صَدْرَ

A word in the Arabic language will either be 1) a verb or 2) a noun or 3) a particle – صَدْرَ is a noun

A noun may either be 1) Definite or indefinite 2) Masculine or feminine 3) Singular, dual or plural 4) Nominative, accusative or genitive

الصَّدْرُ is the natural state of this definite noun and becomes الصََّدْرَ due to it being the: 1) المَفْعُوْلُ بِهِ object of a verb or 2) اسْمُ of إِنَّ or 3) خَبَرُ of كَانَ or 4) المَفْعُوْلُ فِيْهِ adverb of time or place or 5) المَفْعُوْلُ لَهُ noun that gives the reason for doing a thing or 6) المَفْعُوْلُ مَعَهُ noun that comes after the و which means “along with” or 7) المَفْعُوْلُ المُطْلَقُ is the مَصْدَرٌ of the verb occurring in the sentence or 8) الحَالُ adverb of manner or 9) التَّمْيِيْزُ noun that specifies the meaning of a vague word or 10) المُسْتَثْنَى noun that comes after إِلَّا meaning “except” or 11) المُنَادَى noun of calling or 12) it’s a noun of dependent declension a) نَعْتٌ / صِفَطٌ adjectives b) التَّوْكِيْدُ emphatic c) المَعْطُوْفٌ conjunction d) البَدَلُ synonym

الصَّدْرَ loses it’s definite article, becoming صَدْرَ due to it being a possessed noun

Heart, Breast

To return from, come back, proceed, go forward, come to pass, happen, emanate from, strike on the chest, commence

Surah/Below are the 3 verses in the holy Quran containing the possessed nounصَدْرَ
Verse
so, whoever Allah (SWT) wills to guide him, He expands his breast to submissionفَمَنْ يُرِدِ اللهُ أَنْ يَهْدِيَهُ يَشْرَحْ صَدْرَهُ لِلإِسْلَامِ
and whoever He wills to misguide him, He makes his breast narrowوَمَنْ يُرِدْ أَن يُضِلَّهُ يَجْعَلْ صَدْرَهُ ضَيِّقًا
constricted as though he climbs into the skyحَرَجًا كَأَنَّمَا يَصَّعَّدُ فِي السَّمَآءِ
6-125thus Allah (SWT) places the wickedness on those who do not believeكَذَلِكَ يَجْعَلُ اللهُ الرِّجْسَ عَلَى الَّذِيْنَ لَا يُؤْمِنُوْنَ
39-22
94-1Have We not opened up for you your breast?أَلَمْ نَشْرَحْ لَكَ صَدْرَكَ