עמוד:16

Atomic number What makes the atoms of different elements different from one another ? For example , what distinguishes a hydrogen atom from a gold atom ? Today we know that what distinguishes one element from another is the number of protons in its nucleus . For example , in a hydrogen atom there is one proton , whereas in a gold atom there are many more – it consists of 79 protons ! The number of protons in the nucleus of every atom of each and every element is called the element’s atomic number * . In a neutral atom ( one without an electric charge ) the number of protons does not equal the number of electrons . The positive electric charge of the protons balances the negative electric charge of the electrons , and therefore the total electric charge equals 0 . Ions – atoms with an electrical charge When atoms combine with one another and form new matter , electrons sometimes move from one atom to another , changing the number of electrons in the atom . Now the number of electrons in the atom does not equal the number of protons . In such a case , the positive electrical charges ( originating from the protons ) and the negative electrical charges ( originating from the electrons ) do not balance each other , and the atom becomes electrically charged – positive or negative . An atom with an electrical charge is called an ion * . In a negative ion , the number of electrons is greater than the number of protons , and in a positive ion , the number of electrons is less than the number of protons ; Atomic number – the number of protons in an atom ( identical to the number of electrons , in a neutral atom ) . Ion – an atom with an electrical charge . The number of protons in an ion does not equal the number of electrons . Toothpaste contain fluoride ions . Gold

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