Atomic number and periodic table

In the periodic table the elements are arranged in order of increasing atomic number.

The atomic number is shown in the box with the symbol for the element.

1	2
							1H										2He
3Li	4Be											5B	6C	7N	8O	9F	10Ne
11Na	12Mg											13Al	14Si	15P	16S	17Cl	18Ar
19K	20Ca	21Sc	22Ti	23V	24Cr	25Mn	26Fe	27Co	28Ni	29Cu	30Zn	31Ga	32Ge	33As	34Se	35Br	36Kr
37Rb	38Sr	39Y	40Zr	41Nb	42Mo	43Tc	44Ru	45Rh	46Pd	47Ag	48Cd	49In	50Sn	51Sb	52Te	53I	54Xe
55Cs	56Ba	71Lu	72Hf	73Ta	74W	75Re	76Os	77Ir	78Pt	79Au	80Hg	81Tl	82Pb	83Bi	84Po	85At

Why was the atomic number chosen to equal the number of protons?
This is because any atom of an element has the same number of protons.

In contrast
Atoms of the same element may have different numbers of neutrons.

These different forms of atoms of the same element are referred to as isotopes.

Atoms in compounds have different numbers of electrons than in their elemental form.

Consider the compound NaCl (which is common salt).

The number of electrons on sodium in NaCl is fewer than for an atom of sodium.
The number of electrons on chlorine in NaCl is more than for an atom of chlorine.
BUT
The number of protons on sodium in NaCl is the same as an atom of sodium.
The number of protons on chlorine in NaCl is the same as an atom of chlorine.