化学の基本中の基本、元素記号と分子式を攻略しよう！これを覚えればテストで確実に点が取れるし、身の回りの物質がどんな原子でできているかがわかるようになる。世界共通の記号だから、一度覚えれば一生使える知識だよ。
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Updated May 10, 2026
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化学の基礎：元素記号と分子式の覚え方
1 / 6
物質を表す記号と式の基本
身の回りの全ての物質は、実は原子という超小さい粒でできてる。でも毎回「水素原子が2個と酸素原子が1個...」なんて言ってたらめんどくさいよね。
だから世界中の科学者が同じ元素記号と化学式を使ってる。原子は物質の最小の粒で、元素はその原子の種類のこと。
分子は原子がいくつか結びついた粒子で、分子式は分子の成分を記号で表したもの。例えば水（H₂O）は水素原子2個と酸素原子1個でできてる。
ココが重要！ 1文字目は大文字、2文字目は小文字で書くのが絶対ルール。Co（コバルト）とco（一酸化炭素）は全く違う物質だから注意！
覚えておきたい元素記号
テストでよく出る元素記号をまずは完璧にしよう。特に日本語名と記号が全然違うやつは要注意だよ。
覚えやすいもの： 水素（H）、炭素（C）、窒素（N）、酸素（O）、硫黄（S）、塩素（Cl）
ちょっと難しいもの： ナトリウム（Na）、マグネシウム（Mg）、アルミニウム（Al）、ケイ素（Si）、カルシウム（Ca）
超難しいもの： 鉄（Fe）、銅（Cu）、亜鉛（Zn）、銀（Ag）、金（Au）←これらはラテン語が元だから全然違う！
最初の20番まで覚えとけば高校でも楽勝。語呂合わせや繰り返し書いて体で覚えよう。
テスト必勝法！ 大文字・小文字のミスは即減点。ALやcLは間違い、正しくはAlとClだよ。
分子式の書き方と読み方
分子式の読み方は実は超シンプル。右下の小さい数字は「その元素の原子が何個あるか」を表してる。
水分子（H₂O）を例にすると：H（水素）の右下に2→水素原子2個、O（酸素）の右下に数字なし→酸素原子1個（1は省略される）
アンモニア（NH₃） →窒素原子1個、水素原子3個 二酸化炭素（CO₂） →炭素原子1個、酸素原子2個
3CH₄みたいに前に大きい数字があるときは、その分子が3個あるって意味。だからメタン3分子なら炭素原子3個、水素原子12個になる。
ひっかけ注意！ H₂SO₄では、Hが2個、Sが1個、Oが4個。「Sも2個」じゃないからね！
分子をつくる物質とつくらない物質
全ての物質が分子でできてるわけじゃない。これ、意外と知らない人多いんだよね。
分子からなる物質： 酸素（O₂）、窒素（N₂）、二酸化炭素（CO₂）、水（H₂O）、アンモニア（NH₃）など。決まった数の原子がセットになってる。
分子をつくらない物質： 鉄（Fe）、銅（Cu）、銀（Ag）などの金属は原子がバラバラに集まってる。だから化学式は元素記号そのまま。
酸素や水素などの気体は、自然界では原子2個がペアになってる（O₂、H₂）。これ覚えとくとテストで役立つ。
覚えるコツ！ 気体は基本的に分子、金属は基本的に原子だけと考えるとわかりやすい。
例題で理解を深めよう
実際の問題を解いて理解を確認してみよう。パターンを覚えればテストも楽勝だよ。
例題1： アンモニア分子は窒素原子1個と水素原子3個でできてる。分子式は？ →答え：NH₃（窒素1個は数字省略、水素3個だからHに₃）
例題2： CO₂の分子1個にはどんな原子が何個？ →答え：炭素原子1個、酸素原子2個（Cの右下数字なし→1個、O₂→2個）
例題3： メタン（CH₄）が3分子あるとき、各原子は合計何個？ →答え：炭素3個、水素12個（1分子につきC1個H4個、それが3倍）
慣れれば簡単！最初は丁寧に分解して考えよう。
テスト攻略法！ 「原子の数を答えよ」と「元素の種類を答えよ」は全然違う問題。問題文をしっかり読もう。
テスト対策とまとめ
元素記号と分子式は化学の入り口。ここでつまずくと後が大変だから、今のうちに完璧にしよう。
テスト必勝ポイント：
- 主要な元素記号は暗記必須（特にNa、Fe、Cu、Ag、Au）
- 大文字・小文字は絶対間違えない
- 右下の数字は直前の元素にだけかかる
- 「原子」と「元素」の違いを理解する
実生活でも役立つ知識： 食品の成分表示や薬の説明書にも化学式が出てくる。今覚えとけば将来絶対役に立つよ。
分子式の読み書きは練習あるのみ。毎日少しずつでも触れてれば、自然に身につく。
最終チェック！ H₂SO₄、NaCl、CO₂、NH₃...これらがスラスラ読めるようになったら完璧だ！
We thought you’d never ask...
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Where can I download the Knowunity app?
You can download the app from Google Play Store and Apple App Store.
Is Knowunity really free of charge?
