Consider an atom that has an atomic number of 8 and a mass number of 17 If it has a -2 charge it contains electrons.

Classified in Chemistry

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ATOMS

the nucleus which is at the middle of the atom- contains protons and neutrons, has positive charge due to protons, almost the whole mass of the atom concentrated here but it´s tiny compared to the rest pf the atom.

The electrons move around the nucleus in enery levels called shells- they are negatively charged, they´re tiny but they cover a lot of space, the size of their orbit determines how big the atom is and they have virtually no mass.
      PROTONS ARE HEAVY AND POSITIVELY CHARGED 
                  NEUTRONS ARE HEAVY AND NEUTRAL
ELECTRONS ARE TINY AND NEGATIVELY CHARGED 

atomic number tells you how many protons there are (bottom number)

Atoms of the same element all have the same number of protons- so atoms of different elements will have different numbers of protons. Number of neutrons=mass number-atomic number.
Molecules are groups of atoms H2,N2 (molecules made from one element)
Molecules are held together by covalent bonds.

isotopes are different atomic forms of the same element, which have the same number of protons but different numbers of neutrons.

Isotopes have: same atomic number BUT different mass numbers (carbon-12 and carbon-14) 
Elements consist of one type of atom only. Compound is a substance that is made of tow or more elements chemically joined (carbon dioxide). Very difficult to separate the two original elements again. Mixtures can be separated by physical methods 


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