What is element and compound definition?

Any substance that contains only one kind of an atom is known as an element. Because atoms cannot be created or destroyed in a chemical reaction, elements such as phosphorus (P4) or sulfur (S8) cannot be broken down into simpler substances by these reactions. Example: Water decomposes into a mixture of hydrogen and oxygen when an electric current is passed through the liquid. Hydrogen and oxygen, on the other hand, cannot be decomposed into simpler substances. They are therefore the elementary, or simplest, chemical substances - elements. Each element is represented by a unique symbol. The notation for each element can be found on the periodic table of elements. The elements can be divided into three categories that have characteristic properties: metals, nonmetals, and semimetals. Most elements are metals, which are found on the left and toward the bottom of the periodic table. A handful of nonmetals are clustered in the upper right corner of the periodic table. The semimetals can be found along the dividing line between the metals and the nonmetals. Atoms Elements are made up of atoms, the smallest particle that has any of the properties of the element.John Dalton, in 1803, proposed a modern theory of the atom based on the following assumptions. 1. Matter is made up of atoms that are indivisible and indestructible. 2. All atoms of an element are identical. 3. Atoms of different elements have different weights and different chemical properties. 4. Atoms of different elements combine in simple whole numbers to form compounds. 5. Atoms cannot be created or destroyed. When a compound decomposes, the atoms are recovered unchanged Compounds Elements combine to form chemical compounds that are often divided into two categories. Metals often react with nonmetals to form ionic compounds. These compounds are composed of positive and negative ions formed by adding or subtracting electrons from neutral atoms and molecules. Nonmetals combine with each other to form covalent compounds, which exist as neutral molecules. The shorthand notation for a compound describes the number of atoms of each element, which is indicated by a subscript written after the symbol for the element. By convention, no subscript is written when a molecule contains only one atom of an element. Thus, water is H2O and carbon dioxide is CO2. read less