Atomic Numbers

The elements on the modern periodic table are listed in order of increasing atomic number. The number of protons in the nucleus of an atom is its atomic number. The number of protons tells us what an element is (i.e., an element with 6 protons is a carbon atom, no matter how many neutrons may be present). The number of protons determines how many electrons are around the nucleus, and how these electrons are arranged determines most of an element’s chemical behaviour.

1A	2A												3A	4A	5A	6A	7A	8A
	(1)	(2)												(13)	(14)	(15)	(16)	(17)	(18)
			3B		4B	5B	6B	7B	—	8B	—	1B	2B
			(3)		(4)	(5)	(6)	(7)	(8)	(9)	(10)	(11)	(12)
1	1																		2
2	3	4												5	6	7	8	9	10
3	11	12												13	14	15	16	17	18
4	19	20	21		22	23	24	25	26	27	28	29	30	31	32	33	34	35	36
5	37	38	39		40	41	42	43	44	45	46	47	48	49	50	51	52	53	54
6	55	56	57		72	73	74	75	76	77	78	79	80	81	82	83	84	85	86
7	87	88	89		104	105	106	107	108	109	110	111	112	—	114	—	—	—	—
6					58	59	60	61	62	63	64	65	66	67	68	69	70	71
7					90	91	92	93	94	95	96	97	98	99	100	101	102	103

In an atomic number-based periodic table, elements with similar chemical properties tend to fall into the same column (group). For example, all of the elements in Group 1A are soft metals that react violently with water and make 1+ charges. All of the elements in Group 8A are nonreactive monatomic gases at room temperature. In other words, the properties of chemical elements tend to repeat themselves over and over again as their mass goes up.

In Dimitri Mendeleev’s first periodic table, published in 1869, the elements were arranged in order of increasing atomic mass. At the time, the nucleus had not yet been found, and no one knew how the inside of an atom was put together, so atomic mass was the only way to organise the elements. Once the structure of the nucleus was understood, it was clear that the atomic number was what controlled the properties of the elements.