Nucleus (positively charged)

Electron (an electron carries one unit of negative charge)

Pure substances composed of the same element

A pure substance composed of two or more elements is a compound

In order to facilitate communication, it is internationally stipulated that each element is expressed by a unified symbol. This symbol is called an element symbol.

Explain macroscopically

explain microscopically

1. In a compound, calculate the valence of the specified element based on the sum of the positive and negative valence algebras being zero.

2. Clearly mark the valence of the element directly above the element, always with a positive or negative sign.

3. When oxygen appears, the oxygen element is written next.

4. In compounds composed of metals and non-metals, when writing chemical formulas, the element symbols of the metal elements are written in front and the element symbols of non-metal elements are written in the back.