0620 · 2.2Atomic structure and the Periodic Table

Atomic particles and charges

0:00A sodium atom contains eleven electrons, but they are not all arranged in the same shell. Their arrangement connects the atom to its position in the Periodic Table. Its nucleus also tells us which element it is. By the end of this video, you will understand how these different pieces fit together. An element contains only one type of atom. Each element has a name and a chemical symbol, which help us identify it in the Periodic Table.

0:24However, atoms themselves contain smaller particles. Understanding those particles lets us explain the numbers and patterns that appear in the table. An atom has a central nucleus containing protons and neutrons, surrounded by electrons in shells. The nucleus occupies a very small part of the atom. Electrons are found outside it, rather than mixed among its protons and neutrons. A shell diagram shows their arrangement, but it is not drawn to scale.

0:51The three particles differ in their electrical charges and masses. Charge tells us whether a particle is positive, negative or neutral. Mass describes how much matter the particle contains. Keeping these properties separate is important because a particle can have a charge without contributing much to the atom’s mass. A proton has a relative charge of plus one and a relative mass of one. A neutron has a relative charge of zero and a relative mass of one.

1:17An electron has a relative charge of minus one. Its relative mass is about one divided by one thousand eight hundred and forty. Relative masses compare the particles using the proton’s mass as a reference. A neutron has approximately the same mass as a proton. An electron is much lighter, so its mass is often described as negligible at this level. Almost all the mass of an atom is therefore concentrated in its nucleus.

1:41The electron’s small mass does not mean its charge is small. One electron has a negative charge equal in size to the positive charge of one proton. A neutral atom has equal numbers of protons and electrons. Their charges cancel overall, while neutrons contribute no electrical charge.

Proton number and mass number

1:58Proton number, also called atomic number, is the number of protons in the nucleus of an atom. This number identifies the element. Every sodium atom has eleven protons, while every magnesium atom has twelve. Changing the number of protons means changing which element the atom belongs to. For a neutral atom, proton number also tells us the number of electrons. Sodium has eleven protons, so a neutral sodium atom has eleven electrons.

2:25This follows from charge balance, not from a separate rule to memorise. The same reasoning works for every neutral atom, regardless of its element. Mass number, also called nucleon number, is the total number of protons and neutrons in the nucleus of an atom. Protons and neutrons are collectively called nucleons because they are particles in the nucleus. Electrons are not included in this count. Mass number counts particles rather than measuring mass in grams.

2:50To find the number of neutrons, subtract proton number from mass number. The mass number includes both types of nuclear particle. Removing the proton count leaves the neutron count. For an atom with mass number twenty-three and proton number eleven, twenty-three minus eleven gives twelve neutrons. That atom therefore contains eleven protons and twelve neutrons. If it is neutral, it also contains eleven electrons. Notice that the electron count comes from proton number, not mass number.

3:20Neutrons have no charge, so they play no part in the electron count.

Electronic configurations in shells

3:24Electronic configuration describes how electrons are arranged in shells. We write the number in each occupied shell, starting with the innermost shell. Aluminium has the configuration two, eight, three. This means two electrons in the first shell, eight in the second, and three in the third. For elements with proton numbers from one to twenty, use a straightforward filling pattern. Put up to two electrons in the first shell, then up to eight in the second.

3:49Next, put up to eight in the third before beginning the fourth. Potassium and calcium then have one and two electrons respectively in the fourth shell. This is the filling pattern needed for the first twenty elements. In heavier elements the third shell can hold more than eight electrons. You do not need that for the first twenty. Always start with the total electron count, then distribute those electrons from the inside out.

4:12Let us build aluminium’s configuration rather than simply recall it. Its proton number is thirteen, so a neutral atom has thirteen electrons. Putting two in the first shell leaves eleven electrons. Putting eight in the second leaves three, giving the electronic configuration two, eight, three. The configurations on screen all follow that same counting process. Hydrogen has one electron, and helium has two, filling the first shell. Lithium begins the second shell, while sodium begins the third.