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The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
化学の基礎：元素記号と分子式の覚え方
化学の基本中の基本、元素記号と分子式を攻略しよう！これを覚えればテストで確実に点が取れるし、身の回りの物質がどんな原子でできているかがわかるようになる。世界共通の記号だから、一度覚えれば一生使える知識だよ。
物質を表す記号と式の基本
身の回りの全ての物質は、実は原子という超小さい粒でできてる。でも毎回「水素原子が2個と酸素原子が1個...」なんて言ってたらめんどくさいよね。
だから世界中の科学者が同じ元素記号と化学式を使ってる。原子は物質の最小の粒で、元素はその原子の種類のこと。
分子は原子がいくつか結びついた粒子で、分子式は分子の成分を記号で表したもの。例えば水（H₂O）は水素原子2個と酸素原子1個でできてる。
ココが重要！ 1文字目は大文字、2文字目は小文字で書くのが絶対ルール。Co（コバルト）とco（一酸化炭素）は全く違う物質だから注意！
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覚えておきたい元素記号
テストでよく出る元素記号をまずは完璧にしよう。特に日本語名と記号が全然違うやつは要注意だよ。
覚えやすいもの： 水素（H）、炭素（C）、窒素（N）、酸素（O）、硫黄（S）、塩素（Cl）
ちょっと難しいもの： ナトリウム（Na）、マグネシウム（Mg）、アルミニウム（Al）、ケイ素（Si）、カルシウム（Ca）
超難しいもの： 鉄（Fe）、銅（Cu）、亜鉛（Zn）、銀（Ag）、金（Au）←これらはラテン語が元だから全然違う！
最初の20番まで覚えとけば高校でも楽勝。語呂合わせや繰り返し書いて体で覚えよう。
テスト必勝法！ 大文字・小文字のミスは即減点。ALやcLは間違い、正しくはAlとClだよ。
Sign up to see the contentIt's free!
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分子式の書き方と読み方
分子式の読み方は実は超シンプル。右下の小さい数字は「その元素の原子が何個あるか」を表してる。
水分子（H₂O）を例にすると：H（水素）の右下に2→水素原子2個、O（酸素）の右下に数字なし→酸素原子1個（1は省略される）
アンモニア（NH₃） →窒素原子1個、水素原子3個 二酸化炭素（CO₂） →炭素原子1個、酸素原子2個
3CH₄みたいに前に大きい数字があるときは、その分子が3個あるって意味。だからメタン3分子なら炭素原子3個、水素原子12個になる。
ひっかけ注意！ H₂SO₄では、Hが2個、Sが1個、Oが4個。「Sも2個」じゃないからね！
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分子をつくる物質とつくらない物質
全ての物質が分子でできてるわけじゃない。これ、意外と知らない人多いんだよね。
分子からなる物質： 酸素（O₂）、窒素（N₂）、二酸化炭素（CO₂）、水（H₂O）、アンモニア（NH₃）など。決まった数の原子がセットになってる。
分子をつくらない物質： 鉄（Fe）、銅（Cu）、銀（Ag）などの金属は原子がバラバラに集まってる。だから化学式は元素記号そのまま。
酸素や水素などの気体は、自然界では原子2個がペアになってる（O₂、H₂）。これ覚えとくとテストで役立つ。
覚えるコツ！ 気体は基本的に分子、金属は基本的に原子だけと考えるとわかりやすい。
Sign up to see the contentIt's free!
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Improve your grades
Join milions of students
例題で理解を深めよう
実際の問題を解いて理解を確認してみよう。パターンを覚えればテストも楽勝だよ。
例題1： アンモニア分子は窒素原子1個と水素原子3個でできてる。分子式は？ →答え：NH₃（窒素1個は数字省略、水素3個だからHに₃）
例題2： CO₂の分子1個にはどんな原子が何個？ →答え：炭素原子1個、酸素原子2個（Cの右下数字なし→1個、O₂→2個）
例題3： メタン（CH₄）が3分子あるとき、各原子は合計何個？ →答え：炭素3個、水素12個（1分子につきC1個H4個、それが3倍）
慣れれば簡単！最初は丁寧に分解して考えよう。
テスト攻略法！ 「原子の数を答えよ」と「元素の種類を答えよ」は全然違う問題。問題文をしっかり読もう。
Sign up to see the contentIt's free!
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Join milions of students
テスト対策とまとめ
元素記号と分子式は化学の入り口。ここでつまずくと後が大変だから、今のうちに完璧にしよう。
テスト必勝ポイント：
- 主要な元素記号は暗記必須（特にNa、Fe、Cu、Ag、Au）
- 大文字・小文字は絶対間違えない
- 右下の数字は直前の元素にだけかかる
- 「原子」と「元素」の違いを理解する
実生活でも役立つ知識： 食品の成分表示や薬の説明書にも化学式が出てくる。今覚えとけば将来絶対役に立つよ。
分子式の読み書きは練習あるのみ。毎日少しずつでも触れてれば、自然に身につく。
最終チェック！ H₂SO₄、NaCl、CO₂、NH₃...これらがスラスラ読めるようになったら完璧だ！
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
Where can I download the Knowunity app?
You can download the app from Google Play Store and Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
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Most popular content in 化学
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Students love us — and so will you.
4.6/5
App Store
4.7/5
Google Play
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user