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- 1Science - Chemistry Atoms contain three sub-atomic particles called protons, neutrons and electrons. The protons and neutrons are found in the nucleus at the centre of the atom. The nucleus is very much smaller than the atom as a whole. The electrons are arranged in shells around the nucleus.The number of electrons in an atom is always the same as the number of protons, so atoms are electrically neutral overall.
Atoms can lose or gain electrons. When they do, they form charged particles called ions:
if an atom loses one or more electrons, it becomes a positively charged ion
if an atom gains one or more electrons, it becomes a negatively charged ion - 2The number of protons in the nucleus of an atom is called its atomic number: the atoms of a particular element all have the same number of protons the atoms of different elements have different numbers of protons The total number of protons and neutrons in an atom is called its mass number.Different atoms have different masses. Atoms have such a small mass it is more convenient to know their masses compared to each other. Carbon is taken as the standard atom and has a relative atomic mass (Ar) of 12.
- 3The relative atomic mass of an element is a weighted average of the masses of the atoms of the isotopes - because if there is much more of one isotope then that will influence the average mass much more than the less abundant isotope will.The periodic table has eight main groups. For example, Group 1 contains very reactive metals such as sodium (Na), while Group 7 contains very reactive non-metals such as chlorine (Cl).
There are no compounds in the periodic table, because these consist of two or more different elements joined together by chemical bonds. - 4Electrons are arranged in different shells around the nucleus. Each successive shell can only hold a certain number of electrons. The innermost shell is filled first. This shell can contain a maximum of two electrons. The second shell can hold a maximum of eight electrons. When this is filled, electrons go into the third shell, which also holds a maximum of eight electrons. Then the fourth shell begins to fill.The electronic configuration of an atom is a description of how the electrons are arranged. It can be shown as numbers or as a diagram.
- 5Electronic Configuration diagramA diagram of what an atom contains, excluding the protons
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