Atomic Structure and the Periodic Table

Count on it: each 0620 series asks you to count protons, neutrons and electrons from a nuclide symbol, and each series a slice of candidates subtracts the wrong way and gets negative neutrons. Atomic structure is pure bookkeeping (three particles, two numbers, one filling rule), yet it feeds the Periodic Table, isotopes, bonding and Ar, so an error here echoes through four other topics.

The particles in an atom (Core)

An atom has a central nucleus containing protons and neutrons, with electrons arranged in shells (energy levels) around it. Almost all the mass sits in the nucleus; most of the atom is empty space.

Particle	Relative mass	Relative charge	Location
Proton	1	+1	Nucleus
Neutron	1	0	Nucleus
Electron	1/1840 (negligible)	−1	Shells around nucleus

Atoms are electrically neutral because the number of protons equals the number of electrons: the positive and negative charges cancel. Lose or gain electrons and you have an ion, covered in ions and ionic bonds.

Proton number and nucleon number (Core)

Proton number (atomic number) Z = number of protons. It defines the element: every atom with 6 protons is carbon, no exceptions. Nucleon number (mass number) A = protons + neutrons. In the nuclide notation the nucleon number sits on top and the proton number below, before the symbol, for example 23 over 11 Na.

The three counts from a nuclide symbol:

Quantity	How to find it	For 23/11 Na
Protons	= proton number	11
Electrons (neutral atom)	= proton number	11
Neutrons	= nucleon number − proton number	23 − 11 = 12

The Periodic Table is arranged in order of increasing proton number, not mass, a distinction Paper 2 occasionally probes (tellurium before iodine breaks the mass order).

Electronic configuration (Core)

Electrons fill shells from the innermost out: the first shell holds up to 2 electrons, the second up to 8, the third up to 8 for 0620 purposes. Configurations are written as numbers separated by commas. Chlorine (17 electrons): 2,8,7. Calcium (20): 2,8,8,2. The syllabus expects configurations for the first 20 elements.

Two patterns convert configuration into Periodic Table position. The number of outer-shell electrons equals the group number (Group VII chlorine has 7 outer electrons); the number of occupied shells equals the period number (chlorine, three shells, Period 3). Group VIII (the noble gases) have full outer shells (2 for helium, 8 for the rest), which is why they are unreactive. Chemical properties are decided by outer-shell electrons, which is why elements in the same group react alike. That single sentence explains most of topic 8.

Worked exam question

An atom of element X is represented as 40/19 X. (a) Deduce the number of protons, neutrons and electrons in this atom. (3) (b) Write the electronic configuration of X and state its group in the Periodic Table. (2)

Model answer: (a) Protons = 19 (1); neutrons = 40 − 19 = 21 (1); electrons = 19 (1). (b) 2,8,8,1 (1); Group I, because it has one outer-shell electron (1).

Mark-by-mark: each count in (a) is a separate mark, so a slip in one leaves the other two intact. Write all three explicitly. In (b) the configuration must total 19; the group mark follows from the single outer electron, and naming the element (potassium) is acceptable supporting detail but not required. Stating “Group 19” confuses proton number with group and scores zero for that point.

The mistakes that cost marks

1. Neutrons calculated as proton number subtracted from nothing sensible. A − Z is the only correct route. Check the answer is positive and plausible.
2. Configurations that do not add up: 2,8,8 written for chlorine (totals 18, not 17). Always re-add the digits.
3. Electron mass written as 0. It is negligible, not zero. Mark schemes want 1/1840, “negligible” or approximately 1/2000.
4. Confusing group number with proton number, or saying the table is ordered by mass. Order is by proton number; group = outer electrons; period = shells.

How examiners want it phrased

Typical student wording	Accepted mark-scheme wording
”The atomic number is how heavy it is"	"Proton number is the number of protons in the nucleus"
"Electrons go around randomly"	"Electrons are arranged in shells (energy levels) around the nucleus"
"It’s in Group I because it’s a metal"	"It has one electron in its outer shell, so it is in Group I"
"Atoms are neutral because charges balance somehow"	"Number of protons equals number of electrons, so the charges cancel”

The Malaysia note

Most Malaysian international-school students learn this in Year 9 or early Year 10, then sit the real exam up to two years later in the May/June series, long enough for 2,8,8 to blur. SPM students meet the same model labelled “nombor proton” and “nombor nukleon”, so the vocabulary transfers directly. The blur is the danger: ten quick nuclide drills restore it. Bring a past-paper question to a free trial lesson and a Chemistry specialist will show you how examiners chain this content into isotope and bonding marks.

Test yourself

Answer all three from memory before you click. The answers stay hidden until you do.

Q1 (3 marks). An atom of aluminium is represented as 27/13 Al. Deduce the number of protons, neutrons and electrons in this atom.

Show answer

• Protons = 13 [1] • Neutrons = 27 − 13 = 14 [1] • Electrons = 13 [1]

Q2 (2 marks). Sulfur has proton number 16. Write the electronic configuration of a sulfur atom and state its group in the Periodic Table.

Show answer

• 2,8,6 [1] • Group VI, because it has six electrons in its outer shell [1]

Q3 (2 marks). State the relative charge of each of the three subatomic particles, and explain why an atom has no overall charge.

Show answer

• Proton +1, neutron 0, electron −1 [1] • The number of protons equals the number of electrons, so the positive and negative charges cancel [1]