Chemistry 2A/B Topic 1 Atomic Structure and the Periodic Table

Contains cards about atoms and periodic table

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NUCLEUS: in the middle of the atom and contains positively charged protons and neutrally charged neutrons. This gives the nucleus an overall positive charge. Almost the whole mass of the atom is concentrated in the nucleus but it's tiny compared to the whole atom.

ELECTRONS: move around the atom and have a negative charge. They have virtually no mass and occupy shells.

Number of protons = number of electrons. The positive protons cancel out the negative electrons giving the atom a neutral charge.

PROTON...relative mass = 1...relative charge = +1

NEUTRON...relative mass = 1...relative charge = 0

ELECTRON...relative mass = negligible...relative charge = -1

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Electron Shells

Electrons occupy shells (sometimes called energy levels). Lowest energy levels are always filled first - they are closest to the nucleus.

1st shell: 2   2nd shell: 8   3rd shell: 8

Atoms want full outer shells (like the noble gases) so they are more reactive if their outer shell is not yet full, this is why atoms react.

NITROGEN: proton number of 7

so has 7 protons and 7 electrons

1st shell must have 2

5 left to go in 2nd shell

electronic configuration is 2.5

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Elements consist of one type of atom only

Each element has a different number of protons (atomic number)

Mass number = tells you the total number of protons and neutrons in the atom

Atomic number = tells you the number of protons in the atoms (and electrons)

Number of neutrons = mass number - atomic number

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Isotopes and Relative Atomic Mass

Isotopes are the same except for an extra neutron or two

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

Relative atomic mass takes isotopes and their relative abundance into account. Relative abundance means how much there is of each isotope compared to the total amount of the element in the world. This can be a ratio, fraction or percentage.

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Brief History of the Periodic Table

Elements are in order of their atomic mass, and when they were put in order scientists began to notice patterns.

Dmitri Mandeleev left gaps in the table and predicted the properties of undiscovered elements. Since then new elements have been found which fit into the gaps, including a whole group called the noble gases. They were not found before because they don't react with much at all and are colourless gases.


Rows = periods. Period number of each element is the number of electron shells.

Colums = groups. Elements in the same group have similar properties. This is because they have the same number of electrons in their outer shell. Group number is the same as the number of electrons in the outer shell.

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