Over many years, the ideas of chemical structure has changed through observations and the gathering of experimental evidence.
- Atoms consist of a central nucleus surrounded by one or more shells of electrons.
- The nucleus consists of two types of particle. Positively charged protons and neutral neutrons.
- The protons and neutrons are very small. they have a relative mass of 1. The relative mass of an electron is so small it is said to be negligible.
Particle Relative mass Relative charge
Proton 1 +1
Neutron 1 0
Electron 1 -1
Atomic number and mass number
- The number of protons in an atom is its proton number or atomic number.
- The number of nucleons (protons + neutrons) is the mass number.
- Therefore, the number of neutrons is the mass number - proton number.
Sometimes the chemical symbol for an element is written in a way that shows the atomic number by the mass number. An example (Carbon) is given below.
Mass number 12
C
Atomic number 6
^ Carbon has 6 protons and 6 neutrons (i.e. 12 nucleons)
Isotopes
Different elements have different atomic (proton) numbers: no two elements have the same number of protons. however, the number of neutrons in the nucleus is not fixed. Some atoms of the same element have different numbers of neutrons, and so different mass numbers. These different forms of the same element are called isotopes.
14
C
6
^ The carbon-14 isotope has a mass number of 14.
Masses of atoms and molecules
Scientists express the mass of an atom relative to the mass of the carbon atom, Carbon-12. The mass of Carbon-12 is given as 12, its mass number. the mass of another atom is compared with 1/12 of the mass of a Carbon-12 atom. This is called its relative atomic mass, symbol Ar.
When atoms form molecules, the relative mass of the molecule can be calculated by adding together the relative atomic masses of the atoms that make it up. This is called the relative molecular mass, symbol Mr. Relative molecular mass is known as relative formula mass when talking about ionic compounds.
For example, lead sulfate has the formula PbSO4. Is contains 1 atom of lead, 1 atom of sulfur and 4 atoms of oxygen. The Ar of these atoms are:
Lead (Pb) = 207
Sulfur (S) = 32
Oxygen (O)= 16
So the Mr of lead sulfate = 207+320(16 x 4)
= 303
Electronic structure
The electrons in an atom are at different energy levels around the nucleus. the energy levels are sometimes called shells or orbits. Each shell can contain up to a maximum number of electrons:
^ Electronic structure of Nitrogen
The electronic structure is written as the number of electrons in each shell, separated by a comma. For example, the electronic structure of nitrogen is 2,5. The electronic structure of potassium is 2,8,8,1.
Electron configuration is an alternative term for electronic structure.
Electronic structure and the periodic table
The electronic structure of an element from its position in the periodic table.
Masses of atoms and molecules
Scientists express the mass of an atom relative to the mass of the carbon atom, Carbon-12. The mass of Carbon-12 is given as 12, its mass number. the mass of another atom is compared with 1/12 of the mass of a Carbon-12 atom. This is called its relative atomic mass, symbol Ar.
When atoms form molecules, the relative mass of the molecule can be calculated by adding together the relative atomic masses of the atoms that make it up. This is called the relative molecular mass, symbol Mr. Relative molecular mass is known as relative formula mass when talking about ionic compounds.
For example, lead sulfate has the formula PbSO4. Is contains 1 atom of lead, 1 atom of sulfur and 4 atoms of oxygen. The Ar of these atoms are:
Lead (Pb) = 207
Sulfur (S) = 32
Oxygen (O)= 16
So the Mr of lead sulfate = 207+320(16 x 4)
= 303
Electronic structure
The electrons in an atom are at different energy levels around the nucleus. the energy levels are sometimes called shells or orbits. Each shell can contain up to a maximum number of electrons:
- The first shell can contain up to 2 electrons.
- Shells 3 and 4 contain up to 8 electrons.
Nitrogen has an atomic number of 7. From this, we can work out its electronic structure; which is shown in the diagram below. We know the atom has 7 protons and so must have 7 electrons:
^ Electronic structure of Nitrogen
The electronic structure is written as the number of electrons in each shell, separated by a comma. For example, the electronic structure of nitrogen is 2,5. The electronic structure of potassium is 2,8,8,1.
Electron configuration is an alternative term for electronic structure.
Electronic structure and the periodic table
The electronic structure of an element from its position in the periodic table.
- The group number tells you the number of electrons in the outer shell. The exception to this is group 0 (Group 8) which indicated that the outer shell is full (however many electrons it can obtain).
- The period indicates how many shells of electron the atom has.
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