4:42Potassium begins the fourth, so calcium’s twenty electrons are arranged two, eight, eight, two. A useful check is to add the shell numbers together. Calcium’s configuration gives two plus eight plus eight plus two, making twenty electrons. For a neutral atom, that total must equal its proton number. A configuration totalling nineteen cannot describe a neutral calcium atom, even if each individual shell looks reasonable.

Electron arrangements of ions

5:07An ion is an atom or group of atoms with an electrical charge. Here, we only need ions formed from individual atoms to work out their electron arrangements. Losing electrons produces a positive ion, while gaining electrons produces a negative ion. The number of protons stays unchanged during these changes. A sodium ion with a 1+ charge has one fewer electron than a neutral sodium atom. Eleven protons and ten electrons leave one positive charge overall.

5:34Its ten electrons occupy the first two shells, giving two, eight. The third shell is now empty, so it is not included in the configuration. A chloride ion with a 1− charge has one more electron than a neutral chlorine atom. Chlorine’s proton number is seventeen, so the ion has eighteen electrons. Its configuration is two, eight, eight. Eighteen negative charges against seventeen positive charges leave an overall charge of 1−.

6:01The sign of the charge tells you whether electrons were lost or gained. The size of the charge tells you how many. A magnesium ion with a 2+ charge has ten electrons because twelve minus two equals ten. An oxide ion with a 2− charge has gained two electrons, giving eight plus two, which is also ten. Both those ions have the electronic configuration two, eight, but they are not the same element.

6:26Magnesium still has twelve protons, while oxygen still has eight. Electron arrangements can match without the nuclei matching. The element’s identity depends on proton number, not on how many electrons it currently contains. Calcium shows why an emptied shell is left out. A calcium atom has twenty electrons, arranged two, eight, eight, two. A calcium ion with a 2+ charge has lost its two outer electrons, leaving eighteen. Its configuration is two, eight, eight, the same arrangement as an argon atom.

Groups, periods and exam practice

6:55The Periodic Table connects these electron arrangements to rows and columns. Its vertical columns are called groups, and its horizontal rows are called periods. In Groups I to seven, the number of outer shell electrons equals the group number. These groups are labelled with Roman numerals in the convention used here. Aluminium has three electrons in its outer shell, so it belongs to Group III. Chlorine has seven outer shell electrons, so it belongs to Group VII.

7:22Sodium and potassium each have one, placing both in Group I. Group number describes the outer shell, not the total number of electrons in the atom. The number of occupied electron shells equals the period number. Aluminium has three occupied shells, so it is in Period three. Calcium has four, placing it in Period four. Count the occupied shells, not the outer electrons. Group VIII contains the noble gases, which have a full outer electron shell.

7:49Helium’s only shell is full with two electrons. Neon and argon each have eight electrons in their outer shell. The important description is a full outer shell, rather than eight outer electrons in every noble gas. Use the electron arrangement of the neutral atom when identifying an element’s group and period. A sodium ion has only two occupied shells, but sodium remains an element in Period three. Forming an ion does not move the element within the table.

8:14Its position corresponds to the arrangement of the neutral atom. An exam question can bring all these relationships together. Consider a neutral atom with proton number twelve and mass number twenty-four. You are asked to state its particle counts and determine its electronic configuration. You must also identify its group and period, then find the configuration of its ion with a 2+ charge. The atom has twelve protons because its proton number is twelve.

8:40It has twelve neutrons because twenty-four minus twelve equals twelve. It has twelve electrons because it is neutral. Its electronic configuration is two, eight, two. The element is in Group II because its atom has two outer shell electrons. It is in Period three because its atom has three occupied shells. Its ion has ten electrons because it has lost two. The ion’s electronic configuration is two, eight. Common exam mistakes come from confusing what the numbers count.

9:10Mass number includes protons and neutrons, but not electrons. A positive ion has fewer electrons, not extra protons. An emptied shell is never written, so a sodium ion is two, eight and not two, eight, zero. For the first twenty neutral atoms, potassium starts a fourth shell rather than putting a ninth electron into the third. The whole topic connects three kinds of information. Proton number identifies the element, while mass number counts the particles in its nucleus.

9:37Electronic configuration shows how its electrons occupy shells. In Groups I to seven, outer electrons give the group number. Occupied shells give the period, and noble gases have full outer shells.