Atomic structure
YOU SHOULD BE ABLE TO:
■ Describe the structure of atoms.
■ State the properties of electrons, protons and neutrons.
■ Define atomic number, mass number and relative atomic mass.
■ Describe the structure of the first 20 atoms.
■ Show the electronic arrangement of an element, given its atomic number.
QUICK POINTS
Matter consists of particles which may be atoms, molecules or ions.
An atom is the smallest part of an element.
An atom is the smallest part into which an element can be divided and it consists of three main particles: protons, neutrons and electrons.
An atom has two main sections: a nucleus, which is the centre and holds the protons and neutrons, and shells or energy levels, which contain the electrons.
The protons (P) give the nucleus a positive charge, while the electrons are moving in energy levels around the nucleus with a negative charge. Neutrons have no charge.
The overall charge on an atom is zero (-1 + 1 = 0), hence atoms are uncharged.
In an atom, the number of protons in the nucleus is called the atomic number and this is equal to the number of electrons in the shells. This is why the charges cancel out. For example, 612 C has 6 protons, 6 neutrons and 6 electrons.
The mass of an electron is 1/1840 of a proton and is considered negligible. This is why the mass of an atom is derived from the number of protons and neutrons. Mass number = Protons + Neutrons.
QUICK POINT
Remember the masses given in the table are not the actual masses but the relative mass when all three are compared.
Electrons are arranged in energy levels around the nucleus. These electrons are not stationary, but continue to move in different energy levels. The arrangement of electrons in the shells or energy levels is governed by certain rules or guidelines.
Each shell or energy level can hold a fixed number of electrons. When a shell is filled, electrons will enter the next shell with the lowest energy. Electrons start to fill up the shells of lowest energy first; that is the closest one to the nucleus.
The first shell (K shell) can hold a maximum of two electrons.
The next shell (L shell) can hold a maximum of eight electrons.
The third shell (M shell) holds eight electrons for the first 20 elements but can hold up to 18 electrons.
Elements are arranged in the periodic table in order of increasing atomic (proton) number and in groups based on the number of electrons in the outer shell. Elements with filled shells of two or eight electrons are in group 8 or O.
The arrangement of elements in groups influences reactivity and the types of reactions they will participate in.
QUICK CHECK
Why do electrons start filling from the first shell in an atom?
QUICK POINTS
It is the atomic number (protons) which determines the type of atom.
Atoms are arranged on the periodic table based on their atomic number.
The structures of the first 20 elements are shown below. The electrons are arranged using the guidelines for the maximum number of electrons in the shells (energy levels).
FURTHER PRACTICE
Element X has a proton number of 14. Determine the electronic configuration of X and its position in the periodic table. What is the identity of element X?
Use evidence from the table to support the following statements. i. Particle A is an atom. ii. Particles A and C are isotopes. iii. Particle B is an ion.