Note: In the energy level diagrams, the electrons are spread out evenly in the level. Some books show them spread out this way and some show them in pairs. The pairing of electrons is meant to represent that pairs of electrons are in separate orbitals within each energy level. At the middle school level, it is not necessary for students to learn about electron orbitals. This information is offered so that it is clearer to you why electrons are often shown in pairs in energy level diagrams. An orbital defines a region within an energy level where there is a high probability of finding a pair of electrons. There can be a maximum of two electrons in each orbital. This is why the electrons are often shown in pairs within an energy level.

Tell students that the rows across on the periodic table are called periods.

You are viewing: Which Element Contains A Full Second Energy Level

Period 1

• Hydrogen Explain that hydrogen has 1 proton and 1 electron. The 1 electron is on the first energy level.
• Helium Explain that helium has 2 protons and 2 electrons. The 2 electrons are on the first energy level.

Period 2

• Lithium Explain that lithium has 3 protons and 3 electrons. There are 2 electrons on the first energy level and 1 electron on the second. Explain that the first energy level can only have 2 electrons so the next electron in lithium is on the next (second) level.
• Neon Explain that neon has 10 protons and 10 electrons. There are 2 electrons on the first energy level and 8 electrons on the second level.
• Beryllium-fluorine Help students fill in the correct number of electrons in the energy levels for the rest of the atoms in period 2.

Period 3

• Sodium Explain that sodium has 11 protons and 11 electrons. There are 2 electrons on the first energy level, 8 electrons on the second level, and 1 electron on the third energy level. Explain that the second energy level can only have 8 electrons so the next electron in sodium must be on the next (third) level.
• Argon Explain that argon has 18 protons and 18 electrons. There are 2 electrons on the first energy level, 8 electrons on the second level, and 8 electrons on the third energy lev- el. Have students complete the energy level model for argon in their periodic table.
• Magnesium-chlorine Help students fill in the correct number of electrons in the energy levels for the rest of the atoms in period 3.

Period 4

• Potassium Explain that potassium has 19 protons and 19 electrons. There are 2 electrons on the first energy level, 8 electrons on the second level, 8 electrons on the third energy level, and 1 on the fourth energy level. Explain that after the third energy level has 8 electrons, the next electron goes into the fourth level.
• Calcium Help students fill in the correct number of electrons in the energy levels for calcium.

Note: Students may wonder why an energy level can hold only a certain number of electrons. The answer to this is far beyond the scope of a middle school chemistry unit. It involves thinking of electrons as 3-dimensional waves and how they would interact with each other and the nucleus.

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Category: WHICH