CC3 & 4: Atomic structure and the periodic table

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CC3 & 4: Atomic structure and the periodic table

CC3a-Structure of an atom

John Dalton published his atomic theory that said: All matter is made up of tiny particles called atoms, they are tiny hard spheres that cannot be broken down into anything smaller, the atoms in an element are all identical and they cannot be created or destroyed.

Now, his ideas have changed. Atoms were proved not to be solid spheres and measurements of charge and mass show that they contain smaller, negatively charged particles called electrons.

The subatomic particles are arranged with protons and neutrons that are found in the nucleus, with electrons surrounding them in shells. The nucleus has a positive charge due to the protons and almost the entire mass is centered in the nucleus. However the nucleus is very small compared to the rest of the atom. Electrons are negatively charged but their shells cover a lot of space.

The size of the shells determine the size of the atom. Also, each element has a different atomic mass and proton number. The higher the proton number, the more protons are found, meaning that there are more electrons and therefore more shells.

Charge of a proton: +1. Neutron: 0. Electron:-1. Mass of a proton: 1 amu. Neutron: 1 amu. Electron: (extremely close to) 0 amu.

Atoms have no overall charge as they have the same number of protons as electrons.

CC3b-Atomic mass and number

Most of the mass of an atom is found in the nucleus.

The atomic number (proton number) is the amount of protons that are found in an atom.

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

The number of protons=the number of electrons.

To work out protons you can look at the atomic number or the amount of electrons. To work out neutrons you must minus the proton number from the mass number. To work out electrons you must look at the atomic number.

CC3c-Isotopes and relative atomic mass

Isotopes are different forms of the same element. They have the same amount of protons but different neutrons. They have the same atomic number but different mass numbers.

Because there are isotopes for many of the elements, the mass number is the average for the mass numbers of each one. Therefore it isn't a whole number.

To work out the RAM of an element, you must find the average mass of all its atoms. Different isotopes of an element occur in different quantities, or isotopic abundances. RAM=multiply each RAM by its isotopic abundance and add up the results. Divide by the sum of the abundances.

CC4a-Atomic number and the periodic table

Mendeleev sorted about 50 elements into the periodic table based on their mass number and their similarities.

Because Mendeleev wanted to keep elements with similar properties together, he swapped them and that left gaps. He used the other elements in the column with the gaps to predict the properties of undiscovered elements. When they were found they fit the pattern e.g. he predicted the chemical and physical properties of what he called ekasilicon which is now known as germanium.

CC4b-Atomic number and the periodic table

A few elements seemed to end up in the wrong column because the atomic mass Mendeleev had was wrong due to isotopes.

The atomic number is based on the amount of protons in the nucleus.

Now, the elements are ordered in ascending in atomic number. They fit the same pattern as Mendeleev figured out. They are also laid out so that elements with similar chemical properties are found in columns called groups. The group the element is in corresponds to the amount of electrons in the outer shell. E.g. group 1 have 1 and group 7 have 7. The rows are called periods. Each new period has another full outer shell.

Metals are found of the left of the zig-zag line and non metals are found on the right.

CC4c-Electronic configurations and the periodic table

Electronic configuration is the arrangement of electrons in shells around the nucleus of an atom.

Electronic configuration of hydrogen is just 1. Calcium is 2. 8. 8. 2