Atoms and Molecules

The atom is made up of three types of subatomic particles, namely protons, neutrons and electrons.

• Nucleus — the small central region containing protons and neutrons
• Proton (+) — positively charged particle in the nucleus
• Neutron — neutral particle in the nucleus
• Electron (−) — negatively charged particle moving around the nucleus
• Electron shells — orbits of electrons surrounding the nucleus
• Valence shell — the outermost electron shell of an atom
• Valence electrons — the electrons in the valence shell

In the diagram of a neon atom, protons and neutrons are found in a small central region known as the nucleus. The protons and neutrons are collectively known as nucleons.

• The nucleus is surrounded by a "cloud" of electrons, which are in a constant state of motion at nearly the speed of light.
• The electrons are arranged in orbits, also known as electron shells.
• The outermost electron shell of an atom is known as its valence shell. The electrons in the valence shell of an atom are known as valence electrons.

• As protons are positively charged while neutrons are electrically neutral, the nucleus has a net positive charge.
• The electrostatic forces of attraction between the positively charged nucleus and the negatively charged electrons hold the atom together.
• There is an equal number of positively charged protons and negatively charged electrons in an atom. As the number of positive charges and negative charges are the same, the atom is electrically neutral.

In the modern Periodic Table, elements are arranged in order of increasing proton (atomic) number. Each element has its own unique proton (atomic) number.

• The horizontal rows in the Periodic Table are called periods. The period number indicates the number of electron shells in an atom. For example, a neon atom has two electron shells and hence, neon is found in Period 2.
• The vertical columns in the Periodic Table are called groups. There are 18 groups, determined based on the number of valence electrons in an atom. For example, a neon atom has eight valence electrons and hence, neon is found in Group 18.

The zigzag line in the Periodic Table divides the table into metals and non-metals.

• Elements found on the left of the zigzag line are metals.
• Elements found on the right of the zigzag line are non-metals.
• Elements close to the zigzag line are called metalloids — they have properties of both metals and non-metals. The metalloids are boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te) and astatine (At).

Determine the number of each type of subatomic particle in an atom of oxygen and potassium (use the Periodic Table).

Oxygen: The proton (atomic) number is 8, so an oxygen atom has 8 protons. Since the atom is neutral, it also has 8 electrons. The relative atomic mass is ≈ 16, so the number of neutrons = 16 − 8 = 8 neutrons.

Potassium: The proton (atomic) number is 19, so a potassium atom has 19 protons and 19 electrons. The relative atomic mass is ≈ 39, so the number of neutrons = 39 − 19 = 20 neutrons.

• Electron shells are numbered outwards in an increasing order. The first electron shell is the closest to the nucleus.
• For the atoms of the first 20 elements in the Periodic Table, the first electron shell can hold up to two electrons, and the second and third electron shells can hold up to eight electrons each.
• If the valence electron shell is full, the atom has achieved a stable duplet or octet electronic configuration.

The electronic configuration of an atom lists the number of electrons in each electron shell of the atom. The format for writing an electronic configuration is shown below:

The electronic configuration of an atom of an element can be determined from the proton (atomic) number of the element. Some examples are shown in the table below.

An oxygen atom has 8 electrons: two in the first electron shell and six in the second (valence) electron shell.

The position of an element in the Periodic Table can be determined from the electronic configuration of its atom. For example, the nitrogen atom has an electronic structure of 2.5. This shows that the nitrogen atom has two electron shells and five valence electrons. Hence, nitrogen is found in Period 2 and Group 15 of the Periodic Table.

An isotope is specified either by writing the name of the element followed by a hyphen and the mass number, or using the nuclide notation. For example, carbon has three common isotopes as shown in the table below.

• Since carbon-12 is the most stable isotope out of the three, 98.9% of carbon found naturally exists as carbon-12.
• Since the proton number is unique to each element, isotopes of the same element have the same number of protons.
• Isotopes of the same element have the same number of electrons, hence they will gain or lose the same number of valence electrons during a chemical reaction, resulting in identical chemical properties. Isotopes of the same element will undergo chemical reactions to form identical compounds — for example, all isotopes of carbon undergo combustion to form carbon dioxide with chemical formula CO₂.
• However, having a different number of neutrons means that the isotopes have slightly different melting point, boiling point and density. For example, hydrogen-2 has a slightly higher boiling point and density compared to hydrogen-1.

An atom is the smallest particle of an element that has the properties of the element. Elements that are made up of single atoms are known as monoatomic elements. Noble gases, which are found in the last column (Group 18) of the Periodic Table, are monoatomic elements.

Most non-metals are made up of molecules. For example, chlorine exists as chlorine molecules, whereby each molecule is made up of the same type of atoms — two chlorine atoms — chemically combined together.

The chlorine molecule can be represented using the chemical formula, Cl₂:

• Cl — chemical symbol of chlorine
• 2 (subscript) — indicating the number of Cl atoms in the molecule

Chlorine molecules are described as diatomic as each chlorine molecule is made up of two chlorine atoms that are chemically combined together. Elements that exist as diatomic molecules are shown below:

Compounds containing only non-metals are made up of molecules. For example, carbon dioxide is a compound that exists as molecules. Each carbon dioxide molecule is made up of two different types of atoms — a carbon atom and two oxygen atoms — chemically combined together.

The chemical formula of a compound indicates the number and type of atoms present in one of its molecules. For example, the carbon dioxide molecule is represented using the chemical formula, CO₂:

• C — chemical symbol of carbon
• O — chemical symbol of oxygen
• 2 (subscript) — indicating that there are two O atoms for every C atom in the molecule

Water is another example of a compound that exists as molecules. Each water molecule is made up of two hydrogen atoms and an oxygen atom. Hence, the water molecule is represented using the chemical formula, H₂O.

Molecules may be made up of different numbers of atoms. The number of atoms in a molecule determines its type:

Molecules that are made up of four or more atoms are also known as polyatomic molecules. For example, methane (CH₄) and ammonia (NH₃) exist as polyatomic molecules.