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2. Fili in the blanks.

a.    An atom is built of three types of................................................................

b.    A............................carries a positive......................................., while an

electron is............................................

c.    Almost the whole mass of an atom is..........................................in its

d.    The charges of a proton and an...................................are of equal

................................but of different.......................................

e.    The masses of a...........................................and a..........................

are approximately the same.

g. We can imagine an atom as a.................................with a..............

cloud of electrons surrounding a.....................................in the.......

2.2. Atomie number, mass number and relative atomie mass

The atomie number of an atom is defined as the number of protons it contains. Substances madę exclusively of atoms with the same atomie number are called elements. In other words, the atomie number identifies which element is which. The element with atomie number 1 is hydrogen, while the one with atomie number 79 is gold. Since individual atoms are electrically neutral, they contain exactly as many electrons as protons: the hydrogen atom has one electron, the gold atom 79 electrons.

The mass number is the total number of protons and neutrons in an atomie nucleus. The nucleus of hydrogen consists of only one proton. Since there are no neutrons, the mass number of hydrogen is 1. The mass number of gold is 197. This means that apart from 79 protons, the nucleus of the gold atom contains exactly 118 neutrons.

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