Atoms

Question 1

Structure of an atom

Answer 1

Protons, neutrons and electrons

Atoms are very small they have a radius of around 1 x 10-10 metres.

The modern view of the atom is of a positively-charged nucleus containing protons and neutrons with smaller electrons orbiting outside the nucleus

Question 2

Mass number and atomic number

Answer 2

Protons and neutrons are the heaviest particles in an atom as a result they make up most of the mass of the atom. The mass of electrons is often not considered to be significant.

The number of protons is what defines the element, ie an atom with six protons in its nucleus will always be carbon, and uranium will always have 92 protons.

The total number of protons and neutrons is called the mass number and the number of protons is called the atomic number.

In a neutral atom, the number of electrons is always the same as the number of protons. If the atom becomes ionised however the number of electrons will change. An ion is an atom that has lost or gained one or more electrons.

Question 3

Atoms and isotopes

Answer 3

An elements atomic number defines it. An element with 17 protons will always be chlorine.

However an elements mass numbers can vary which means that it can have different numbers of neutrons. So although chlorine usually has a mass number of 35 which means it has 18 neutrons it can also have a mass number of 37 which means it has 20 neutrons. The different types of chlorine are called isotopes.

Question 4

Ions and excited atoms

Answer 4

Normally, atoms are neutral. They have the same number of protons in the nucleus as they have electrons orbiting around the nucleus.

Atoms can lose or gain electrons due to collisions or other interactions. When they do form charged particles called ions:

If the atom loses one or more electrons it becomes a positively charged ion

If the atom gains one or more electrons it becomes a negatively charged ion