chemical formula, chemical equation, proton, neutron, element, electron, atomic nucleus, molecule, cation, anion, chemical compound, chemical reaction, chemical bonds, ion, molecule, atomic number
An atomis a collection of matterconsisting ofa positively charged core(the ________________) which contains ____________and ____________ and which maintainsa number of electrons to balancethe positive charge in the nucleus. The atom is also the smallest portioninto which an ____________can be divided and still retainits properties, made up of a dense, positively charged nucleus surrounded by a system of ____________.
The most basic chemical substancesare the chemical elements. They are building blocks of all other substances. An element is a class of atoms which have the same number of protons in the nucleus. This number is known as the ___________ ____________of the element. For example, all atoms with 6 protons in their nuclei are atoms of the chemical element carbon, and all atoms with 92 protons in their nuclei are atoms of the element uranium. Each chemical element is made up of only one kind of atom. The atoms of one element differfrom those of all other elements. Chemists use letters of the alphabet as symbols for the elements. In total, 117 elements have been observed as of 2007, of which 94 occur naturally on Earth. Others have been produced artificially.
An ____________is an atom or a moleculethat has lost or gainedone or more electrons. Positively charged ____________(e.g. sodiumcation Na+) and negatively charged ___________(e.g. chlorideCl−) can form neutral salts(e.g. sodium chlorideNaCl). Electrical forcesat the atomic level create _____________ __________that join two or more atoms together, forming ____________. Some molecules consist of atoms of a single element.
Oxygenmolecules, for example, are made up of two oxygen atoms. Chemists represent the oxygen molecule O2. The 2 indicates the number of atoms in the molecule.
When atoms of two or more different elements bond together, they form a ____________________. Water is a compound made up of two hydrogenatoms and one oxygen atom. The __________ _________for a water molecule is H2O.
Compounds are formed or broken down by means of ____________ __________. All chemical reactions involvethe formationor destructionof chemical bonds. Chemists use ___________ ___________to express what occursin chemical reactions. Chemical equations consist of chemical formulas and symbols that show the substances involved inchemical change.
For example, the equation C + O2 = CO2 expresses the chemical change that occurs when one carbonatom reacts, or bonds, with an oxygen molecule. The reaction produces one molecule of carbon dioxide, which has the formula CO2.
Read the text again paying attention to the words in bold. Practice their pronunciation.
Match the words from the box with the pictures.
| hydrogen atom; atom; carbon dioxide molecule; ions; oxygen molecule; neutral salt
a__________ b__________ c__________
d__________ e___________ f__________