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A step-by-step guide to drawing visual representations of atoms for the first 20 elements.
If you enlarged an atom to the size of a football stadium, the nucleus would be like a marble in the center, yet it holds almost all the mass—how do we map such a strange, empty space?
Every atom has a dense center called the nucleus. This is where the 'heavy lifting' happens. Inside, you find protons (), which have a positive charge, and neutrons (), which are neutral. To find the number of protons, look at the Atomic Number on the Periodic Table. To find the neutrons, subtract the Atomic Number from the Atomic Mass (rounded to the nearest whole number). For example, if an element has an Atomic Mass of and an Atomic Number of , it has neutrons. The nucleus is tiny, but it contains of the atom's mass!
Let's map the nucleus of Helium (): 1. Find the Atomic Number: . This means there are protons. 2. Find the Atomic Mass: . Round to . 3. Calculate Neutrons: neutrons. 4. Draw a small circle and write and inside.
Quick Check
If an atom has an Atomic Number of 11 and an Atomic Mass of 23, how many neutrons are in the nucleus?
Answer
12 neutrons ()
Electrons () don't just fly around randomly; they live in energy levels or shells. Think of these like seats in a theater. The 'front row' (the shell closest to the nucleus) is small and can only hold 2 electrons. Once that is full, electrons move to the second shell, which can hold 8 electrons. The third shell also holds 8 electrons for the first 20 elements. This is often called the Octet Rule because 'oct' means eight. Electrons always fill the shells from the inside out!
Lithium () has an Atomic Number of , so it has electrons: 1. Fill the 1st shell: Place electrons. (The shell is now full). 2. Fill the 2nd shell: Place the remaining electron (). 3. Result: Two shells are used; the inner is full, and the outer has one electron.
Quick Check
An atom has 10 electrons. How many shells will be used, and how many electrons will be in the outermost shell?
Answer
2 shells will be used; the second (outermost) shell will have 8 electrons.
The electrons in the very last shell are called valence electrons. These are the most important parts of the atom because they determine how the atom reacts with others. If the outer shell is full (like in Neon or Argon), the atom is 'happy' and stable (a Noble Gas). If it's not full, the atom will try to bond with others to fill it. By counting the protons in the center or the total electrons in a neutral Bohr model, you can identify exactly which element you are looking at on the Periodic Table.
Argon () has an Atomic Number of . Let's map its shells: 1. Shell 1: electrons (Full). 2. Shell 2: electrons (Full). Total so far: . 3. Shell 3: electrons (Full). 4. Because all shells are full, Argon is a stable, non-reactive gas.
What is the maximum number of electrons that can fit in the second energy shell?
If a Bohr model shows 7 protons in the nucleus, which element is it?
Neutrons have a negative charge and are found in the electron shells.
Review Tomorrow
In 24 hours, try to sketch the Bohr model for Oxygen (Atomic Number 8) from memory and check if your shells follow the 2-8-8 rule.
Practice Activity
Pick any element between Atomic Number 11 (Sodium) and 20 (Calcium) and practice calculating the neutrons and drawing the three shells.