Atoms and atomic theory

English in chemistry 1 – 2017 
TS. Nguyễn Tuyết Phương – TS. Trần Thu Phương 
 
 
7 
Lesson
 3. 
ATOMS AND ELECTRON CONFIGURATION
 
3.1 Listening: Drawing electron configuration diagrams 
Complete the paragraph by filling the blanks 
 
You need to be familiar with the structure of an atom before going any further, and you may 
wish to look at the lesson “Parts of an atom, their charge, and their mass” first. 
Electrons are arranged in energy levels or shells around the …(1)… of an atom. The actual 
orbit shapes are a bit too complicated to cover in such a short lesson, but on average, the 
orbital radius increases as the energy levels increase. In our diagrams, the shell nearest the 
nucleus is going to represent the lowest energy level or shell, and we draw a …(2)… to 
depict each shell. We use a dot or a cross to represent each electron, and we represent the 
nucleus by the chemical symbol. Each electron in an atom is in a particular shell, and the 
electrons must occupy the …(3)… available shell nearest the nucleus. So when we are 
drawing the electron configuration, we have to fill up each shell in turn, starting with the 
lowest. 
We’ll take a lithium atom as an example. With an atomic number of three, it must have 
three electrons to …(4)… the positive charge of the three protons in the nucleus. It’s worth 
remembering that the atomic number tells you the total number of electrons in a neutral 
atom. So, we put the first electron into the first shell, and the second. However, this shell 
can only contain a maximum of two electrons. That’s one of the rules that you need to 
remember. The third electron, therefore, must go into the next shell which we draw as a 
larger circle around the outside of the first. And that completes the electron …(5)… of a 
lithium atom. 
The same process of filling up shells apply to larger atoms. You always start with lowest 
available shells, and whenever a shell is full, the next electron to be …(6)… goes to the next 
shell. And that’s pretty much it. All you need to know is how many electrons can occupy 
each shell. And at this level, you only need to know the rules for the first twenty elements. 
The maximum numbers are as follows:
- The first shell can …(7)… a maximum of two electrons 
- The second shell up to eight electrons. 
- The third shell also has a maximum of eight electrons. 
- And the nineteenth and twentieth electrons go into the fourth shell. 

English in chemistry 1 – 2017 
TS. Nguyễn Tuyết Phương – TS. Trần Thu Phương 
 
 
8 
And this would actually represent a calcium …(8)…. Note that as we fill up each level, 
we’ve been drawing the electrons evenly spaced out on each shell. This is to keep the 
diagram tidy and make it easier to count electrons. And what’s more, there’s a short-hand 
way of …(9)… this down: “2.8.8.2”. The first number represents the inner shell or lowest 
energy level. The full stop shows how the electrons are separated between the shells. The 
second number tells us how many electrons are in the next shell, followed by the third and 
fourth shells. And so now it’s your turn. I’d like you to draw the electron configuration of a 
…(10)… atom. Pause the video, find a piece of paper, and have a go. 
And here is what it should look like. It has a full first shell, and four electrons in the outer 
shell. Did you get it right? 
Now, how would you write the electron configuration in numbers for this atom? Pause the 
video again or rewind if you need to have a think about this. 
The answer is that it would be …(11)…: two dot four. And now that you have covered the 
basics of drawing electron configurations, you’ll be ready to tackle our lesson “Electron 
configuration of the first twenty elements”. 

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