 (alkaline earth): Be, Mg — 2 outer electrons
   • Group 7 (halogens): F — 7 outer electrons, very reactive non-metals
4. The staircase: Boron (B) is the only metalloid among the first 12 — it separates metals (left) from non-metals (right).
5. Gases: H, He, N, O, F, Ne are gases at room temperature. The rest are solids.
6. Electron shells: Period 1 = 1 shell (max 2 e⁻), Period 2 = 2 shells (2,1-8), Period 3 = 3 shells (2,8,1-2).

Practice

Test your knowledge with the interactive quiz: 👉 ⚛️ First 12 Elements Quiz